**Processes during Thermal Treatment**

The processes discussed in this chapter include the mechanisms and temperature ranges of chemical reactions and phase transformations, which take place during the treatment of strontium aluminate raw materials as the mixture of strontium carbonate and alumina. Fig.1 shows the equilibrium phase composition during the preparation of calcium (a), strontium (b) and barium aluminate clinker (c). These systems show many similarities, therefore alumina react with carbonate in the molar ratio close to one. Therefore, is interesting to compare the changes of equilibrium composition which take place with increasing temperature for clinkers and pure carbonates (d).

**Figure 1.** Calculated equilibrium composition during the thermal treatment of calcium (a), strontium (b) and barium aluminate cement (c) and thermal decomposition of pure carbonates (d).

© 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

It can be seen that strontium dialuminate phase should be formed first via the reaction of alumina with strontium oxide after the thermal decomposition of strontium carbonate. The maximum amount of strontium dialuminate occurs at the temperature of 540 °C. The amount of SrO∙2Al2O3 phase decreases with increasing temperature and formed amount of strontium aluminate. This behaviour is in agreement with experimental results (Chapter 4.3), but the temperature of the process is lower. On the other hand, the differences between the equilibrium temperature and the temperature determined by experiment are common fact for heteroge‐ neous systems, where the kinetic obstacles e.g. related to the nucleation of product occur. The content of strontium aluminate reaches the maximum at the temperature of 675 °C. Except for strontium hexaaluminate, the content of other phases from SrO – Al2O3 system increases with increasing temperature.

The similar behaviour can be observed for calcium aluminate cement, where higher aluminate phases (SrA2 and SrA6) are formed prior to calcium aluminate. On the contrary the formation of higher aluminate phases did not occur during the formation of barium aluminate. The formation of all three aluminates depends on the thermal decomposition of carbonates where the equilibrium constant of the process is not changed by the activity of formed clinker minerals. The mechanism, kinetics and thermodynamics of the process of thermal decompo‐ sition of strontium carbonate are discussed in Chapter 4.2.

**Figure 3.** Temperature dependence of equilibrium composition in the systems with higher content of CaO (a) and SrO

The α-Al2O3 modification of alumina can be used for the synthesis of strontium aluminate clinker. On the other hand, the most stable and well crystalline phases are also less reactive. That often means a long time synthesis which must be performed at high temperatures. Due the thermodynamic reasons, it is better to use **aluminium hydroxides** (Al(OH)<sup>3</sup> or Al2O3⋅3H2O), **oxyhydroxides** (AlO(OH) or Al2O3⋅H2O) or the **transitional alumina oxides** (Fig.4) for the preparation of strontium aluminate as the main clinker mineral for strontium

Individual Al(OH)<sup>3</sup> polymorphs differ in the terms of stacking sequences of layers which are bound together by weak hydrogen bonds. The following main types of aluminium hydroxides

**• Gibbsite (hydrargyllite**, Fig.5(a)) is the monoclinic *γ*-Al(OH)3 (P21/n, *a*=8.684 Å, *b*=5.078 Å, c=9.736 Å, *β*=94.54°). The crystals mostly show tabular pseudohexagonal habit. The structure of gibbsite is often drawn as sheets of *hcp* layers with open packing between successive sheets. In the lateral extension of hexagonal closed packed sheets each Al3+ion is coordinated by six hydroxyl (OH) groups. Each OH group is coordinated by two Al3+ions with one vacant

shows a perfect cleavage parallel to the basal plane (001). The layers are arranged in the AA-

**• Bayerite** (Fig.5(b)) is the monoclinic α-Al(OH)3 (P21/a, *a*=5.0626 Å, *b*=8.6719 Å, c=9.4254 Å, *β*=90.26°). It mostly occurs in very fine fibers in radiating hemispherical aggregates and sometimes forms flaky tabular crystals with the size of about 0.1 mm. The crystal lattice of bayerite is composed of layers of hydroxyl groups similar to those in gibbsite. These layers are arranged in the AB-AB-AB sequence, so that hydroxyl groups of the third layer lie in

6+ ring. Gibbsite

Processes during Thermal Treatment 99

octahedral site. Six octahedra, each sharing two edges, yield [Al6(OH)12]

BB-AA sequence. Gibbsite was firstly described in 1820 [336,337].

(b).

aluminate cements.

and oxohydroxides are [325-335]:

**1. Thermal treatment of alumina**

The equilibrium composition in the mixture of ternary system CaO – SrO – Al2O3, which consists of CaCO3, SrCO3 and Al2O3 in the molar ratio ½ : ½ : 1 is shown in Fig.2. Increasing amount of CaO and SrO in the raw material (Fig.3) supports the formation of C2SrA and SrA in the reaction mixture, respectively.

**Figure 2.** Influence of temperature on the equilibrium composition in the ternary system.

It was observed that increasing amount of glassy phase led to greenish tinge of fired clinker while the material treated to the temperature at which the glassy phase could not be formed was white. On the other hand, increasing amount of glassy phase increased the demand on milling of the clinker to strontium aluminate cement.

**Figure 3.** Temperature dependence of equilibrium composition in the systems with higher content of CaO (a) and SrO (b).

### **1. Thermal treatment of alumina**

It can be seen that strontium dialuminate phase should be formed first via the reaction of alumina with strontium oxide after the thermal decomposition of strontium carbonate. The maximum amount of strontium dialuminate occurs at the temperature of 540 °C. The amount of SrO∙2Al2O3 phase decreases with increasing temperature and formed amount of strontium aluminate. This behaviour is in agreement with experimental results (Chapter 4.3), but the temperature of the process is lower. On the other hand, the differences between the equilibrium temperature and the temperature determined by experiment are common fact for heteroge‐ neous systems, where the kinetic obstacles e.g. related to the nucleation of product occur. The content of strontium aluminate reaches the maximum at the temperature of 675 °C. Except for strontium hexaaluminate, the content of other phases from SrO – Al2O3 system increases with

98 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

The similar behaviour can be observed for calcium aluminate cement, where higher aluminate phases (SrA2 and SrA6) are formed prior to calcium aluminate. On the contrary the formation of higher aluminate phases did not occur during the formation of barium aluminate. The formation of all three aluminates depends on the thermal decomposition of carbonates where the equilibrium constant of the process is not changed by the activity of formed clinker minerals. The mechanism, kinetics and thermodynamics of the process of thermal decompo‐

The equilibrium composition in the mixture of ternary system CaO – SrO – Al2O3, which consists of CaCO3, SrCO3 and Al2O3 in the molar ratio ½ : ½ : 1 is shown in Fig.2. Increasing amount of CaO and SrO in the raw material (Fig.3) supports the formation of C2SrA and SrA

It was observed that increasing amount of glassy phase led to greenish tinge of fired clinker while the material treated to the temperature at which the glassy phase could not be formed was white. On the other hand, increasing amount of glassy phase increased the demand on

sition of strontium carbonate are discussed in Chapter 4.2.

**Figure 2.** Influence of temperature on the equilibrium composition in the ternary system.

milling of the clinker to strontium aluminate cement.

in the reaction mixture, respectively.

increasing temperature.

The α-Al2O3 modification of alumina can be used for the synthesis of strontium aluminate clinker. On the other hand, the most stable and well crystalline phases are also less reactive. That often means a long time synthesis which must be performed at high temperatures. Due the thermodynamic reasons, it is better to use **aluminium hydroxides** (Al(OH)<sup>3</sup> or Al2O3⋅3H2O), **oxyhydroxides** (AlO(OH) or Al2O3⋅H2O) or the **transitional alumina oxides** (Fig.4) for the preparation of strontium aluminate as the main clinker mineral for strontium aluminate cements.

Individual Al(OH)<sup>3</sup> polymorphs differ in the terms of stacking sequences of layers which are bound together by weak hydrogen bonds. The following main types of aluminium hydroxides and oxohydroxides are [325-335]:


the depressions between hydroxyl positions of the second layer. Bayerite was firstly described in 1925 [338].

**Phase pH of PZC/ IEP**

Bayerite

Gibbsite 8.1 – 9.8 (10) [342-344] 4.85 2.34

Nordstrandite ? 4.72 2.42 Doyleite … … 2.48

[344]

Boehmite 7.3 – 7.5 [343] 8.9 [343] 9.4 [344]

**Table 1.** Properties of strontium aluminate polymorphs.

and fine chemical processes [349,350].

alumina phases [325].

5.4 – 9.7 [347] 8.1 [345] 9.2

**d001 Density Al2O3 H2O [Å] [g·cm-3] [%]**

6.12 3.03 85.0 15.0

4.79 2.53

Pseudoboehmite 9.2 [346,344] … 77.7-84.0 16-22.3 Diaspore 7.9 [343] 6.9 – 8.8 [347] … 3.38 85.0 15.0 Amorphous 9.4-9.5 [348] … … … …

The thermal treatment of these compounds leads to the dehydration and subsequent formation of the most stable modification of α-Al2O3. The process is complicated due to the formation of different **transitional aluminas**. The industrial importance of transitional aluminas is the application of many phases as catalysts or carriers (support) for catalysts for petrochemical

**Figure 4.** Temperature transformation of hydroxides or oxohydroxides to corundum via the formation of transitional

Significant effects of pH value on the composition, structure, morphology, and phase trans‐ formation of precipitated aluminium hydroxide can be observed. During the increase of pH value from 5 to 11, the aluminum hydroxides precipitating from the solution vary from amorphous aluminum hydroxide through γ-AlOOH to α-Al(OH)3; and the corresponding morphology of the aluminum hydroxide particles changes from the ultrafine floccules through 50 nm blowballs up to irregular agglomerates with the average diameter of 150 nm. The aluminum hydroxides precipitating at different pH values have different transformation

sequences toward α-Al2O3 and different formation temperatures of α-Al2O3 [351]:

65.4 36.4

Processes during Thermal Treatment 101


Some selected properties of Al(OH)3 polymorphs including the point of zero charge are listed in Table 1. The experimental methods used to determine the PZC/IEP are described using the following abbreviations [343]:


<sup>1</sup> Van Nordstrad proposed the name Bayerite II for this polymorph of Al(OH)3.


**Table 1.** Properties of strontium aluminate polymorphs.

the depressions between hydroxyl positions of the second layer. Bayerite was firstly

100 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

**• Boehmite** is rhombic *γ*-AlO(OH) (*Amam*, *a*=3.6936 Å, *b*=12.214 Å, *c*=2.8679 Å) which has the same structure as *γ*-FeO(OH). The structure of boehmite consists of double layers of oxygen octahedra partially filled with Al3+cations. The stacking arrangement of the three oxygen layers is such that the double octahedral layer is in cubic closed packing. Within the double layer one can distinguish two different types of oxygen. Each oxygen atom in between the double layer is shared by four other octahedra, while oxygen atoms on the outer side are only shared by two octahedra. These outer oxygen atoms are hydrogen-bound to two other similarly coordinated oxygen atoms in the neighboring double layers above and below. The stacking of the layers is such that the hydroxyl groups of one layer are located over the

**• Pseudoboehmite** (**gelatinous boehmite**) contains higher amount of water than boehmite

*γ*=104.74° and Z=2) polymorph of gibbsite, bayerite and nordstrandite. Doyleite was firstly

**• Nordstrandite** (Fig.5(c)) is triclinic polymorph of Al(OH)3 (P1, *a*=6.148 Å, *b*=6.936 Å, *c*=5.074 Å, α=79.76°, *β*=99.06, *γ*=83.3°) synthesized and identified by van Nordstrand at al. in 1956

**• Diaspore** is orthorhombic α-AlO(OH) (*PBnm*, *a*=4.379 Å, *b*=9.421 Å, *c*=2.844 Å and Z=4)

**• Amorphous gel forms** precipitate from aluminium salts. According to applied conditions,

Some selected properties of Al(OH)3 polymorphs including the point of zero charge are listed in Table 1. The experimental methods used to determine the PZC/IEP are described using the

**•** CIP (common intersection point) of potentiometric titration curves obtained at three or more

**•** Intersection (intersection point) of potentiometric titration curves obtained at two ionic

**•** pH (natural pH of the dispersion), e.g., mass titration and potentiometric titration at one

**•** IEP (isoelectric point) obtained by means of electrophoresis, electroosmosis, or electroa‐

which is rare and occurs in the metamorphic and sedimentary bauxite ores.

. This polymorph occurs in nature and can be prepared by the crystallization of

¯or P1,*a*=5.002 Å, *b*=5.175 Å, *c*=4.98 Å, α=97.5°, *β*=118.6,

described in 1925 [338].

**• Doyleite** is triclinic Al(OH)3 (*P*1

described in 1985 [340].

following abbreviations [343]:

electrolyte concentration;

strengths);

coustic method.

ionic strengths or equivalent methods;

[341]1

depression of hydroxyl groups in the adjacent layer.

(*γ*-AlO(OH)⋅*n*H2O, where n ranges from 0.08 to 0.62) [339].

alumina gels at high pH and the presence of chelating agent.

various crystalline forms mentioned above can be prepared.

1 Van Nordstrad proposed the name Bayerite II for this polymorph of Al(OH)3.

The thermal treatment of these compounds leads to the dehydration and subsequent formation of the most stable modification of α-Al2O3. The process is complicated due to the formation of different **transitional aluminas**. The industrial importance of transitional aluminas is the application of many phases as catalysts or carriers (support) for catalysts for petrochemical and fine chemical processes [349,350].

**Figure 4.** Temperature transformation of hydroxides or oxohydroxides to corundum via the formation of transitional alumina phases [325].

Significant effects of pH value on the composition, structure, morphology, and phase trans‐ formation of precipitated aluminium hydroxide can be observed. During the increase of pH value from 5 to 11, the aluminum hydroxides precipitating from the solution vary from amorphous aluminum hydroxide through γ-AlOOH to α-Al(OH)3; and the corresponding morphology of the aluminum hydroxide particles changes from the ultrafine floccules through 50 nm blowballs up to irregular agglomerates with the average diameter of 150 nm. The aluminum hydroxides precipitating at different pH values have different transformation sequences toward α-Al2O3 and different formation temperatures of α-Al2O3 [351]:

**• Amorphous aluminum hydroxide** precipitating at the pH of 5-6 is transformed to α-Al2O3 at 950 °C via the following transformation sequence:

**2. High-temperature transition alumina phases** (*δ*-and *η*-Al2O3).

The beta modifications of alumina including *β*-and *β*''-alumina (three blocks structure) are not polymorphs of Al2O3, but they are binary or more complex solid solutions of alumina with different metal ions. The beta alumina group of oxides has closely-packed slabs (spinel block) and loosely-packed layers (conduction planes) with mobile ions. The spinel block consists of four layers of oxygen ions with aluminium ions in both octahedral and tetrahedral interstices.

Processes during Thermal Treatment 103

**Figure 6.** Stacking sequence of β´and β´´ alumina in the unit cell shown in the (11 2¯ 0) projection [**352**].

crystal structures (Fig.6):

alumina is Na2O 11Al2O3.

cells for the measurement of thermodynamic data [353-358].

From this viewpoint the following phases can be recognized:

Two neighboring spinel blocks are bound via the conduction plane. There are two different

**a. Hexagonal***β***-alumina** (P63/mmc; *a*=0.559 nm, *c*=2.261nm) where the conduction plane is sandwiched in between two spinel blocks (two-block structure). The ideal formula of *β*-

**b. Rhombohedral***β´´*-alumina (R3m; *a*=0.560 nm, *c*=3.395 nm) where two conduction slabs are separated by three spinel blocks. Since they accommodate higher amount of sodium ions, the structure of *β*´´-Al2O3 shows higher conductivity than *β*-Al2O3. The phase is stable up to the temperature of 1600 °C. The ideal formula of *β*´´-alumina is Na2O (Al2O3)6+x. These materials can be used as solid electrolytes (BASE), sodium heat engine or alkali metal thermoelectric converters for direct thermoelectric energy conversion, gas sensors, galvanic

**•** AM – *β and/ orβ´´*-alumina where AM denotes Alkali Metals (Li, Na, K, Rb…). Sodium *β*-or *β*´´-alumina (Na2O1+x 11Al2O3 where *x*=0.25 – 0.55) exhibits high sodium conductivity at relatively low temperatures (300 – 350 °C). The most convectional technique for the production is solid-state synthesis based on the thermal treatment of Na2CO3 and α-Al2O3

*Amorphous hydroxide → amorphous alumina → α-Al2O3*

**• Boehmite** precipitating at pH=7 transforms to α-Al2O3 at 950 °C via the following path:

*γ-AlOOH → γ-Al2O3→* α*-Al2O3*

**• Bayerite** precipitating at the pH values between 8 and 11, is transformed to α-Al2O3 at 1000 °C via the transformation sequence:

$$\alpha\text{-Al(OH)}\_3 \rightarrow \mathfrak{y}\text{-Al}\_2\text{O}\_3 \rightarrow \mathfrak{e}\text{-Al}\_2\text{O}\_3 \rightarrow \mathfrak{η}\text{-Al}\_2\text{O}\_3 \rightarrow \alpha\text{-Al}\_2\text{O}\_3$$

The temperature relationships between transitional alumina phases are complicated. The course of the process depends on the type of initial hydroxide or oxohydroxide due to their structural similarities, experimental conditions and content of admixtures (Fig.4). Some uncertainty still remains about the numbers and transition sequence between these phases. The course of the phase transitions seems to be given by the requirement to attain the closest structural similarity between original and newly formed phase. The structural similarity between the structures of original and newly formed phase is termed as **topotaxy**. The final product of the process is α-Al2O3 in all cases.

**Figure 5.** The structure of gibbsite (a), bayerite (b) and nordstrandite (c).

Two general groups of transitional alumina phases can be recognized [325]:


Another classification of transitional alumina phases defines:

**1. Low-temperature transition alumina phases** (*γ*-and *η*-Al2O3);

## **2. High-temperature transition alumina phases** (*δ*-and *η*-Al2O3).

**• Amorphous aluminum hydroxide** precipitating at the pH of 5-6 is transformed to α-Al2O3

102 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

**• Boehmite** precipitating at pH=7 transforms to α-Al2O3 at 950 °C via the following path:

**• Bayerite** precipitating at the pH values between 8 and 11, is transformed to α-Al2O3 at 1000

The temperature relationships between transitional alumina phases are complicated. The course of the process depends on the type of initial hydroxide or oxohydroxide due to their structural similarities, experimental conditions and content of admixtures (Fig.4). Some uncertainty still remains about the numbers and transition sequence between these phases. The course of the phase transitions seems to be given by the requirement to attain the closest structural similarity between original and newly formed phase. The structural similarity between the structures of original and newly formed phase is termed as **topotaxy**. The final

at 950 °C via the following transformation sequence:

*Amorphous hydroxide → amorphous alumina → α-Al2O3*

α*-Al(OH)<sup>3</sup> → γ-Al2O3 → ε-Al2O3 → η-Al2O3 →* α*-Al2O3*

*γ-AlOOH → γ-Al2O3→* α*-Al2O3*

°C via the transformation sequence:

product of the process is α-Al2O3 in all cases.

**Figure 5.** The structure of gibbsite (a), bayerite (b) and nordstrandite (c).

orthorhombic) *δ*-Al2O3 (*delta*) polymorphs.

Another classification of transitional alumina phases defines:

**1. Low-temperature transition alumina phases** (*γ*-and *η*-Al2O3);

Al2O3 (*chi*) are included.

Two general groups of transitional alumina phases can be recognized [325]:

**1. Transitional alumina with face-cantered cubic arrangement of oxygen anions** including cubic *γ*-Al2O3 (*gamma*) and *η*-Al2O3 (*eta*), monoclinic *θ*-Al2O3 (*theta*) and tetragonal (or

**2. Transitional alumina with hexagonal close packed arrangement** where trigonal poly‐ morph of α-Al2O3 (*alpha, corundum*), orthorhombic **κ**-Al2O3 (*kappa*) and hexagonal **χ**- The beta modifications of alumina including *β*-and *β*''-alumina (three blocks structure) are not polymorphs of Al2O3, but they are binary or more complex solid solutions of alumina with different metal ions. The beta alumina group of oxides has closely-packed slabs (spinel block) and loosely-packed layers (conduction planes) with mobile ions. The spinel block consists of four layers of oxygen ions with aluminium ions in both octahedral and tetrahedral interstices.

**Figure 6.** Stacking sequence of β´and β´´ alumina in the unit cell shown in the (11 2¯ 0) projection [**352**].

Two neighboring spinel blocks are bound via the conduction plane. There are two different crystal structures (Fig.6):


These materials can be used as solid electrolytes (BASE), sodium heat engine or alkali metal thermoelectric converters for direct thermoelectric energy conversion, gas sensors, galvanic cells for the measurement of thermodynamic data [353-358].

From this viewpoint the following phases can be recognized:

**•** AM – *β and/ orβ´´*-alumina where AM denotes Alkali Metals (Li, Na, K, Rb…). Sodium *β*-or *β*´´-alumina (Na2O1+x 11Al2O3 where *x*=0.25 – 0.55) exhibits high sodium conductivity at relatively low temperatures (300 – 350 °C). The most convectional technique for the production is solid-state synthesis based on the thermal treatment of Na2CO3 and α-Al2O3 mixture. Ion exchange in molten salt is further used for the preparation of other ion-alumina phases and their mixtures [359-361].

Fig.7 shows the temperature dependence of the synthesis of molar unit of strontium aluminate phase using different sources of alumina. These results clearly indicate that *γ*-Al2O3 is the most reactive oxide form for the synthesis of strontium aluminate while thermodynamically most stable α-Al2O3 reacts less willingly. Aluminum hydroxides and oxide-hydroxides are even more reactive, but they are the subject of dehydroxylation and phase transformation before sufficient temperature is reached. Therefore we need to identify aluminium hydroxide or oxide-hydroxide which form *γ*-Al2O3 within the temperature interval where the formation of

Processes during Thermal Treatment 105

**Figure 7.** Temperature dependence of ΔrG° for the formation of structure unit of SrAl2O4 from different sources of

Diaspore (α-AlO(OH)) is directly transformed to the lesser reactive form of α-Al2O3 (Fig.4) within the temperature range from 350 to 500 °C (please compare Fig.8(a) and (c)). From this viewpoint, it is not appropriate to use it for the synthesis. In the temperature interval where the formation of strontium aluminate takes place, the thermal transformations of gibbsite,

These transitional alumina phases are all more reactive than corundum, but the question is which of the transitional alumina phases is the most reactive or which transition sequence is optimal and will prevail the for synthesis of strontium aluminate clinker. In the assumption that the phases which require the minimum structural changes (or have the highest structural similarity) are formed predominantly, the bayerite leading to the monoclinic Θ-Al2O3 should be considered as the most reactive phase. On the other hand, there is another factor such as alumina source availability. From this point of view, the gibbsite as the main product of Bayer

The course of synthesis of strontium aluminate clinker can be significantly changed by the source of Al2O3. Regardless of raw material prices, the usage of thermodynamically most stable

2 Corundum for ceramics purposes is often produced from bauxite via Bayer process (Chapter 2.1.2).

) as the source of aluminium oxide requires the highest

bayerite and boehmite lead to *κ*-Al2O3, Θ-Al2O3 and *δ*-Al2O3, respectively.

strontium aluminate proceeds.

process is promising raw material.

modification of α-Al2O3 (corundum2

temperatures and the longest time for the synthesis.

alumina.


## **1.1. Influence of the source of alumina**

It can be concluded that there are several ways to produce strontium aluminate clinker from raw material bases:


Using alumina is promising way for the preparation of high purity product, where the course of solid state reaction (Eq.43), the phase composition and the properties of product can be easily controlled. Moreover, transition aluminas, hydroxides, oxohydroxides or gels have an advantage in higher reactivity.

Using bauxite may increase the content of Fe in the strontium aluminate cement which decreases the thermal stability of materials especially after the reduction conditions. In principle the same production process as for alumina cement can be used.

The activated aluminas, i.e. acidic, neutral and basic aluminas (no definite chemical composi‐ tions; made by adding various amounts of water to activated aluminas) are used extensively as adsorbents because of their affinity for water and other polar molecules; and as catalysts because of their large surface area and appropriate pore structure. As adsorbents, they are used for drying gases and liquids; and in adsorption chromatography. The catalytic properties may be attributed to the presence of surface active sites (primarily OH– , O2–, and Al3+ions). Such catalytic applications include the sulphur recovery from H2S (Clauss catalysis); the dehydra‐ tion of alcohols, the isomerization of olefins; and as the catalyst support in petroleum refining [91].

Fig.7 shows the temperature dependence of the synthesis of molar unit of strontium aluminate phase using different sources of alumina. These results clearly indicate that *γ*-Al2O3 is the most reactive oxide form for the synthesis of strontium aluminate while thermodynamically most stable α-Al2O3 reacts less willingly. Aluminum hydroxides and oxide-hydroxides are even more reactive, but they are the subject of dehydroxylation and phase transformation before sufficient temperature is reached. Therefore we need to identify aluminium hydroxide or oxide-hydroxide which form *γ*-Al2O3 within the temperature interval where the formation of strontium aluminate proceeds.

mixture. Ion exchange in molten salt is further used for the preparation of other ion-alumina

104 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

**•** AEM – *β and/ orβ´´*-alumina where AE denotes Alkaline Earth Elements=Ca, Sr, Ba. The structure consists of two spinel block and an intermediate layer. Large cations such as Ba2+occupy the nine-fold coordination sites in intermediate layer of b-alumina structure. Small cations, such as Sr2+or Ca2+are located in the twelve-fold coordination sites of MP

**•** REE – *β and/ orβ´´*-alumina where REE denotes Rare Earth Elements such as Gd3+, Eu3+, Nd3+

It can be concluded that there are several ways to produce strontium aluminate clinker from

**3.** The mixture of strontium carbonate with aluminium hydroxides, oxohydroxides or gels;

Using alumina is promising way for the preparation of high purity product, where the course of solid state reaction (Eq.43), the phase composition and the properties of product can be easily controlled. Moreover, transition aluminas, hydroxides, oxohydroxides or gels have an

Using bauxite may increase the content of Fe in the strontium aluminate cement which decreases the thermal stability of materials especially after the reduction conditions. In

The activated aluminas, i.e. acidic, neutral and basic aluminas (no definite chemical composi‐ tions; made by adding various amounts of water to activated aluminas) are used extensively as adsorbents because of their affinity for water and other polar molecules; and as catalysts because of their large surface area and appropriate pore structure. As adsorbents, they are used for drying gases and liquids; and in adsorption chromatography. The catalytic properties may

catalytic applications include the sulphur recovery from H2S (Clauss catalysis); the dehydra‐ tion of alcohols, the isomerization of olefins; and as the catalyst support in petroleum refining

, O2–, and Al3+ions). Such

principle the same production process as for alumina cement can be used.

be attributed to the presence of surface active sites (primarily OH–

, Pb2+mobile ions were used.

phases and their mixtures [359-361].

structure in aluminum oxide [362].

**•** M – *β and/ orβ´´*-alumina where Ag+

**1.1. Influence of the source of alumina**

**1.** The mixture of strontium carbonate with alumina;

**4.** The mixture of strontium carbonate with bauxite;

**2.** The mixture of strontium carbonate with transitional aluminas;

… [363]

raw material bases:

**5.** The advanced techniques.

advantage in higher reactivity.

[91].

**Figure 7.** Temperature dependence of ΔrG° for the formation of structure unit of SrAl2O4 from different sources of alumina.

Diaspore (α-AlO(OH)) is directly transformed to the lesser reactive form of α-Al2O3 (Fig.4) within the temperature range from 350 to 500 °C (please compare Fig.8(a) and (c)). From this viewpoint, it is not appropriate to use it for the synthesis. In the temperature interval where the formation of strontium aluminate takes place, the thermal transformations of gibbsite, bayerite and boehmite lead to *κ*-Al2O3, Θ-Al2O3 and *δ*-Al2O3, respectively.

These transitional alumina phases are all more reactive than corundum, but the question is which of the transitional alumina phases is the most reactive or which transition sequence is optimal and will prevail the for synthesis of strontium aluminate clinker. In the assumption that the phases which require the minimum structural changes (or have the highest structural similarity) are formed predominantly, the bayerite leading to the monoclinic Θ-Al2O3 should be considered as the most reactive phase. On the other hand, there is another factor such as alumina source availability. From this point of view, the gibbsite as the main product of Bayer process is promising raw material.

The course of synthesis of strontium aluminate clinker can be significantly changed by the source of Al2O3. Regardless of raw material prices, the usage of thermodynamically most stable modification of α-Al2O3 (corundum2 ) as the source of aluminium oxide requires the highest temperatures and the longest time for the synthesis.

<sup>2</sup> Corundum for ceramics purposes is often produced from bauxite via Bayer process (Chapter 2.1.2).

**Figure 8.** Formation of strontium aluminate clinker from the mixture of SrCO3 with corundum (a), gibbsite, bayerite (c) and diaspore (d)

## **1.2. Thermal decomposition of Al(OH)3**

Thermal decomposition of gibbsite can be described by the reaction [364,365]:

$$2\text{ Al(OH)}\_{3}(\text{s}) \rightarrow \text{Al}\_{2}\text{O}\_{3}(\text{s}) + 3\text{ H}\_{2}\text{O}(\text{g})\tag{1}$$

It is generally accepted that hydrothermal conditions favor the formation of boehmite from gibbsite. The pathway of the dehydration is affected by the particle size, partial pressure of water vapor and heating rate. On the contrary to the dehydration sequence in Fig.4, there are

Processes during Thermal Treatment 107

**b.** Large crystal (> 100 μm) in wet air under the pressure higher than atmospheric;

**Figure 9.** Influence of conditions on the reaction pathway for the dehydration of gibbsite [367].

**2. Thermal decomposition of strontium carbonate**

When mechanically activated by vigorous grinding, many crystalline materials, including gibbsite, are known to lose their long-range order and become X-ray amorphous. Amorphous phase retains its water content and becomes "gel-like". The thermal dehydration of this amorphous phase broadens the characteristic gibbsite dehydroxylation endotherm and lowers its temperature to about 150 – 200°C. The dehydrated product is also X-ray amorphous and is thermally converted to α-alumina (Fig.4) either via *η*-alumina or via the *γ*-*δ*-*η* sequence [364]. The dehydration of boehmite is a topotactic process, which can be described by the following

The mechanism of boehmite transformation to *γ*-Al2O3 involves the elimination of water formed by protons and hydroxyl groups and the migration of Al cations. The latter is the rate-

The thermal decomposition of strontium carbonate should be described by simple first order

23 2 2 AlOOH(s) Al O (s)+H O(g) ® (2)

at least three ways for the dehydration of gibbsite [367]:

This transformation sequences are shown in Fig.9.

**c.** Flash calcination of gibbsite.

decomposition reaction [73,368]:

limiting step.

chemical equation:

**a.** Small crystals (< 10 μm) in dry air at atmospheric pressure;

The published data related to the kinetic triplet, which includes the activation energy, the frequency factor and the kinetic factor of the process, are listed in Table.2.


**Table 2.** Thermal decomposition of aluminas.

It is generally accepted that hydrothermal conditions favor the formation of boehmite from gibbsite. The pathway of the dehydration is affected by the particle size, partial pressure of water vapor and heating rate. On the contrary to the dehydration sequence in Fig.4, there are at least three ways for the dehydration of gibbsite [367]:


**1.2. Thermal decomposition of Al(OH)3**

**Kinetic triplet data**

and diaspore (d)

**Ea [kJ·mol-1] A [s-1] n**

<sup>175</sup> 1.20∙10<sup>9</sup> *<sup>m</sup>* = 0,205

**Table 2.** Thermal decomposition of aluminas.

Thermal decomposition of gibbsite can be described by the reaction [364,365]:

106 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

frequency factor and the kinetic factor of the process, are listed in Table.2.

108.5 2.93·109 ~1 Gibbsite Kissinger Equation [365] 198 --- --- Boehmite Staring Equation [368]

The published data related to the kinetic triplet, which includes the activation energy, the

**Figure 8.** Formation of strontium aluminate clinker from the mixture of SrCO3 with corundum (a), gibbsite, bayerite (c)

3 23 2 2 Al(OH) (s) Al O (s)+3 H O(g) ® (1)

**Note Reference**

*<sup>n</sup>* = 0,738 Diaspore Šesták-Berggren [366]

This transformation sequences are shown in Fig.9.

**Figure 9.** Influence of conditions on the reaction pathway for the dehydration of gibbsite [367].

When mechanically activated by vigorous grinding, many crystalline materials, including gibbsite, are known to lose their long-range order and become X-ray amorphous. Amorphous phase retains its water content and becomes "gel-like". The thermal dehydration of this amorphous phase broadens the characteristic gibbsite dehydroxylation endotherm and lowers its temperature to about 150 – 200°C. The dehydrated product is also X-ray amorphous and is thermally converted to α-alumina (Fig.4) either via *η*-alumina or via the *γ*-*δ*-*η* sequence [364].

The dehydration of boehmite is a topotactic process, which can be described by the following decomposition reaction [73,368]:

$$\text{2 AlCOOH(s)} \rightarrow \text{Al}\_2\text{O}\_3(\text{s}) + \text{H}\_2\text{O(g)}\tag{2}$$

The mechanism of boehmite transformation to *γ*-Al2O3 involves the elimination of water formed by protons and hydroxyl groups and the migration of Al cations. The latter is the ratelimiting step.

#### **2. Thermal decomposition of strontium carbonate**

The thermal decomposition of strontium carbonate should be described by simple first order chemical equation:

$$\text{Ca-SrCO}\_3\text{(s)} \xrightarrow{\text{g20rC}} \text{β-SrCO}\_3\text{(s)} \xrightarrow{\text{l}} \text{SrO(s)} + \text{CO}\_2\text{(g)}\tag{3}$$

From the thermodynamic point of view, there is only simple chemical reaction in the system:

$$\sum\_{i=1}^{N} \nu\_i \mathbb{C}\_i = 0$$
 
$$\text{(4)}$$

*i i dG S dT V dp d* =- + +å

<sup>0</sup> *i i* ån m

From the consideration mentioned above the Gibbs energy of chemical reaction reaches the

 x

ln

°. From the combination of Eqs.12 and 13 follows:

 n

ln 0 *r ii <sup>G</sup> RT a i i* D= + = å å nm

ln 0 *r r G G RT a i i* D =D + = ån

> ln *<sup>r</sup> G RT a i i* D =- ån

ln *<sup>i</sup>* ln *r i G RT a RT K* n

> exp *<sup>r</sup> <sup>G</sup> <sup>K</sup> RT*

0 *<sup>i</sup>*

æ ö ¶ ¶ ¶ D= = = = ç ÷ ç ÷ ¶ ¶¶ è ø å å (12)

*ii i* = + *RT a* <sup>o</sup> (13)

<sup>o</sup> (14)

<sup>o</sup> (15)

<sup>o</sup> (16)

æ ö -D <sup>=</sup> ç ÷ è ø (18)

D =- Õ =- <sup>o</sup> (17)

is the activity. For the standard state, the

n m

=*ν*<sup>i</sup>

The equilibrium state is required to fulfill the following conditions:

minimum for the equilibrium state and should be written as:

° is the standard chemical potential and *a*<sup>i</sup>

,

. The chemical potential is defined as follows:

m m

x

*r i i T p i G G <sup>n</sup> <sup>G</sup> <sup>n</sup>*

where μi

where *dn*<sup>i</sup>

where μi

value of *a*<sup>i</sup>

/*dξ*=*ν*<sup>i</sup>

=1 and μi

=μi

where *K* is the equilibrium constant of reaction 4.

chemical reaction where *ν*<sup>i</sup>

is the chemical potential, *dn*<sup>i</sup>

mn x

mols of species *i* react, the value of *dξ*=1.

(9)

Processes during Thermal Treatment 109

d*ξ* and *ξ* is the extent of the reaction. For complete

, 0 *T p dG* = (10)

= (11)

where *ν*<sup>i</sup> is the stoichiometric coefficient for species *C*<sup>i</sup> . According to the convection:


The change in the number of moles for the reaction 4 is then equal to *Σν*<sup>i</sup> *.*

Since pure condensed phases (SrCO3 and SrO) which participate in reaction 3 do not form solid solution, their activities are equal to one (*a*SrCO3=1 and *a*SrO=1) if pure solid phase at given temperature and pressure is considered as standard state. In the other words, the activities of condensed phases do not influence the value of equilibrium constant and *Σν*<sup>i</sup> includes gas species (CO2) only.

The Gibbs energy (*G*) of open homogeneous system is described by the general equation:

$$G = f(T, p, n\_1, n\_2, \dots \\
n\_i) \tag{5}$$

where *T* is the temperature, *p* is the pressure and *n*1, *n*2,...*n*<sup>i</sup> denote the number of moles of chemical species of the reaction system. The expression for total differential of Eq.5 is given as the following relation:

$$dG = \left(\frac{\partial G}{\partial T}\right)\_{p, u\_i} dT + \left(\frac{\partial G}{\partial p}\right)\_{T, u\_i} dp + \sum \left(\frac{\partial G}{\partial u\_i}\right)\_{p, T, u\_{j \neq i}} dn\_i \tag{6}$$

Based on the relations:

$$
\left(\frac{\partial G}{\partial T}\right)\_{p,u\_i} = -S \; ; \qquad \left(\frac{\partial G}{\partial p}\right)\_{T,u\_i} = V \; ; \qquad \left(\frac{\partial G}{\partial u\_i}\right) p, T, u\_{j \star i} = \mu\_i \tag{7}
$$

Eq.6 can be written as:

$$dG = -S\,dT + V\,dp + \sum \mu\_i dn\_i\tag{8}$$

$$dG = -S\,dT + V\,dp + \sum \mu\_l \nu\_l\,d\xi \tag{9}$$

where μi is the chemical potential, *dn*<sup>i</sup> =*ν*<sup>i</sup> d*ξ* and *ξ* is the extent of the reaction. For complete chemical reaction where *ν*<sup>i</sup> mols of species *i* react, the value of *dξ*=1.

The equilibrium state is required to fulfill the following conditions:

930°C <sup>T</sup> α-SrCO (s) β-SrCO (s) SrO(s)+CO (g) 33 2 ¾¾¾® ¾¾® (3)

å <sup>=</sup> (4)

1 2 ( , , , ,... ) *G fTpn n ni* = (5)

*i*

*ji i*

ç ÷ ¶ ¶¶ èø èø è ø (7)

m¹

(8)

¹

ç ÷ ¶¶ ¶ èø èø è ø <sup>å</sup> (6)

. According to the convection:

*.*

denote the number of moles of

includes gas

From the thermodynamic point of view, there is only simple chemical reaction in the system:

0

=0 for specimens which do not participate in the reaction.

Since pure condensed phases (SrCO3 and SrO) which participate in reaction 3 do not form solid solution, their activities are equal to one (*a*SrCO3=1 and *a*SrO=1) if pure solid phase at given temperature and pressure is considered as standard state. In the other words, the activities of

The Gibbs energy (*G*) of open homogeneous system is described by the general equation:

chemical species of the reaction system. The expression for total differential of Eq.5 is given as

, , , , *i i j i*

*p n T n i pTn GG G dG dT dp dn Tp n*

; ; ,,

æö æö ¶¶ ¶ æ ö = ++ ç÷ ç÷ ç ÷

1

is the stoichiometric coefficient for species *C*<sup>i</sup>

where *T* is the temperature, *p* is the pressure and *n*1, *n*2,...*n*<sup>i</sup>

, ,

*i i*

*p n T n i G GG <sup>S</sup> V pTn T pn*

æö æö ¶ ¶¶ æ ö ç÷ ç÷ <sup>=</sup> -= = ç ÷

*i i dG S dT V dp dn* =- + +å

m

The change in the number of moles for the reaction 4 is then equal to *Σν*<sup>i</sup>

condensed phases do not influence the value of equilibrium constant and *Σν*<sup>i</sup>

< 0 for reactants;

> 0 for products;

where *ν*<sup>i</sup>

**•** The value of *ν*<sup>i</sup>

**•** The value of *ν*<sup>i</sup>

**•** The value of *ν*<sup>i</sup>

species (CO2) only.

the following relation:

Based on the relations:

Eq.6 can be written as:

*N i i i* n *C* =

108 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

$$dG\_{T,p} = 0\tag{10}$$

$$
\sum \nu\_i \mu\_i = 0 \tag{11}
$$

From the consideration mentioned above the Gibbs energy of chemical reaction reaches the minimum for the equilibrium state and should be written as:

$$
\Delta\_r \mathbf{G} = \left(\frac{\partial \ G}{\partial \ \ \ \ \ \ \xi}\right)\_{T, p} = \sum\_{\mathcal{D}} \frac{\partial \ G}{\partial \ n\_i} \frac{\partial}{\partial \ \ \ \ \xi} = \sum \nu\_i \mu\_i = 0 \tag{12}
$$

where *dn*<sup>i</sup> /*dξ*=*ν*<sup>i</sup> . The chemical potential is defined as follows:

$$
\mu\_i = \mu\_i^\circ + RT \ln a\_i \tag{13}
$$

where μi ° is the standard chemical potential and *a*<sup>i</sup> is the activity. For the standard state, the value of *a*<sup>i</sup> =1 and μi =μi °. From the combination of Eqs.12 and 13 follows:

$$
\Delta\_r G = \sum \nu\_i \mu\_i^\* + RT \sum \nu\_i \ln \sigma\_i = 0 \tag{14}
$$

$$
\Delta\_r G = \Delta\_r G^\circ + RT \sum \nu\_i \ln a\_i = 0 \tag{15}
$$

$$
\Delta\_r G^\circ = -RT\sum \nu\_i \ln a\_i \tag{16}
$$

$$
\Delta\_r G' = -RT\ln\prod a\_i^{\nu\_i} = -RT\ln K \tag{17}
$$

$$\mathcal{K} = \exp\left(\frac{-\Lambda\_r G}{RT}\right) \tag{18}$$

where *K* is the equilibrium constant of reaction 4.

From the definition of Gibbs energy:

$$G = H - TS\tag{19}$$

Where *A*, *B*, *C* and *D* are the constants for given species.

partial pressure which is given by the Dalton law (*P*<sup>i</sup>

derived from Eq.20:

where *P*\*, *P°*, *P* and *P*<sup>i</sup>

of *K* should be expressed as follows:

Therefore the following can be derived:

reaction is supposed:

Another option for the calculation of dependence of Δr*G*° is to use the equation which was

*r*

The activity of the ideal mixture of gasses is given by the following relation:

*G*

o

2

o

\* == = o o (26)

=*n*<sup>i</sup> /*Σn*<sup>j</sup>

are the relative pressure, the standard pressure, the pressure and the

) and *x*<sup>i</sup>

Õ Õ <sup>å</sup> (27)

*<sup>P</sup> Kn P <sup>n</sup>* = = (28)

*G RT K RT pCO* D =- =- <sup>o</sup> (29)

=*P*∙*x*<sup>i</sup>

*i i*

n

2

MeO+CO MeCO , where Me=Ca, Mg, Sr, Ba, Fe ... 2 3 ® (30)

(25)

. Therefore, the value

Processes during Thermal Treatment 111

*r*

*p*

*i i i i P Px <sup>a</sup> P x P P*

\* \*

 n

2 2 2 \* \* *CO CO CO*

\* ln ln *<sup>r</sup>*

In order to explain the influence of temperature and relative pressure of carbon dioxide on the thermal stability of SrCO3 as well as other carbonates that are important for the chemistry of strontium aluminate cement (Fig.11) was constructed. For this purpose, the following type of

*j*

ç ÷ è ø

<sup>å</sup> æ ö

*i i i i i i i i j*

<sup>å</sup> ç ÷ = = ç ÷

n n

*<sup>P</sup> K xP n <sup>n</sup>*

The thermodynamic equilibrium constant of the process 3 is then:

*T H T T* æ ö <sup>D</sup> ç ÷ ¶ <sup>D</sup> ç ÷ = ç ÷ ¶ è ø

where *H* and *S* are enthalpy and entropy, the law for reaction Gibbs energy can then be written as:

$$
\Delta\_r G^\circ = \Delta\_r H^\circ - T\Delta\_r S^\circ \tag{20}
$$

**Figure 10.** Temperature dependence of the thermodynamic potentials for species (a) and reaction (b).

The temperature dependence of Δr*G*° (Fig.10) can be calculated using the following equations:

$$
\Delta\_r H'(T\_2) = \Delta\_r H'(T\_1) + \int\_{T\_1}^{T\_{a \to \theta}} \Delta c\_p^\alpha \, dT + \Delta\_{\alpha \to \beta} H' + \int\_{T\_{a \to \theta}}^{T\_2} \Delta c\_p^\beta \, dT \tag{21}
$$

$$
\Delta\_r S^\*(T\_2) = \Delta\_r S^\*(T\_1) + \int\_{T\_1}^{T\_{a \to \theta}} \frac{\Delta c\_p^\alpha}{T} \, dT + \frac{\Delta\_{a \to \theta} H^\*}{T\_{a \to \theta}} + \int\_{T\_{a \to \theta}}^{T\_2} \frac{\Delta c\_p^\beta}{T} \, dT \tag{22}
$$

where Δr*H*° and Δr*S*° are the standard reaction enthalpy and entropy, respectively, Δα→β*H*° and Δα→β*H*°/ *T*=Δα→β*S*° are the enthalpy and entropy related to the α→*β* phase transformation and Δ*c*p is the isobaric heat capacity:

$$
\Delta \mathcal{L}\_p = \left(\frac{\partial \ln\_r H^\circ}{\partial \, T}\right)\_p = \sum \nu\_i \mathcal{c}\_{p,i} \tag{23}
$$

The temperature dependence of heat capacity is given by law:

$$c\_{p,l} = A + B\,T + \frac{C}{T^2} + DT^2 \tag{24}$$

Where *A*, *B*, *C* and *D* are the constants for given species.

From the definition of Gibbs energy:

as:

*G H TS* = - (19)

*G H TS* oo o (20)

where *H* and *S* are enthalpy and entropy, the law for reaction Gibbs energy can then be written

D =D - D *rr r*

110 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

**Figure 10.** Temperature dependence of the thermodynamic potentials for species (a) and reaction (b).

1

1

a b

®

D =D + + + ò ò

*r p i pi p <sup>H</sup> c c <sup>T</sup>*

*p i*, 2 *<sup>C</sup> c A B T DT*

 æ ö ¶ D D= = ç ÷ ç ÷ ¶ è ø

<sup>å</sup> <sup>o</sup>

a b

®

*rr p p*

D =D + D +D + D ò ò ®

*H T H T c dT H c dT*

*ST ST dT dT*

a

a

2 1 () ()

2 1 () ()

The temperature dependence of heat capacity is given by law:

*r r*

Δ*c*p is the isobaric heat capacity:

The temperature dependence of Δr*G*° (Fig.10) can be calculated using the following equations:

*T T*

*T T*

*T T*

*T T*

where Δr*H*° and Δr*S*° are the standard reaction enthalpy and entropy, respectively, Δα→β*H*° and Δα→β*H*°/ *T*=Δα→β*S*° are the enthalpy and entropy related to the α→*β* phase transformation and

a b

*p p*

,

2

n

o

*c c H*

® ® D D D

a b

> a b

*TT T*

2

o o <sup>o</sup> (21)

 b

 b

*<sup>T</sup>* =+ + + (24)

(23)

a b

2

o o (22)

a b

®

®

Another option for the calculation of dependence of Δr*G*° is to use the equation which was derived from Eq.20:

$$\left(\frac{\partial \ ^{\Lambda\_r G}\text{C}}{\partial \ T}\right)\_p = -\frac{\Lambda\_r H^\top}{T^2} \tag{25}$$

The activity of the ideal mixture of gasses is given by the following relation:

$$\mathbf{a}\_{i} = \frac{P\_{i}}{P^{\*}} = \frac{P^{\*}\mathbf{x}\_{i}}{P^{\*}} = P^{\*}\mathbf{x}\_{i} \tag{26}$$

where *P*\*, *P°*, *P* and *P*<sup>i</sup> are the relative pressure, the standard pressure, the pressure and the partial pressure which is given by the Dalton law (*P*<sup>i</sup> =*P*∙*x*<sup>i</sup> ) and *x*<sup>i</sup> =*n*<sup>i</sup> /*Σn*<sup>j</sup> . Therefore, the value of *K* should be expressed as follows:

$$\mathcal{K} = \prod\_{i} \boldsymbol{\mu}\_{i}^{\nu\_{i}} \boldsymbol{\mathcal{P}}^{\sum\_{i} \nu\_{i}} = \prod\_{i} \boldsymbol{n}\_{i}^{\nu\_{i}} \left(\frac{\boldsymbol{\mathcal{P}}^{\star}}{\sum\_{j} \boldsymbol{n}\_{j}}\right)^{\sum\_{i} \nu\_{i}} \tag{27}$$

The thermodynamic equilibrium constant of the process 3 is then:

$$K = n\_{CO\_2} \frac{P^\*}{n\_{CO\_2}} = P\_{CO\_2}^\* \tag{28}$$

Therefore the following can be derived:

$$
\Delta\_r G^\circ = -RT\ln K = -RT\ln p\_{CO\_2}^\circ \tag{29}
$$

In order to explain the influence of temperature and relative pressure of carbon dioxide on the thermal stability of SrCO3 as well as other carbonates that are important for the chemistry of strontium aluminate cement (Fig.11) was constructed. For this purpose, the following type of reaction is supposed:

$$\text{MeO} \bullet \text{CO}\_2 \rightarrow \text{MeCO}\_3 \text{ , where } \text{Me=Ca, Mg}\_2 \text{Sr, Ba, Fe} \dots \text{ } \tag{30}$$

All reactions are balanced for 1 mol of CO2 hence the standard Gibbs energy for the reaction is given by the formula:

$$
\Delta\_r G^\circ = RT \ln P\_{CO\_2}^\circ \tag{31}
$$

phase transformation. Strontium carbonate shows the transformation of orthorhombic (α) to hexagonal (*β*) modification at 925 - 935 °C. From the diagram we can read the value which is necessary for the thermal decomposition of SrCO3, below the temperature of phase transfor‐ mation. The extrapolation of line between the point on the *0K* axis and the point on the temperature dependence of Δr*G*° for 925 °C to the axis of relative pressure of CO2 provides the value of 0.9965. Therefore the partial pressure of CO2 lower than 100.97 kPa is needed. On the contrary, the equilibrium temperature of thermal decomposition can be increased by increas‐

In some cases the cross point between the temperature dependences of Δr*G*° for two carbonates occurrs which means the chemical reaction. For example, at the temperature of 1330 °C there

= 1.013:

*CO*<sup>2</sup>

The value of Δr*G*°(c) > 0 below the equilibrium temperature (reaction 35 shows opposite

The reaction 3 shows that the phase transformation of orthorhombic α-SrCO3 to the hexagonal structure of *β*-SrCO3 proceeds prior to the thermal decomposition of strontium carbonate. This process shows sharp endothermic effect which is well visible at DTA (Fig.12). The equilibrium

*d d dg dg dg* 0

 a

 a b

*dg s dT v dp* aa

direction), while Δr*G*°(c) < 0 if the temperature is higher than 1330 °C.

 a b

 m m

ab

mm

. For example, for the temperature of 1400 °C the attainment of *P* \*

MnO+CO MnCO (a) 2 3 ® (33)

ZnO+CO ZnCO (b) 2 3 ® (34)

ZnO+MnCO MnO+ZnCO (c) 3 3 ® (35)

ooo Δ G (c)=Δ G (b)-Δ G (a) rrr (36)

= Þ = Þ = Þ= (37)

=- + (38)

*CO*<sup>2</sup> of

Processes during Thermal Treatment 113

ing the value of *P* \*

*CO*<sup>2</sup>

is an equilibrium of two reactions under *P* \*

Therefore the following reaction can be supposed:

of both phases means that:

where:

1.002, i.e. the partial pressure of 101,53 kPa is required.

The value of Gibbs energy which is necessary for one mol of CO2 to expand from standard to the equilibrium pressure related to given temperature is:

$$
\Delta G = -RT\ln\frac{P}{P^\circ} = -RT\ln P^\circ = -(R\ln P^\circ)\,T\tag{32}
$$

The lines representing the constant relative pressure form the *P\*CO2* scale of diagram in Fig.11.

**Figure 11.** Thermal stability of carbonates.

The temperature of spontaneous thermal decomposition of carbonate can be found as the cross point of the temperature dependence of Δr*G*° with the line for *P* \* *CO*<sup>2</sup> =1. For strontium carbonate we can read the temperature of 1175 °C. The change in line direction means the phase transformation. Strontium carbonate shows the transformation of orthorhombic (α) to hexagonal (*β*) modification at 925 - 935 °C. From the diagram we can read the value which is necessary for the thermal decomposition of SrCO3, below the temperature of phase transfor‐ mation. The extrapolation of line between the point on the *0K* axis and the point on the temperature dependence of Δr*G*° for 925 °C to the axis of relative pressure of CO2 provides the value of 0.9965. Therefore the partial pressure of CO2 lower than 100.97 kPa is needed. On the contrary, the equilibrium temperature of thermal decomposition can be increased by increas‐ ing the value of *P* \* *CO*<sup>2</sup> . For example, for the temperature of 1400 °C the attainment of *P* \* *CO*<sup>2</sup> of 1.002, i.e. the partial pressure of 101,53 kPa is required.

In some cases the cross point between the temperature dependences of Δr*G*° for two carbonates occurrs which means the chemical reaction. For example, at the temperature of 1330 °C there is an equilibrium of two reactions under *P* \* *CO*<sup>2</sup> = 1.013:

$$\text{MnO} \vdash \text{CO}\_2 \rightarrow \text{MnCO}\_3 \qquad \text{(a)}\tag{33}$$

$$\text{ZnO} \bullet \text{CO}\_2 \rightarrow \text{ZnCO}\_3 \qquad \text{(b)}\tag{34}$$

Therefore the following reaction can be supposed:

$$\text{ZnO} \star \text{MnO}\_3 \rightarrow \text{MnO} \star \text{ZnCO}\_3 \quad \text{(c)}\tag{35}$$

$$
\Delta\_{\mathbf{r}} \mathbf{G}^{0} \text{(c)} = \Delta\_{\mathbf{r}} \mathbf{G}^{0} \text{(b)} \text{-}\Delta\_{\mathbf{r}} \mathbf{G}^{0} \qquad \text{(a)}\tag{36}
$$

The value of Δr*G*°(c) > 0 below the equilibrium temperature (reaction 35 shows opposite direction), while Δr*G*°(c) < 0 if the temperature is higher than 1330 °C.

The reaction 3 shows that the phase transformation of orthorhombic α-SrCO3 to the hexagonal structure of *β*-SrCO3 proceeds prior to the thermal decomposition of strontium carbonate. This process shows sharp endothermic effect which is well visible at DTA (Fig.12). The equilibrium of both phases means that:

$$
\mu^{\alpha} = \mu^{\beta} \implies d\,\mu^{\alpha} = d\mu^{\beta} \implies dg^{\alpha} = dg^{\beta} \implies dg = 0 \tag{37}
$$

where:

All reactions are balanced for 1 mol of CO2 hence the standard Gibbs energy for the reaction

The value of Gibbs energy which is necessary for one mol of CO2 to expand from standard to

The lines representing the constant relative pressure form the *P\*CO2* scale of diagram in Fig.11.

The temperature of spontaneous thermal decomposition of carbonate can be found as the cross

carbonate we can read the temperature of 1175 °C. The change in line direction means the

*CO*<sup>2</sup>

=1. For strontium

point of the temperature dependence of Δr*G*° with the line for *P* \*

\* ln *<sup>r</sup>*

112 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

\* \* ln ln ( ln ) *<sup>P</sup> G RT RT P R P T*

*P*

the equilibrium pressure related to given temperature is:

2

*G RT PCO* D =<sup>o</sup> (31)

D =- =- =- <sup>o</sup> (32)

is given by the formula:

**Figure 11.** Thermal stability of carbonates.

$$d\lg^a = -s^a \, dT + v^a \, dp \tag{38}$$

$$d\mathbf{g}^{\rho} = -\mathbf{s}^{\rho} \, dT + \mathbf{v}^{\rho} \, dp \tag{39}$$

determined from the value of kinetic factor (*n*=2.73 ± 0.03) obtained via the extrapolation of the dependence of *n* on Θ to the nearly isothermal conditions (Θ → 0). The frequency factor

s-1.

Processes during Thermal Treatment 115

as well.

can be then calculated from the intercept of Kissinger plot with *y*-axis to be 1.13 107

**Figure 13.** Kissinger plot (a) and extrapolation of kinetic factor to nearly isothermal conditions of the process (b).

The total Gram-Schmidt reconstruction data show sharp peak related to the thermal decom‐ position of strontium carbonate. The peak was integrated and used to calculate the degree of

The rate determining step of the thermal decomposition of SrCO3 is recognized from the slope

the reaction interface of contracted sphere (shrinking core, R3: 1-(1-α)1/3). The activation energy required for decarbonation of SrCO3 has the value of 238.6 kJ.mol-1. The frequency factor that

4 Another and more often applied option is to use TG data in order to evaluate the kinetics of the process. On the other hand, there is one great advantage of EGA, as it is possible to monitor the required product (e.g. H2O, CO2, organics such

) on reciprocal temperature (Fig.15) as the rate of reaction on

s-1. Since usual error in the assessment of activation

The kinetics should be determined via the model fitting procedure using EGA data<sup>4</sup>

**Figure 14.** Using EGA to evaluate the kinetics of thermal decomposition of SrCO3.

of the dependence of *ln* (*g*(α)/*T*<sup>2</sup>

was calculated from the intercept is 2.32 106

as hydrocarbons, acetone, etc.) according to the selected wavelength.

conversion.

From that the Clapeyron law can be derived:

$$-s^{\mu} \,\, dT + \upsilon^{\mu} \,\, dp = -s^{\beta} \,\, dT + \upsilon^{\beta} \,\, dp \tag{40}$$

$$(\mathbf{s}^{\mathcal{J}} - \mathbf{s}^{\alpha}) \, dT = \frac{\Delta\_{a \to \rho} h}{T} \, dT = (\mathbf{v}^{\mathcal{J}} - \mathbf{v}^{\alpha}) \, dp \tag{41}$$

$$\frac{dp}{dT} = \frac{\Lambda\_{\alpha \to \beta} h}{T \text{ } \Delta v} \tag{42}$$

#### **2.1. Kinetics of thermal decomposition of SrCO3**

The behaviour of pure strontium carbonate during the thermal treatment is discussed in this chapter. The mechanism, the kinetics and the thermodynamic stability of activated complex of the process of thermal decomposition were assessed by non-isothermal TG-DTA and EGA assessment using the model-free Kissinger kinetic approach (Eq.112 in Chapter 1). 30 mg of strontium carbonate were heated to the temperature of 1200 °C using the heating rate from 5 to 25 °C min-1. The kinetics of the process was evaluated from the shift of DTA peak with the heating rate3 . It should be pointed that the peak temperature is higher than the temperature of α→*β* transformation, therefore the kinetics results are related to the thermal decomposition of high temperature (hexagonal) polymorph of SrCO3.

**Figure 12.** Typical plot of simultaneous TG-DTA and EGA analysis of strontium carbonate.

The slope of Kissinger plot (Fig.13(a)) enables to calculate the activation energy for the thermal decomposition of strontium carbonate (Eq.3) to be 223.7 kJ mol-1. The kinetic factor was calculated according to the relation in Eq.113 in Chapter 1. The mechanism of the process was

<sup>3</sup> Other options are to use the peak temperature of DTG or the EGA peak.

determined from the value of kinetic factor (*n*=2.73 ± 0.03) obtained via the extrapolation of the dependence of *n* on Θ to the nearly isothermal conditions (Θ → 0). The frequency factor can be then calculated from the intercept of Kissinger plot with *y*-axis to be 1.13 107 s-1.

*dg s dT v dp* bb

114 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

*s dT v dp s dT v dp*

() ( ) *h s s dT dT v v dp <sup>T</sup>*

> *dp h dT T v* Da b

The behaviour of pure strontium carbonate during the thermal treatment is discussed in this chapter. The mechanism, the kinetics and the thermodynamic stability of activated complex of the process of thermal decomposition were assessed by non-isothermal TG-DTA and EGA assessment using the model-free Kissinger kinetic approach (Eq.112 in Chapter 1). 30 mg of strontium carbonate were heated to the temperature of 1200 °C using the heating rate from 5 to 25 °C min-1. The kinetics of the process was evaluated from the shift of DTA peak with the

of α→*β* transformation, therefore the kinetics results are related to the thermal decomposition

The slope of Kissinger plot (Fig.13(a)) enables to calculate the activation energy for the thermal decomposition of strontium carbonate (Eq.3) to be 223.7 kJ mol-1. The kinetic factor was calculated according to the relation in Eq.113 in Chapter 1. The mechanism of the process was

. It should be pointed that the peak temperature is higher than the temperature

a b®

 bb

> b a

aa

b a

**2.1. Kinetics of thermal decomposition of SrCO3**

of high temperature (hexagonal) polymorph of SrCO3.

**Figure 12.** Typical plot of simultaneous TG-DTA and EGA analysis of strontium carbonate.

3 Other options are to use the peak temperature of DTG or the EGA peak.

heating rate3

From that the Clapeyron law can be derived:

 b

=- + (39)


<sup>D</sup> - = =- (41)

® <sup>=</sup> <sup>D</sup> (42)

**Figure 13.** Kissinger plot (a) and extrapolation of kinetic factor to nearly isothermal conditions of the process (b).

The kinetics should be determined via the model fitting procedure using EGA data<sup>4</sup> as well. The total Gram-Schmidt reconstruction data show sharp peak related to the thermal decom‐ position of strontium carbonate. The peak was integrated and used to calculate the degree of conversion.

**Figure 14.** Using EGA to evaluate the kinetics of thermal decomposition of SrCO3.

The rate determining step of the thermal decomposition of SrCO3 is recognized from the slope of the dependence of *ln* (*g*(α)/*T*<sup>2</sup> ) on reciprocal temperature (Fig.15) as the rate of reaction on the reaction interface of contracted sphere (shrinking core, R3: 1-(1-α)1/3). The activation energy required for decarbonation of SrCO3 has the value of 238.6 kJ.mol-1. The frequency factor that was calculated from the intercept is 2.32 106 s-1. Since usual error in the assessment of activation

<sup>4</sup> Another and more often applied option is to use TG data in order to evaluate the kinetics of the process. On the other hand, there is one great advantage of EGA, as it is possible to monitor the required product (e.g. H2O, CO2, organics such as hydrocarbons, acetone, etc.) according to the selected wavelength.

energy usually reaches few percents of the determined value, the good agreement with the model free and model fitting method has been achieved.

The schematic energy curve in Fig.16 illustrates the method used for the estimation of activation energy for the thermal decomposition of strontium carbonate according to congru‐

Processes during Thermal Treatment 117

**Figure 16.** Schematic representation of energy curve for the thermal decomposition of SrCO*3*.

The temperature dependence of equimolar activation energy on the heating rate is shown in Fig.17. The transitions between individual carbonate polymorphs lead to the step change in

The thermodynamic parameters of activated complex were calculated from Eqs.117 – 120 in

ent dissociative vaporization mechanism.

the value of activation energy.

**Figure 17.** Dependence of Ea

Te on temperature.

**2.3. Thermodynamics of thermal decomposition**

Chapter 1 and are listed in Table 4.

**Figure 15.** Determination of the activation energy for the most probable mechanism of the process.

These results are also in agreement with experimental as well as calculated5 results published in literature, namely 233 and 238.7 kJ mol-1 [369], respectively.

#### **2.2. Calculation of activation energy from thermodynamic data**

The thermodynamic data required for the calculation of theoretical value of activation energy and the values of calculated equimolar (*E*<sup>a</sup> Te) and isobaric activation energy (*E*<sup>a</sup> Ti) for the process of thermal decomposition of strontium carbonate are listed in Table 3.


**Table 3.** Thermodynamic data for the calculation of theoretical value of activation energy.

<sup>5</sup> The method for the calculation of theoretical value of activation energy is described in Chapter 1.6.5. and the example in the next Chapter.

The schematic energy curve in Fig.16 illustrates the method used for the estimation of activation energy for the thermal decomposition of strontium carbonate according to congru‐ ent dissociative vaporization mechanism.

**Figure 16.** Schematic representation of energy curve for the thermal decomposition of SrCO*3*.

The temperature dependence of equimolar activation energy on the heating rate is shown in Fig.17. The transitions between individual carbonate polymorphs lead to the step change in the value of activation energy.

**Figure 17.** Dependence of Ea Te on temperature.

energy usually reaches few percents of the determined value, the good agreement with the

116 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

**Figure 15.** Determination of the activation energy for the most probable mechanism of the process.

The thermodynamic data required for the calculation of theoretical value of activation energy

Δ<sup>f</sup> *H*° [kJ∙mol-1] SrCO3(s) -1144.481 -1131.506 -1100.920 -1086.320 -1071.720

Δ<sup>c</sup> *H*° SrO(g→s) -568.435 -566.920 -565.426 -563.961 -562.532

Ti 512 509 489 484 480

5 The method for the calculation of theoretical value of activation energy is described in Chapter 1.6.5. and the example

**Table 3.** Thermodynamic data for the calculation of theoretical value of activation energy.

SrO(g) 11.895 15.638 19.403 23.202 27.050 SrO(s) -556.539 -551.283 -546.023 -540.759 -535.482 CO2(g) -360.097 -354.601 -349.006 -343.328 -337.580

Te) and isobaric activation energy (*E*<sup>a</sup>

**1000 1100 1200 1300 1400**

796.297 792.542 771.317 766.194 761.190

512.062 509.083 488.604 484.214 479.924

256 255 244 242 240

results published

Ti) for the process

These results are also in agreement with experimental as well as calculated5

in literature, namely 233 and 238.7 kJ mol-1 [369], respectively.

and the values of calculated equimolar (*E*<sup>a</sup>

**Parameter Species Temperature [K]**

Chapter 1

Chapter 1

Chapter 1

Δ<sup>r</sup> *H*° Eq.121 in

Δrc *H*° Eq.121 in

Te Eq.121 in

in the next Chapter.

*E*a

*E*a

**2.2. Calculation of activation energy from thermodynamic data**

of thermal decomposition of strontium carbonate are listed in Table 3.

model free and model fitting method has been achieved.

#### **2.3. Thermodynamics of thermal decomposition**

The thermodynamic parameters of activated complex were calculated from Eqs.117 – 120 in Chapter 1 and are listed in Table 4.


**Table 4.** Thermodynamic data of activated complex.

The plot of *ln k* vs 1/*T* of course gives the Arrhenius plot with the same activation energy (Fig. 18) as was calculated by Kissinger method. Therefore non-isothermal data can be used to calculate the isothermal rate constant (*k*) of the investigated process.

**Figure 19.** Mechanism of formation of strontium heaxaaluminate [371].

proposed by An et al. [370] for the alumina–calcia system) [371].

may be formed by the process [371]:

**•** Reaction between Sr and A:

**•** Reaction between SrA and A:

**•** Reaction between SrA2 and A:

One possibility is that SrA6 nucleates at the interfaces between alumina and SrA particles and the reaction proceeds by solid-state diffusion through the reactant phase. However, if the surfaces of SrA6 and alumina grains are already wet by a liquid phase, the transformation to SrA6 would, by necessity, proceed via the solution-precipitation reaction. The reaction by solidstate diffusion results in the formation of equiaxed SrA6 grains, while the solution-precipitation favors the development of plate like grains. It is proposed that localized melting take place as a result of low temperature eutectic reaction in the SrO-Al2O3 system and it is the eutectic liquid which plays a dominant role in affecting the SrA6 reaction mechanism (similar model is

The difference in the final microstructure lies in the extent to which the solid-state reaction occurs between SrA and SrA6 prior to wetting by liquid phase. This depends on a variety of factors including the particle size, the packing density and the uniform dispersion (mixing) of alumina–strontia powders. Just a slight difference in the above factors could have an appre‐ ciable effect on the subsequent wetting behaviour and the microstructure development. SrA6

<sup>6</sup> Sr+6 A SrA ® (44)

Processes during Thermal Treatment 119

<sup>6</sup> SrA+5 A SrA ® (45)

2 6 *SrA A SrA* + ® 4 (46)

**Figure 18.** Arrhenius plot calculated from the data of activated complex.

#### **3. Formation of strontium aluminate**

During the early stages of sintering, mono-strontium aluminate is formed by the solid-state reaction between strontia and alumina in the powder compact:

$$\text{SrCO}\_3 + \text{Al}\_2\text{O}\_3 \rightarrow \text{SrAl}\_2\text{O}\_4 + \text{CO}\_2(\text{g}) \tag{43}$$

Hexaaluminate must be formed by further reaction of strontium aluminate (SrA) and alumina. That can proceed by two distinct mechanisms as is shown in Fig.19.

**Figure 19.** Mechanism of formation of strontium heaxaaluminate [371].

One possibility is that SrA6 nucleates at the interfaces between alumina and SrA particles and the reaction proceeds by solid-state diffusion through the reactant phase. However, if the surfaces of SrA6 and alumina grains are already wet by a liquid phase, the transformation to SrA6 would, by necessity, proceed via the solution-precipitation reaction. The reaction by solidstate diffusion results in the formation of equiaxed SrA6 grains, while the solution-precipitation favors the development of plate like grains. It is proposed that localized melting take place as a result of low temperature eutectic reaction in the SrO-Al2O3 system and it is the eutectic liquid which plays a dominant role in affecting the SrA6 reaction mechanism (similar model is proposed by An et al. [370] for the alumina–calcia system) [371].

The difference in the final microstructure lies in the extent to which the solid-state reaction occurs between SrA and SrA6 prior to wetting by liquid phase. This depends on a variety of factors including the particle size, the packing density and the uniform dispersion (mixing) of alumina–strontia powders. Just a slight difference in the above factors could have an appre‐ ciable effect on the subsequent wetting behaviour and the microstructure development. SrA6 may be formed by the process [371]:

**•** Reaction between Sr and A:

**T** Δ**H#** Δ**S#** Δ**G# K# ν k [K] [J·mol-1] [J·K-1·mol-1] [J·mol-1] [s-1]** 295.15 221.24 -123.99 251.86 8.41·10-45 6.22·1012 5.23·10-32 1000 215.42 -128.48 343.34 1.16·10-18 2.09·1013 2.42·10-5 1100 214.59 -129.12 356.38 1.19·10-17 2.29·1013 2.73·10-4 1200 213.76 -129.76 369.42 8.30·10-17 2.50·1013 2.08·10-3 1300 212.93 -130.40 382.46 4.29·10-16 2.71·1013 1.16·10-2 1400 212.10 -131.34 395.50 1.75·10-15 2.92·1013 5.11·10-2

118 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

The plot of *ln k* vs 1/*T* of course gives the Arrhenius plot with the same activation energy (Fig. 18) as was calculated by Kissinger method. Therefore non-isothermal data can be used to

During the early stages of sintering, mono-strontium aluminate is formed by the solid-state

Hexaaluminate must be formed by further reaction of strontium aluminate (SrA) and alumina.

3 23 24 2 SrCO +Al O SrAl O +CO (g) ® (43)

calculate the isothermal rate constant (*k*) of the investigated process.

**Figure 18.** Arrhenius plot calculated from the data of activated complex.

reaction between strontia and alumina in the powder compact:

That can proceed by two distinct mechanisms as is shown in Fig.19.

**3. Formation of strontium aluminate**

**Table 4.** Thermodynamic data of activated complex.

$$\text{Sr} \star \text{6} \to \text{SrA}\_{\text{6}} \tag{44}$$

**•** Reaction between SrA and A:

$$\text{SrA} + \text{5A} \rightarrow \text{SrA}\_6 \tag{45}$$

**•** Reaction between SrA2 and A:

$$\text{SrA}\_2 + 4\text{ A} \to \text{SrA}\_6\tag{46}$$

**•** Reaction between SrA, SrA2 and A:

$$\text{SrA} + \text{SrA}\_2 + \text{9 A} \to 2\text{SrA}\_6 \tag{47}$$

yet and its diffraction lines can be detected up to the temperature of 1000 °C. The features of

The comparison of X-ray diffraction pattern of raw materials (a) and fired strontium aluminate clinker (b) is shown in Fig.21. There are two main product of thermal treatment of raw equimolar mixture of SrCO3 with Al2O3 to the temperature of 1600 °C, namely strontium aluminate and tri-strontium aluminate. Using quantitative X-ray diffraction Rietveld analysis, the content of strontium aluminate and tri-strontium aluminate phase in prepared strontium

The comparison of infrared spectra of raw materials and product (Fig.22) shows disappearing

band of antisymmetric stretching at 1465 cm-1 *ν*3, weak band of symmetric stretching at 1072 cm-1 *ν*1, out of plane bending at 856 cm-1 *ν*2 and antisymmetric bending vibration at 702 cm-1 (*ν*4). There are also three combination bands 2*ν*1+*ν*2, *ν*1+*ν*4 and *ν*1+*ν*4. Corundum in raw mixture shows four main absorption bands of stretching of (AlO6) octahedra at 639, 591, 489 and 448

On the contrary, the SrAl2O4 structure is built up from (AlO4) tetrahedra with the Td symmetry and the structural lattice channels are occupied by Sr2+ions [377]. The bands appearing in the spectral regions from 900 to 780 cm-1 and from 650 to 550 cm-1 belong to antisymmetric and symmetric stretching of (AlO4) tetrahedra. The bands related to the doublet of bending of O-

Simultaneous TG-DTA (a) and EGA (b) analyses of the mixture of strontium carbonate and alumina (Fig.23) show sharp exothermic peak at the temperature of 967 °C, which divides the huge endothermic effect of thermal decomposition of strontium carbonate occurring within the temperature range from 810 to 1020 °C. The CO2 bands appear on EGA. That means, that

degenerated modes *ν*2 and *ν*4 and amount of vibration modes in infrared spectrum is increasing. Furthermore, the intensity of fully symmetric stretching mode *ν*1 is increasing (this mod is not IR active for carbonates from the group of calcite).

2- anion is reduced to Cs for carbonates from group of aragonite. It leads to the splitting of

[183,186,372-374] including strong

Processes during Thermal Treatment 121

2-anion with D3h symmetry6

free SrO gradually disappear at the temperature of 1250 °C.

aluminate clinker is assessed to be 96 % and 4 %, respectively.

**Figure 21.** Initial (a) and final composition (b) of strontium aluminate clinker [379].

absorption bands of planar CO3

Al-O are located at 446 and 418 cm-1.

6 The D3h symmetry of CO3

cm-1 [375,376].

$$\text{2SrA} \star \text{SrA}\_2 + \text{14 A} \rightarrow \text{3SrA}\_6 \tag{48}$$

The phases SA and SA2 are both the reaction intermediates and are minimized with the formation of SA6. Similar reaction intermediates were proposed for the formation of calcium aluminates [371].

#### **3.1. Thermal treatment of raw meal**

The processes occurring during the formation of strontium aluminate clinker can be observed by HT-XRD analysis of raw meal upon heating. Fig.20 shows the changes in the phase composition of equimolar mixture of strontium aluminate and alumina. The diffraction lines of SrCO3 start disappearing at the temperature of 800 °C. SrO formed via the thermal decom‐ position of SrCO3 reacts with alumina and the diffraction line of tristrontium aluminate appears at 825 °C. For this reason the diffraction lines of alumina disappear at the same temperature

**Figure 20.** High-temperature X-ray diffraction analysis of raw materials upon heating with the heatinf rate of 10 °C min-1 [379].

The features of the main clinker phase, i.e. strontium aluminate, are recognized at the tem‐ perature of 875 °C. The process of thermal decomposition of strontium carbonate is not finished yet and its diffraction lines can be detected up to the temperature of 1000 °C. The features of free SrO gradually disappear at the temperature of 1250 °C.

The comparison of X-ray diffraction pattern of raw materials (a) and fired strontium aluminate clinker (b) is shown in Fig.21. There are two main product of thermal treatment of raw equimolar mixture of SrCO3 with Al2O3 to the temperature of 1600 °C, namely strontium aluminate and tri-strontium aluminate. Using quantitative X-ray diffraction Rietveld analysis, the content of strontium aluminate and tri-strontium aluminate phase in prepared strontium aluminate clinker is assessed to be 96 % and 4 %, respectively.

**Figure 21.** Initial (a) and final composition (b) of strontium aluminate clinker [379].

**•** Reaction between SrA, SrA2 and A:

**3.1. Thermal treatment of raw meal**

aluminates [371].

temperature

min-1 [379].

2 6 SrA+SrA +9 A 2 SrA ® (47)

2 6 2 SrA+SrA +14 A 3 SrA ® (48)

The phases SA and SA2 are both the reaction intermediates and are minimized with the formation of SA6. Similar reaction intermediates were proposed for the formation of calcium

120 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

The processes occurring during the formation of strontium aluminate clinker can be observed by HT-XRD analysis of raw meal upon heating. Fig.20 shows the changes in the phase composition of equimolar mixture of strontium aluminate and alumina. The diffraction lines of SrCO3 start disappearing at the temperature of 800 °C. SrO formed via the thermal decom‐ position of SrCO3 reacts with alumina and the diffraction line of tristrontium aluminate appears at 825 °C. For this reason the diffraction lines of alumina disappear at the same

**Figure 20.** High-temperature X-ray diffraction analysis of raw materials upon heating with the heatinf rate of 10 °C

The features of the main clinker phase, i.e. strontium aluminate, are recognized at the tem‐ perature of 875 °C. The process of thermal decomposition of strontium carbonate is not finished The comparison of infrared spectra of raw materials and product (Fig.22) shows disappearing absorption bands of planar CO3 2-anion with D3h symmetry6 [183,186,372-374] including strong band of antisymmetric stretching at 1465 cm-1 *ν*3, weak band of symmetric stretching at 1072 cm-1 *ν*1, out of plane bending at 856 cm-1 *ν*2 and antisymmetric bending vibration at 702 cm-1 (*ν*4). There are also three combination bands 2*ν*1+*ν*2, *ν*1+*ν*4 and *ν*1+*ν*4. Corundum in raw mixture shows four main absorption bands of stretching of (AlO6) octahedra at 639, 591, 489 and 448 cm-1 [375,376].

On the contrary, the SrAl2O4 structure is built up from (AlO4) tetrahedra with the Td symmetry and the structural lattice channels are occupied by Sr2+ions [377]. The bands appearing in the spectral regions from 900 to 780 cm-1 and from 650 to 550 cm-1 belong to antisymmetric and symmetric stretching of (AlO4) tetrahedra. The bands related to the doublet of bending of O-Al-O are located at 446 and 418 cm-1.

Simultaneous TG-DTA (a) and EGA (b) analyses of the mixture of strontium carbonate and alumina (Fig.23) show sharp exothermic peak at the temperature of 967 °C, which divides the huge endothermic effect of thermal decomposition of strontium carbonate occurring within the temperature range from 810 to 1020 °C. The CO2 bands appear on EGA. That means, that

<sup>6</sup> The D3h symmetry of CO3 2- anion is reduced to Cs for carbonates from group of aragonite. It leads to the splitting of degenerated modes *ν*2 and *ν*4 and amount of vibration modes in infrared spectrum is increasing. Furthermore, the intensity of fully symmetric stretching mode *ν*1 is increasing (this mod is not IR active for carbonates from the group of calcite).

model, i.e. Al2O3 particle surrounded by SrCO3, where the formation of strontium aluminate is limited by the diffusion of Sr2+ions into the disappearing alumina core cannot explain the observed behaviour. Therefore, formed Sr3A is actually the mesophase during the process of

Processes during Thermal Treatment 123

The cooling zone in (Fig.23) showed an exothermic effect of reversible transformation of hexagonal high-temperature modification of SrAl2O4 to low-temperature monoclinic phase at the temperature of 650 °C. During repeated cycle of heating, the exothermic transformation took place at higher temperature. The temperature hysteresis of this transformation increased linearly with the heating rate (Fig.25). Therefore, the limit value of 9.10 ± 0.07 °C should be

**Figure 25.** Temperature hysteresis in the polymorphic transformation (a) [379] between monoclinic (b) and hexgonal

The recorded XPS (X-ray photoelectron spectroscopy) spectrum of strontium (Sr3d) in prepared strontium aluminate cement is presented in Fig.26. The results indicate that the

reaction [378].

**Figure 24.** Mechanism of formation of strontium aluminate [379].

estimated from the linear fit for Θ → 0 [379].

strontium aluminate (c) [24].

**Figure 22.** Initial (a) and final infrared spectra (b) of strontium aluminate clinker.

formed SrAl2O4 phase covered the surface of decomposed SrCO3 particle and this layer slowed down the diffusion of CO2 from the reaction interface to disappearing SrCO3 core. Therefore, the thermal decomposition of SrCO3 was suppressed. The volume changes caused by the decarbonation and recrystallization of products led to the formation of cracks, which enabled easy diffusion of CO2 through the layer of the product. The mass of the sample was reduced by 17.7 wt. % during this process.

**Figure 23.** Thermal analysis of strontium aluminate clinker: simultaneous TG-DTA (a) and EGA (b) assessment using the heating rate of 10 °C min-1.

The reaction interface (Fig.24) abundance of SrO (outer side of formed strontium aluminate layer) or Al2O3 (inner side) component led to the formation of tri-strontium aluminate (Sr3A) and strontium hexaaluminate (SrA6). The equilibrium composition of strontium aluminate (SrA) was then established via the opposite direction diffusion of Sr2+and Al3+ions. The opposite model, i.e. Al2O3 particle surrounded by SrCO3, where the formation of strontium aluminate is limited by the diffusion of Sr2+ions into the disappearing alumina core cannot explain the observed behaviour. Therefore, formed Sr3A is actually the mesophase during the process of reaction [378].

**Figure 24.** Mechanism of formation of strontium aluminate [379].

formed SrAl2O4 phase covered the surface of decomposed SrCO3 particle and this layer slowed down the diffusion of CO2 from the reaction interface to disappearing SrCO3 core. Therefore, the thermal decomposition of SrCO3 was suppressed. The volume changes caused by the decarbonation and recrystallization of products led to the formation of cracks, which enabled easy diffusion of CO2 through the layer of the product. The mass of the sample was reduced

122 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

**Figure 22.** Initial (a) and final infrared spectra (b) of strontium aluminate clinker.

**Figure 23.** Thermal analysis of strontium aluminate clinker: simultaneous TG-DTA (a) and EGA (b) assessment using

The reaction interface (Fig.24) abundance of SrO (outer side of formed strontium aluminate layer) or Al2O3 (inner side) component led to the formation of tri-strontium aluminate (Sr3A) and strontium hexaaluminate (SrA6). The equilibrium composition of strontium aluminate (SrA) was then established via the opposite direction diffusion of Sr2+and Al3+ions. The opposite

by 17.7 wt. % during this process.

the heating rate of 10 °C min-1.

The cooling zone in (Fig.23) showed an exothermic effect of reversible transformation of hexagonal high-temperature modification of SrAl2O4 to low-temperature monoclinic phase at the temperature of 650 °C. During repeated cycle of heating, the exothermic transformation took place at higher temperature. The temperature hysteresis of this transformation increased linearly with the heating rate (Fig.25). Therefore, the limit value of 9.10 ± 0.07 °C should be estimated from the linear fit for Θ → 0 [379].

**Figure 25.** Temperature hysteresis in the polymorphic transformation (a) [379] between monoclinic (b) and hexgonal strontium aluminate (c) [24].

The recorded XPS (X-ray photoelectron spectroscopy) spectrum of strontium (Sr3d) in prepared strontium aluminate cement is presented in Fig.26. The results indicate that the structure of SrAl2O4 includes two binding states of strontium in the structure of strontium aluminate distributed in the ratio close to 1:2, similarly as was described for the structure CaAl2O4 where two Ca(I) and Ca(II) had octaedric coordination while Ca(III) was coordinated by six O2-ions in the trigonal antiprism [380].

**3.3. Sintering of strontium aluminate clinker**

respectively.

kinetic factor (Chapter 4.3.2).

and isothermal condition in reaction zone (d).

Heating microscopy (Fig.28) showed the expansion of specimen up to 113 % of its original height. For the temperatures higher than 1050 °C the growth of specimen decreased. The initial temperatures of solid-state and liquid state sintering were determined to be 1450 and 1550 °C,

Processes during Thermal Treatment 125

**Figure 28.** Behaviour of raw material specimen during heating with the heating rate of 5 °C min-1 [379].

The information on the mechanism and kinetics of the processes occurring during the synthesis of strontium aluminate clinker were used for the calculation of correct conditions (Fig.29) during the thermal treatment from kinetic triplet, i.e. activation energy, frequency factor and

**Figure 29.** Formation of strontium aluminate using single ramp (a), fast ramp (b), slow heating upon reaction zone (c)

**Figure 26.** XPS spectrum of strontium from prepared strontium aluminate clinker.

#### **3.2. Kinetic of formation of strontium aluminate**

The Kissinger plot related to the kinetics of strontium aluminate and tri-strontium aluminate phase formed from the mixture of raw materials is shown in Fig.27.

**Figure 27.** Kissinger plot related to the formation of SrA and Sr3A clinker phase [379].

The apparent activation energy and the frequency factor relating to the formation of Sr3A are 551 kJ mol-1 and 1.69 1023 s-1, respectively. The value of kinetic exponents calculated according to Eq.113 in Chapter 1 is 4.0, hence the process is driven by constant nucleation rate of a new phase. SrA shows the activation energy and the frequency factor of 218 kJ⋅mol-1 and 1.63 107 s-1, respectively. The value of kinetic exponent is equal to 5.2. Therefore, the crystallization of SrA phase is driven by increasing nucleation rate of a new phase [379].

#### **3.3. Sintering of strontium aluminate clinker**

structure of SrAl2O4 includes two binding states of strontium in the structure of strontium aluminate distributed in the ratio close to 1:2, similarly as was described for the structure CaAl2O4 where two Ca(I) and Ca(II) had octaedric coordination while Ca(III) was coordinated

124 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

The Kissinger plot related to the kinetics of strontium aluminate and tri-strontium aluminate

The apparent activation energy and the frequency factor relating to the formation of Sr3A are 551 kJ mol-1 and 1.69 1023 s-1, respectively. The value of kinetic exponents calculated according to Eq.113 in Chapter 1 is 4.0, hence the process is driven by constant nucleation rate of a new phase. SrA shows the activation energy and the frequency factor of 218 kJ⋅mol-1 and 1.63 107

respectively. The value of kinetic exponent is equal to 5.2. Therefore, the crystallization of SrA

s-1,

by six O2-ions in the trigonal antiprism [380].

**Figure 26.** XPS spectrum of strontium from prepared strontium aluminate clinker.

phase formed from the mixture of raw materials is shown in Fig.27.

**Figure 27.** Kissinger plot related to the formation of SrA and Sr3A clinker phase [379].

phase is driven by increasing nucleation rate of a new phase [379].

**3.2. Kinetic of formation of strontium aluminate**

Heating microscopy (Fig.28) showed the expansion of specimen up to 113 % of its original height. For the temperatures higher than 1050 °C the growth of specimen decreased. The initial temperatures of solid-state and liquid state sintering were determined to be 1450 and 1550 °C, respectively.

**Figure 28.** Behaviour of raw material specimen during heating with the heating rate of 5 °C min-1 [379].

The information on the mechanism and kinetics of the processes occurring during the synthesis of strontium aluminate clinker were used for the calculation of correct conditions (Fig.29) during the thermal treatment from kinetic triplet, i.e. activation energy, frequency factor and kinetic factor (Chapter 4.3.2).

**Figure 29.** Formation of strontium aluminate using single ramp (a), fast ramp (b), slow heating upon reaction zone (c) and isothermal condition in reaction zone (d).

### **4. The influence of degree of saturation**

The influence of saturation degree (*SD*, Eq.36 in Chapter 2) on the equilibrium composition of the product during the thermal treatment of strontium aluminate raw mixture consisting of SrCO3 and Al2O3 is shown in Fig.30. Lower values of saturation of raw materials by strontium oxide support the formation of strontium dialuminate7 while the amount of tri-strontium aluminate and strontium oxide increases with increasing SDSrO. These phases significantly contribute to the heat release during mixing the clinker with water. In order to avoid the danger of overheating of mixture, the saturation degree ≥ 100 % shouldn't be used. The theoretical value, which was estimated as the highest allowed value of saturation degree for which the equilibrium amount of SrO is inconsiderable is close to 95 %.

**5. Final treatment of strontium aluminate cement**

**Figure 32.** SEM picture of fine ground strontium aluminate cement.

Chapter 2.8.

particles.

phase.

Grinding and fine milling of strontium aluminate clinker (Fig.31) prepared via the calcination of pellets (a) or fine raw material powder (b) provided cement with the median of particle size of 7.52 μm (Fig.33), which was used for the hydration experiments described in the next chapter as well as for other applications described in Chapters 6-8. Using pressed pellets may influence the behaviour of raw material during the thermal treatment as was already discussed in

Processes during Thermal Treatment 127

**Figure 31.** Strontium aluminate clinker prepared from pelletized (a) and fine powdered (b) mixture of raw materials.

The SEM picture of strontium aluminate cement (Fig.32) shows larger subhedral particles with some glass-like faces surrounded by aggregates consisting of closely associated smaller

The particle-size distribution (PSD) and the cumulative distribution function of prepared binder are shown in Fig.33. The multimodal particle size distribution in ground clinker should be explained by the content of other phases of different grindability (Fig.21) and of glassy

**Figure 30.** Influence of SDSrO on the calculated equilibrium composition of reaction products: 73.8% (a), 88.6 (b), 98.4 (c), 109.4 (d) and 131.2 (e).

<sup>7</sup> Formation SrA6 is preferred from the formation of SrA2 during thermal treatment of raw meal (please see discussion in Chapter 1.5).

## **5. Final treatment of strontium aluminate cement**

**4. The influence of degree of saturation**

oxide support the formation of strontium dialuminate7

equilibrium amount of SrO is inconsiderable is close to 95 %.

The influence of saturation degree (*SD*, Eq.36 in Chapter 2) on the equilibrium composition of the product during the thermal treatment of strontium aluminate raw mixture consisting of SrCO3 and Al2O3 is shown in Fig.30. Lower values of saturation of raw materials by strontium

126 Strontium Aluminate - Cement Fundamentals, Manufacturing, Hydration, Setting Behaviour and Applications

aluminate and strontium oxide increases with increasing SDSrO. These phases significantly contribute to the heat release during mixing the clinker with water. In order to avoid the danger of overheating of mixture, the saturation degree ≥ 100 % shouldn't be used. The theoretical value, which was estimated as the highest allowed value of saturation degree for which the

**Figure 30.** Influence of SDSrO on the calculated equilibrium composition of reaction products: 73.8% (a), 88.6 (b), 98.4

7 Formation SrA6 is preferred from the formation of SrA2 during thermal treatment of raw meal (please see discussion in

(c), 109.4 (d) and 131.2 (e).

Chapter 1.5).

while the amount of tri-strontium

Grinding and fine milling of strontium aluminate clinker (Fig.31) prepared via the calcination of pellets (a) or fine raw material powder (b) provided cement with the median of particle size of 7.52 μm (Fig.33), which was used for the hydration experiments described in the next chapter as well as for other applications described in Chapters 6-8. Using pressed pellets may influence the behaviour of raw material during the thermal treatment as was already discussed in Chapter 2.8.

**Figure 31.** Strontium aluminate clinker prepared from pelletized (a) and fine powdered (b) mixture of raw materials.

The SEM picture of strontium aluminate cement (Fig.32) shows larger subhedral particles with some glass-like faces surrounded by aggregates consisting of closely associated smaller particles.

**Figure 32.** SEM picture of fine ground strontium aluminate cement.

The particle-size distribution (PSD) and the cumulative distribution function of prepared binder are shown in Fig.33. The multimodal particle size distribution in ground clinker should be explained by the content of other phases of different grindability (Fig.21) and of glassy phase.

**Chapter 5**

**Hydration and Setting Behaviour of Strontium**

Hydration of cement generally means complex changes and reactions which occur when the cement paste is prepared by mixing of anhydrous cement with water. Setting means stiffening without significant development of compressive strength which typically occurs within a few hours after mixing of cement with water. Hardening process means gradual significant development of compressive strength during curing in applied environmental conditions

As was already mentioned in Chapter 2.6, cement slurries are complex reactive systems. They continuously change, physically and chemically. The microstructural and chemical evolution of cement slurry from the first minutes after mixing of cement powder with water until the beginning of setting can be observed by a variety of physicochemical methods including thermal analysis calorimetry [286-393], X-ray diffraction analysis [386,394,744], infrared Raman and Mössbauer spectroscopy [386,395,396], electron microscopy [273,275,394-399] as well as via liquid phase analysis [400,401], porosity and specific surface of cement stone measurement [402], ultrasonic pulse velocity [403-406], impedance spectroscopy measurement

The knowledge of the setting characteristics of concrete is quite important in the field of concrete construction. They will help in scheduling of various stages involved in concrete construction operations such as transporting, placing, compacting and finishing of concrete. This information is a necessity when deciding whether to use a retarding admixture or

The introduction of superplasticizers in early 1960s had great effect on the development in concrete technology. Superplasticized cement can be prepared with lower water to cement ratio, has improved workability, high strength and high durability without affecting the setting and hardening behaviour. In chemical terms, superplasticizers are organic polyelectrolytes which belong to the category of polymeric dispersants. Today, superplasticizers are considered

**1. Class F**: High-range water reducers: the most important group are sulphonated synthetic polymers such as poly-*β*-naphtalene sulphonate and sodium polymelamine sulphonate;

> © 2014 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

[407,408] and electrical conductivity/ resistivity measurement [409,410].

as an integral component of High-Performance concretes [412].

**2. Class G**: High-range water reducers and retarders.

ASTM C494 recognizes the following classes of superplasticizers [412]:

**Aluminate Cements**

accelerator or not [411].

(storage in air or water, temperature) [263,265,270].

**Figure 33.** Particle size distribution of strontium aluminate clinker [379].

## **6. Clinker quality evaluation**

The specific surface, the particle size distribution (Chapter 3.5 and Chapter 6.1.2), the content of major and minor clinker phases (Chapter 2.5) in cement are the main parameters which can be used for the evaluation of quality of strontium aluminate clinker similarly to other binders such as AC and PC. These parameters define the proportion of fine and coarse particles in cement. This proportion controls the water demand, the setting and hydration reactions [381-385].

**Figure 34.** The clinker phase (Sr3A, SrA, SrA2 and SrA6) and alumina (a) and strontium aluminate clinker with high con‐ tent of glassy phase (b) under visible and UV light.

Therefore, low firing temperature or rough milling procedure can be used in order to reduce initial high reactivity of SrA cement with water. The phosphorescence of calcinate can be ap‐ plied to evaluate the firing process (estimate the amount of unrecated alumina, Fig.34). Fur‐ thermore, the phosphorescence under UV light can be used for marking of cement or for preparation of cement layer or concretes of special properties.
