**Expansive Strontium Aluminate Cements**

## **1. Expansive cements**

Cement hydration is a complex set of exothermic chemical reactions. Each chemical compound may react with water in different ways, each reaction has its own volume stoichiometry, and the reaction (or hydration) products may undergo the transition to different forms. There are several mechanisms by which a cement paste changes its volume (shrinks or swells) during the hydration process under sealed conditions, or without the moisture transfer from paste to the environment. The major mechanisms of total shrinkage include [12,278,673-676]:


Autogenous shrinkage is a phenomenon in which cementitious materials shrink at constant temperature without any changes in weight [677]. It is well-known that the volume changes in a cement paste or concrete mentioned above may occur simultaneously. It is the overall shrinkage behaviour that controls the concrete performance [278]. Furthermore, the influence of this phenomenon on total shrinkage depends on the size of specimen [414]. Most chemical shrinkage measurements reported are often performed at short duration and suggested water to cement ratio of 0.40 as required by ASTM C 1608 [678].

Autogenous shrinkage in concrete with low water-cement ratio is the major factor that generates larger shrinkage and early cracking [673,677,679]. A small increase in the chemical shrinkage at "infinite time" was found with increasing water-cement ratio. The influence of temperature was found to be twofold: Increasing temperature caused increasing rate of the development of chemical shrinkage and the decrease of the chemical shrinkage at "infinite time" [680]. The mineral admixtures in concrete affect the concrete shrinkage in different ways. Fly ash and limestone powder can reduce the autogenous shrinkage while silica fume may lead to increased shrinkage [679,681].

© 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.

At very early age and during the setting, two phenomena are directly responsible for the shrinkage of material in sealed conditions [648]:

(3CaO·3Al2O3·CaSO4, C4A3*S*¯

2-and Ca2+to SO<sup>4</sup>

SO<sup>4</sup>

,

2-ratio [700-703].

**2. Ettringite and analog of ettringite**

been known since 1874 [637,704-707].

Columns: {*Ca*<sup>6</sup> *Al*(*OH* )

H2O molecules [706-709].

structure are Ca2+, Al3+, SO<sup>4</sup>

Channels: {(*SO*<sup>4</sup>

have two distinct structural components:

2-)<sup>3</sup> ×2 *<sup>H</sup>*2*O*}6-

<sup>6</sup> <sup>2</sup> ×24 *H*2*O*}6<sup>+</sup>

aluminate (3CaO·Al2O3, C3A) in S-type cement [12,693,697].

*<sup>α</sup>* or *β* anhydrites (CaSO4, C*S*¯) [699,370] are used as the sources of SO<sup>4</sup>

so-called "kleinite") is present in K-type cement [12,693-697],

2-ions. The decreasing

Expansive Strontium Aluminate Cements 221

2-and one by two


monocalcium aluminate (CaO·Al2O3, CA) in M-type cement [12,693,697] and tricalcium

Gypsum (CaSO4·2H2O, C *S*¯ H2) [698],*α* or *β* hemihydrates (CaSO4·0.5H2O, C *S*¯ H0.5) [699] and

dissolution rate of anhydrite results inthe lack of sulphate ions necessary for the formation andstability of ettringite, which is sensible to such factors as temperature, pH, Al(OH)<sup>4</sup>

Ettringite (3CaO⋅Al2O3⋅3CaSO4⋅32H2O, AFt as calcium aluminate or ferrite tri-substituted hydrates of general formula 3CaO⋅(Al,Fe)2O3⋅3CaX2⋅nH2O) is formed in hydrated cement at early hydration stage. The AFt designation was first suggested by Smolczyk in 1961 in order to distinguish the high sulfate hydrate phases from the low sulfate phases, which are collec‐ tively termed AFm (3CaO⋅Al2O3⋅CaSO4⋅12H2O). From its natural occurrences, ettringite has

Ettringite, like other AFt phases, forms hexagonal prismatic or acicular crystals. The crystals

The columns consist of AI(OH)6 octahedra alternating with triangular groups of edgesharing CaO8 polyhedra. The column alignment is along the c-axis of the trigonal-hexago‐ nal unit cell. Eight oxygensatoms in the polyhedra come from the Ca coordination with four OH ions shared with the AI(OH)6 octahedra and from the coordination with four H2O molecules. The hydrogen atoms from the molecules of water form the cylindrical surface of the columns. The channels contain four sites per formula unit of the column structure

Both calcium sulfoaluminate hydrates (AFt and AFm) have the ability to combine with a number of cations and anions, which makes them important with regard to the waste immo‐ bilization in cementitious matrices [710-712]. Ions available for the substitution in the ettringite

. They are listed in Table 1.

which contains six calcium atoms. Three of these sites are occupied by SO<sup>4</sup>

2-and OH-

Reported analogs of ettringite are listed in Table 2.


Drying shrinkage is relatively slow process. The driving force is the loss of water from the pores due to lower humidity prevailing outside the concrete. The diffusion of water through the material depends mainly on its porous structure, on the size and shape of the specimen, on the duration of drying and on prevailing atmospheric conditions [414,682].

The shrinkage can be compensated by several compounds acting through different mecha‐ nisms, but the principle remains similar-the incidence of expansion pressure on the setting paste. It can be achieved via the formation of new crystalline phases, predominantly (3CaO⋅Al2O3⋅3CaSO4⋅32H2O, C6AS3H32, AFt), swelling gel of portlandite (Ca(OH)2,CH [12,688]) or brucite (Mg(OH)2,MH [12,683-687,690]), or by the osmotic pressure generated by semi-permeable membrane of hydration products on the surface of anhydrous grains under suitable conditions.

The expansion admixture has to produce the expansive stress after the start of setting, at the stage when the paste attains certain rigidity to be able to transfer the expansion stress as a plastic material. If ettringite occurs homogeneously and immediately (within hours) after the start of setting, it does not cause any significant localized disruptive action (early ettringite formation, EEF). In the case of the expansion, which appears after the solidifica‐ tion (Delayed Ettringite Formation, DEF) it reduces the mechanical performance, or in extreme case, it leads to the material failure. DEF occurs when the constituents of con‐ crete provide initial source of sulphates as a result of undesirable heating of the concrete to a temperatures higher than 65-70 °C. It has been found that the expansion time is inversely proportional to the fineness of expansive component and directly proportional to the amount of added sulfate [12,688-691,738,739].

The mechanisms mentioned above are based on the formation of ettringite where the expan‐ sion occurs due to the crystallization pressure as well as the swelling pressure which is caused by the selective absorption of ions combined with large number of water molecules on tiny ettringite crystals. The growing mechanism, the size and the shape of ettringite crystals are important factors, once the shrinkage compensation or the expansion effect shall be reached. The needle-like form of ettringite crystals growing on the surface of C3A grains by the topochemical process after the solidification of cement slurry can convert the resulting strain into the desired volume expansion. In the case that ettringite crystals precipitate from the solution in the pores between grains of clinker phases, the expansion or shrinkage compen‐ sation effect does not occur [12].

Three types of expansive cements are defined by ASTM C 845 (Standard Specification for Expansive Hydraulic Cement) – K, M and S type [692]. The main aluminate phase that is responsible for the formation of ettringitevaries with the cement type. The Klein's compound (3CaO·3Al2O3·CaSO4, C4A3*S*¯ , so-called "kleinite") is present in K-type cement [12,693-697], monocalcium aluminate (CaO·Al2O3, CA) in M-type cement [12,693,697] and tricalcium aluminate (3CaO·Al2O3, C3A) in S-type cement [12,693,697].

Gypsum (CaSO4·2H2O, C *S*¯ H2) [698],*α* or *β* hemihydrates (CaSO4·0.5H2O, C *S*¯ H0.5) [699] and *<sup>α</sup>* or *β* anhydrites (CaSO4, C*S*¯) [699,370] are used as the sources of SO<sup>4</sup> 2-ions. The decreasing dissolution rate of anhydrite results inthe lack of sulphate ions necessary for the formation andstability of ettringite, which is sensible to such factors as temperature, pH, Al(OH)<sup>4</sup> - to SO<sup>4</sup> 2-and Ca2+to SO<sup>4</sup> 2-ratio [700-703].

## **2. Ettringite and analog of ettringite**

At very early age and during the setting, two phenomena are directly responsible for the

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

**2.** Le Chatelier's contraction caused by the density difference between hydrates on one hand

Drying shrinkage is relatively slow process. The driving force is the loss of water from the pores due to lower humidity prevailing outside the concrete. The diffusion of water through the material depends mainly on its porous structure, on the size and shape of the specimen,

The shrinkage can be compensated by several compounds acting through different mecha‐ nisms, but the principle remains similar-the incidence of expansion pressure on the setting paste. It can be achieved via the formation of new crystalline phases, predominantly (3CaO⋅Al2O3⋅3CaSO4⋅32H2O, C6AS3H32, AFt), swelling gel of portlandite (Ca(OH)2,CH [12,688]) or brucite (Mg(OH)2,MH [12,683-687,690]), or by the osmotic pressure generated by semi-permeable membrane of hydration products on the surface of anhydrous grains under

The expansion admixture has to produce the expansive stress after the start of setting, at the stage when the paste attains certain rigidity to be able to transfer the expansion stress as a plastic material. If ettringite occurs homogeneously and immediately (within hours) after the start of setting, it does not cause any significant localized disruptive action (early ettringite formation, EEF). In the case of the expansion, which appears after the solidifica‐ tion (Delayed Ettringite Formation, DEF) it reduces the mechanical performance, or in extreme case, it leads to the material failure. DEF occurs when the constituents of con‐ crete provide initial source of sulphates as a result of undesirable heating of the concrete to a temperatures higher than 65-70 °C. It has been found that the expansion time is inversely proportional to the fineness of expansive component and directly proportional to the

The mechanisms mentioned above are based on the formation of ettringite where the expan‐ sion occurs due to the crystallization pressure as well as the swelling pressure which is caused by the selective absorption of ions combined with large number of water molecules on tiny ettringite crystals. The growing mechanism, the size and the shape of ettringite crystals are important factors, once the shrinkage compensation or the expansion effect shall be reached. The needle-like form of ettringite crystals growing on the surface of C3A grains by the topochemical process after the solidification of cement slurry can convert the resulting strain into the desired volume expansion. In the case that ettringite crystals precipitate from the solution in the pores between grains of clinker phases, the expansion or shrinkage compen‐

Three types of expansive cements are defined by ASTM C 845 (Standard Specification for Expansive Hydraulic Cement) – K, M and S type [692]. The main aluminate phase that is responsible for the formation of ettringitevaries with the cement type. The Klein's compound

on the duration of drying and on prevailing atmospheric conditions [414,682].

shrinkage of material in sealed conditions [648]: **1.** Capillary depression in the porous space;

and cement and water on the other hand.

amount of added sulfate [12,688-691,738,739].

sation effect does not occur [12].

suitable conditions.

Ettringite (3CaO⋅Al2O3⋅3CaSO4⋅32H2O, AFt as calcium aluminate or ferrite tri-substituted hydrates of general formula 3CaO⋅(Al,Fe)2O3⋅3CaX2⋅nH2O) is formed in hydrated cement at early hydration stage. The AFt designation was first suggested by Smolczyk in 1961 in order to distinguish the high sulfate hydrate phases from the low sulfate phases, which are collec‐ tively termed AFm (3CaO⋅Al2O3⋅CaSO4⋅12H2O). From its natural occurrences, ettringite has been known since 1874 [637,704-707].

Ettringite, like other AFt phases, forms hexagonal prismatic or acicular crystals. The crystals have two distinct structural components:

Columns: {*Ca*<sup>6</sup> *Al*(*OH* ) <sup>6</sup> <sup>2</sup> ×24 *H*2*O*}6<sup>+</sup>

Channels: {(*SO*<sup>4</sup> 2-)<sup>3</sup> ×2 *<sup>H</sup>*2*O*}6-

The columns consist of AI(OH)6 octahedra alternating with triangular groups of edgesharing CaO8 polyhedra. The column alignment is along the c-axis of the trigonal-hexago‐ nal unit cell. Eight oxygensatoms in the polyhedra come from the Ca coordination with four OH ions shared with the AI(OH)6 octahedra and from the coordination with four H2O molecules. The hydrogen atoms from the molecules of water form the cylindrical surface of the columns. The channels contain four sites per formula unit of the column structure which contains six calcium atoms. Three of these sites are occupied by SO<sup>4</sup> 2-and one by two H2O molecules [706-709].

Both calcium sulfoaluminate hydrates (AFt and AFm) have the ability to combine with a number of cations and anions, which makes them important with regard to the waste immo‐ bilization in cementitious matrices [710-712]. Ions available for the substitution in the ettringite structure are Ca2+, Al3+, SO<sup>4</sup> 2-and OH- . They are listed in Table 1.

Reported analogs of ettringite are listed in Table 2.


According to the stability field diagramof the system CaO–Al2O3–SO3–H2O (Fig.1(a)) estab‐ lished by Hampson and Bailey [720], high sulphate concentrations stabilize gypsum and only when its concentration is low, then ettringite becomes the stable phase [735]. The thermody‐ namic investigation of the CaO-Al2O3-CaSO4-H2O (Fig.1(b)) system at 25 °C [721], 50 °C and 85 °C was performed by Damidot and Glasser [722]. The effect of Na2O and K2O on the

Expansive Strontium Aluminate Cements 223

**Figure 1.** Ettringite stability in alkaline environments [720] (a) and three-dimensional representation of CaO-Al2O3-Ca‐

**•** Hydrogarnet: 3CaO⋅Al2O3⋅6H2O (other calcium alumina hydrates are also possible but they

The reaction of cement minerals with sulphate and carbonate ions can lead to the formation of thaumasite (Ca6[Si(OH)6]2(CO3)2(SO4)2 24H2O or CaSiO3 CaCO3 CaSO4 15H2O) at tempera‐ tures below 15 °C. This reaction can destroy the C-S-H matrix and result in the disintegration of concrete (thaumasite sulfate attack, TSA). Ettringite and thaumasite have very similar crystal structures. Thaumasite is hexagonal (*a*=11.54 Å, *a*=10.401 Å) with the columns of [Ca3Si(OH)<sup>6</sup>

**•** Ettringite (triphase or AFt in cement nomenclature): 3CaO⋅Al2O3⋅3CaSO4⋅32H2O;

equilibrium state is disused in works [721] and [723], respectively.

The following solid phases are possible in this system [721]:

are metastable with respect to hydrogarnet);

**•** Monosulfate (monophase or AFm): 3CaO⋅Al2O3⋅CaSO4⋅12H2O;

SO4-H2O system [721] (b).

**•** Gypsum: CaSO4⋅2H2O;

**•** Portlandite: Ca(OH)2;

**•** Gibbsite: Al(OH)3.

**Table 1.** Ion substitutions in ettringite minerals according to [706].


**Table 2.** Reported analogs of ettringite.

According to the stability field diagramof the system CaO–Al2O3–SO3–H2O (Fig.1(a)) estab‐ lished by Hampson and Bailey [720], high sulphate concentrations stabilize gypsum and only when its concentration is low, then ettringite becomes the stable phase [735]. The thermody‐ namic investigation of the CaO-Al2O3-CaSO4-H2O (Fig.1(b)) system at 25 °C [721], 50 °C and 85 °C was performed by Damidot and Glasser [722]. The effect of Na2O and K2O on the equilibrium state is disused in works [721] and [723], respectively.

**Figure 1.** Ettringite stability in alkaline environments [720] (a) and three-dimensional representation of CaO-Al2O3-Ca‐ SO4-H2O system [721] (b).

The following solid phases are possible in this system [721]:


**Ion Name/ formula Reference**

(4CaO·Al2O3· ½ B2O3·12H2O, B-AFm) [737]

**Site Ion size [Å] Site Ion size [Å] Site Ion size [Å] Site Ion size [Å]**

SO4 2-

VO4 3- BrO3 - NO3 - MnO4 - CrO3 - SO3 2- IO3 -

0.62 – 3.71

OH-

0.62 – 1.38

0.42 – 0.76

Sr2+ 1.26 Cr3+ B(OH)4 - O2-

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

Chromate ettringite (3CaO·Al2O3·CaCrO4·nH2O) [736,715] Chromate monophase (3CaO·Al2O3·CaCrO4·15H2O) [736]

(3CaO·Al2O3·CaSeO4·xH2O) [712,718]

(Ca6Al2(CO3)3(OH)12·26H2O) [719]

(Ca6[Al1-xFex(OH)6]2(SO4)3·26H2O) [708]

Selenate ettringite(Ca6[Al(OH)6]2(SeO4)3·31.5H2O) [712,716,717]

[737,713,714]

(3CaO·Al2O3·Ca(B(OH)4)2·Ca(OH)2·36H2O), B-AFt)

(3CaO· Al2O3·Ca(B(OH)4)2·2Ca (OH)2·36H2O)

B(OH)4 -

Ca2+ 1.12

0.90 – 1.42

Al3+

Ba2+ 1.42 Si4+ CO3 2- Pb2+ Fe3+ Cl-Cd2+ Mn3+ OH-Co2+ Ni3+ CrO4 2- Ni2+ Co3+ AsO4 3- Zn2+ Ti3+ SeO4 2-

CrO4 2-

SeO4

High-boro-ettringite

**Table 1.** Ion substitutions in ettringite minerals according to [706].

Low-boro-ettringite

Selenatemonophase

Al3+ Solid solution of Al- and Fe-ettringite

HBO3 2- Boro-monophase

CO3 2- Carbonate ettringite

**Table 2.** Reported analogs of ettringite.


The reaction of cement minerals with sulphate and carbonate ions can lead to the formation of thaumasite (Ca6[Si(OH)6]2(CO3)2(SO4)2 24H2O or CaSiO3 CaCO3 CaSO4 15H2O) at tempera‐ tures below 15 °C. This reaction can destroy the C-S-H matrix and result in the disintegration of concrete (thaumasite sulfate attack, TSA). Ettringite and thaumasite have very similar crystal structures. Thaumasite is hexagonal (*a*=11.54 Å, *a*=10.401 Å) with the columns of [Ca3Si(OH)<sup>6</sup> 12H2O]4+and sulfate and carbonate ions in an ordered arrangement in channels between the columns. Si is octahedrally coordinated by oxygen [724-730].

2+ 2- Ca + SO + 2 H O CaSO × 2H O 4 2 ¬¾® 4 2 (2)

¬¾® (3)

Expansive Strontium Aluminate Cements 225

¬¾® (4)

(5)

(6)

2- ettringite either

¬¾®

At elevated pH (~12) the dissolution of gypsum is promoted while ettringite remains stable. Gypsumis used afterwards as a source of sulphate in the synthesis of ettringite according to

4 42 2

At low sulphate concentrations the ettringite synthesis proceeds following the reaction given

4 4 2

The reaction of tricalcium aluminate (3CaO⋅Al2O3) with gypsum (CaSO4⋅2H2O) is known to

Ettringite appears as a hydrate phase of Portland cement in the first stage of hydration process. In stage one, the aluminate phase reacts with water in the presence of calcium sulfate to form

23 4 2 2

Ettringite can also be formed through the reaction of ferrite hydrate phase. Ettringite is formed within minutes after the start of cement hydration. Its concentration increases to a maximum within about 1 day and then it begins to decrease and may even approach zero. As ettringite disappears, mono-sulfate (AFm) hydrate phase takes its place due to lower sulfate concentra‐ tion in the pore solutions. The ettringite produced in this first hydration stage tends to be in the form of stubby, prismatic crystals close to the aluminate surface as well as away from these

23 4 2 23 2

Structurally, AFm phases are similar to the mineral hydrocalumite, the crystals of which are morphologically recognizable as thin hexagonal plates (Fig.2). Basal spacing in AFm crystal

3CaO× Al O × 3CaSO × 32H O+ 2 3 CaO× Al O + 4 H O

( )

®

23 4 2 3CaO× Al O + 3 CaSO × 2H O+ 26 H O

3CaO× Al O × 3CaSO × 32H O

surfaces. Depending on the cement composition and availability of CO3

persists or is converted into monosulfate [7,12,706,736,737]:

( )

3 3CaO× Al O × CaSO ×12H O

23 4 2

2+ - -

2+ - 2- -

23 4 2 3 Ca + 2 Al(OH) + 3 SO + 4 OH + 26 H O

3CaO× Al O × 3CaSO × 32H O

be rapid and complete at room temperature (within 30 to 60 min) [705].

3 Ca + 2 Al(OH) + 3 CaSO × 2H O+ 4 OH + 20 H O

23 4 2

3CaO× Al O × 3CaSO × 32H O

the reaction given by:

by the equation:

ettringite [706]:

**Figure 2.** SEM image of ettringite crystals prepared by the saccharate method andcharacteristic crystal morphologies of AFt (a) and AFm (b) [737].

Most synthesis experiments reported in literature are based on the "saccharate" method reported originally by Carlson and Berman [731]. A CaO supersaturated, 10 % sucrose solution is mixed with Na2O Al2O3 and Na2SO4 in the stoichiometric amounts. Sodium sulfate can be replaced by oxyanion salt to obtain substituted ettringite [713,732]. The same process without the addition of sucrose is referred to as the "paste reaction" method [713].

Ettringite can be also synthesized by mixing the calcium hydroxide slurry and the aluminium sulphate solution [733,734]:

$$\begin{aligned} \text{(6 Ca(OH)}\_{2} + \text{Al}\_{2}\text{(SO}\_{4})\_{3} \cdot 16\text{H}\_{2}\text{O} \star 10\text{H}\_{2}\text{O} \rightarrow\\ \text{Ca}\_{6}[\text{Al(OH)}\_{6}]\_{2}\text{(SO}\_{4})\_{3} \times 26\text{H}\_{2}\text{O} \end{aligned} \tag{1}$$

At high sulphate concentrations, gypsuminitially controls the maximum concentration of sulphate in solution by the reaction [375]:

$$\text{Ca}^{2+} + \text{SO}\_4^{2-} + 2\text{H}\_2\text{O} \longleftarrow \text{CaSO}\_4 \times 2\text{H}\_2\text{O} \tag{2}$$

At elevated pH (~12) the dissolution of gypsum is promoted while ettringite remains stable. Gypsumis used afterwards as a source of sulphate in the synthesis of ettringite according to the reaction given by:

12H2O]4+and sulfate and carbonate ions in an ordered arrangement in channels between the

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

**Figure 2.** SEM image of ettringite crystals prepared by the saccharate method andcharacteristic crystal morphologies

Most synthesis experiments reported in literature are based on the "saccharate" method reported originally by Carlson and Berman [731]. A CaO supersaturated, 10 % sucrose solution is mixed with Na2O Al2O3 and Na2SO4 in the stoichiometric amounts. Sodium sulfate can be replaced by oxyanion salt to obtain substituted ettringite [713,732]. The same process without

Ettringite can be also synthesized by mixing the calcium hydroxide slurry and the aluminium

2 2 43 2 2

At high sulphate concentrations, gypsuminitially controls the maximum concentration of

× ®

(1)

6 Ca(OH) + Al (SO ) 16H O+ 10 H O

the addition of sucrose is referred to as the "paste reaction" method [713].

6 62 43 2

Ca [Al(OH) ] (SO ) × 26H O

columns. Si is octahedrally coordinated by oxygen [724-730].

of AFt (a) and AFm (b) [737].

sulphate solution [733,734]:

sulphate in solution by the reaction [375]:

$$\begin{aligned} \text{3Ca}^{2+} + 2\text{ Al(OH)}\_{4}^{\cdot} + \text{3CaSO}\_{4} \times 2\text{H}\_{2}\text{O} + 4\text{ OH}^{\cdot} + 20\text{ H}\_{2}\text{O} &\longleftrightarrow \\ \text{3CaO} \times \text{Al}\_{2}\text{O}\_{3} \times 3\text{CaSO}\_{4} \times 32\text{H}\_{2}\text{O} \end{aligned} \tag{3}$$

At low sulphate concentrations the ettringite synthesis proceeds following the reaction given by the equation:

$$\begin{aligned} \text{3Ca}^{2+} + 2\text{ Al(OH)}\_{4}^{-} + 3\text{ SO}\_{4}^{2-} + 4\text{ OH}^{-} + 26\text{ H}\_{2}\text{O} &\longleftrightarrow \\ \text{3CaO} \times \text{Al}\_{2}\text{O}\_{3} \times 3\text{CaSO}\_{4} \times 32\text{H}\_{2}\text{O} \end{aligned} \tag{4}$$

The reaction of tricalcium aluminate (3CaO⋅Al2O3) with gypsum (CaSO4⋅2H2O) is known to be rapid and complete at room temperature (within 30 to 60 min) [705].

Ettringite appears as a hydrate phase of Portland cement in the first stage of hydration process. In stage one, the aluminate phase reacts with water in the presence of calcium sulfate to form ettringite [706]:

$$\begin{aligned} 3\text{CaO} \times \text{Al}\_2\text{O}\_3 + 3\text{CaSO}\_4 \times 2\text{H}\_2\text{O} + 26\text{H}\_2\text{O} &\to\\ 3\text{CaO} \times \text{Al}\_2\text{O}\_3 \times 3\text{CaSO}\_4 \times 32\text{H}\_2\text{O} \end{aligned} \tag{5}$$

Ettringite can also be formed through the reaction of ferrite hydrate phase. Ettringite is formed within minutes after the start of cement hydration. Its concentration increases to a maximum within about 1 day and then it begins to decrease and may even approach zero. As ettringite disappears, mono-sulfate (AFm) hydrate phase takes its place due to lower sulfate concentra‐ tion in the pore solutions. The ettringite produced in this first hydration stage tends to be in the form of stubby, prismatic crystals close to the aluminate surface as well as away from these surfaces. Depending on the cement composition and availability of CO3 2- ettringite either persists or is converted into monosulfate [7,12,706,736,737]:

$$\begin{aligned} \text{3CaO} \star \text{Al}\_2\text{O}\_3 \star \text{3CaSO}\_4 \star \text{32H}\_2\text{O} &\star 2\text{(3CaO} \star \text{Al}\_2\text{O}\_3) + 4\text{H}\_2\text{O} \longleftarrow \\ \text{3(3CaO} \star \text{Al}\_2\text{O}\_3 \star \text{CaSO}\_4 \times 12\text{H}\_2\text{O}) \end{aligned} \tag{6}$$

Structurally, AFm phases are similar to the mineral hydrocalumite, the crystals of which are morphologically recognizable as thin hexagonal plates (Fig.2). Basal spacing in AFm crystal structure depends on the type of anion, such as CO3 2-, OH- , Cl and SO4 2-, within the interlayer region.

**3. Expansive strontium aluminate cements**

**Figure 3.** Effect of gypsum on the hydration of strontium aluminate cement.

least during initial stages of hydration.

The additions of sulfate bearing phases such as anhydrite to strontium aluminate cement can be performed in order to prepare expansive cements based on strontium aluminate.The effect of gypsum onto the hydration of SrAC was investigated by isothermal calorimetric experiment using the blend of strontium aluminate cement with 2% and 5% of CaSO4⋅2H2O (Fig.3).

In comparison with pure SrAC, the changes of heat flow indicate that gypsum has a significant effect on the course of hydration of strontium aluminate cement. The calorimetric data indicate the changes in the mechanism of hydration process and those caused by the formation of ettringite (Figs.5, 7 and 8). Fast precipitation of ettringite leads to the deficiency of [Al(OH)4]

ions and Sr2+ ions which substitute for Ca2+ ions in the structure of {Ca6[Al(OH)6]2⋅24H2O}6+ columns. That explains the formation of SrAHx (Fig.5(b)) and the absence of gibbsite. It is obvious that the intensity of effect related to the formation of SrAH<sup>x</sup> increases with the amount of gypsum added to SrAC. The addition of gypsum changes the main hydration products at

The diffusion barrier formed on the surface of strontium aluminate phase leads to a broad hydration effect where the time and temperature related to the maximum of released heat decreases with increasing content of gypsum. Increasing amount of added gypsum leads increased formation of SrAHx and ettringite while the amounts of formed Sr3AH6 and gibbsite decrease. Fig.3(b) shows that the heat released during the first 50 hours of hydration of strontium aluminate cement decreases exponentially with increasing content of gypsum. This behavior can provide the explanation for the hydration of the blend of strontium aluminate

The hydration of the samples after the calorimetric assessment was stopped by repeated washing of ground samples in acetone in order to remove water followed by the evaporation

cement with SrAC (please see the discussion related to Fig.7 in Chapter 7).


Expansive Strontium Aluminate Cements 227

There are two different types of DEF-related damages depending on the sulfate source [738-740]:


Several forms of ettringite phase which may be formed duringthe hydration of Portland cement are listed in Table 3.


**Table 3.** Forms of ettringite [741].

The formation of ettringite, which induces swelling, may also cause failures in stabilization of soil, where within the mechanical treatment CaO, Ca(OH)2 or cement are used. Ettringite shows very high water retention potential. Therefore, as ettringite grows in stabilized soil, its presence enhances the water retention of soil, causing that the sample exerts more force in order to take in the water–sulfate solution from lower soil strata [742]. Although lime treatment can be effective in stabilizing gypseous soils and in enhancing their mechanical properties, the amount of added lime must be adjusted according to the mineralogical composition and the gypsum content of soil [743].

## **3. Expansive strontium aluminate cements**

structure depends on the type of anion, such as CO3

region.

[738-740]:

**Ettringite**

Early primary

Late primary

Delayed

Secondary

meable concrete.

cement are listed in Table 3.

**Time of formation**

First hour of hydration

First days or weeks of hydration

Months after thermal curing

Months after thermal curing

**Table 3.** Forms of ettringite [741].

gypsum content of soil [743].

External Any time Migration of SO3 from

2-, OH-

There are two different types of DEF-related damages depending on the sulfate source

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

**1.** External (ESA) sulfate attack, which occurs when environmental sulfate (from water or soil) penetrates into a concrete structure. It can be avoided or reduced by using imper‐

**2.** Internal (ISA) sulfate attack, which occurs also in sulfate free-environment by latesulfate ions release from either cement or gypsum contaminated in aggregates. The ISA-induced damage caused by the late sulfate release from the cement can be related either to the

Several forms of ettringite phase which may be formed duringthe hydration of Portland

thermal decomposition of ''primary'' ettringite or to sulfate release from C-S-H.

Normal conditions of

Excessive amount of gypsum

After curing at temperatures

Recrystallization of ettringite in formed cracks upon wetting and drying

The formation of ettringite, which induces swelling, may also cause failures in stabilization of soil, where within the mechanical treatment CaO, Ca(OH)2 or cement are used. Ettringite shows very high water retention potential. Therefore, as ettringite grows in stabilized soil, its presence enhances the water retention of soil, causing that the sample exerts more force in order to take in the water–sulfate solution from lower soil strata [742]. Although lime treatment can be effective in stabilizing gypseous soils and in enhancing their mechanical properties, the amount of added lime must be adjusted according to the mineralogical composition and the

hydration

is cement

, Cl-

**Conditions of formation Al2O3 source Texture Expansive**

outside Mono-sulphate Microcrystalline yes

>70 °C Mono-sulphate Microcrystalline yes

C3A Microcrystalline no

C3A Microcrystalline yes

Ettringite Microcrystalline No (?)

and SO4

2-, within the interlayer

**behaviour**

The additions of sulfate bearing phases such as anhydrite to strontium aluminate cement can be performed in order to prepare expansive cements based on strontium aluminate.The effect of gypsum onto the hydration of SrAC was investigated by isothermal calorimetric experiment using the blend of strontium aluminate cement with 2% and 5% of CaSO4⋅2H2O (Fig.3).

**Figure 3.** Effect of gypsum on the hydration of strontium aluminate cement.

In comparison with pure SrAC, the changes of heat flow indicate that gypsum has a significant effect on the course of hydration of strontium aluminate cement. The calorimetric data indicate the changes in the mechanism of hydration process and those caused by the formation of ettringite (Figs.5, 7 and 8). Fast precipitation of ettringite leads to the deficiency of [Al(OH)4] ions and Sr2+ ions which substitute for Ca2+ ions in the structure of {Ca6[Al(OH)6]2⋅24H2O}6+ columns. That explains the formation of SrAHx (Fig.5(b)) and the absence of gibbsite. It is obvious that the intensity of effect related to the formation of SrAH<sup>x</sup> increases with the amount of gypsum added to SrAC. The addition of gypsum changes the main hydration products at least during initial stages of hydration.

The diffusion barrier formed on the surface of strontium aluminate phase leads to a broad hydration effect where the time and temperature related to the maximum of released heat decreases with increasing content of gypsum. Increasing amount of added gypsum leads increased formation of SrAHx and ettringite while the amounts of formed Sr3AH6 and gibbsite decrease. Fig.3(b) shows that the heat released during the first 50 hours of hydration of strontium aluminate cement decreases exponentially with increasing content of gypsum. This behavior can provide the explanation for the hydration of the blend of strontium aluminate cement with SrAC (please see the discussion related to Fig.7 in Chapter 7).

The hydration of the samples after the calorimetric assessment was stopped by repeated washing of ground samples in acetone in order to remove water followed by the evaporation of solvent and drying upon silica-gel in desiccator. The storage of sample in the desiccator also restricts the course of carbonation [409,744,745].

to the dehydration of gel phase and of main product of hydration (Sr3AH3) decreases (Chapter

**Figure 5.** X-ray analysis of cement stone after 50 h of hydration: sample with 2 % (a) and 5 % (b) of gypsum.

**Figure 6.** Infrared spectrum of hydrated cement stone of sample containing 2 % (a) and 5 % (b) of gypsum.

Infrared spectrum (Fig.6) shows the features typical for hydrated strontium aluminate cement (tri-strontium aluminate hexahydrate and strontium carbonate) which are described in

The results of X-ray diffraction analysis of cement stone after 50 hours of hydration at the temperature of 25°C is shown in Fig.5. Higher intensity of diffraction lines of unreacted strontium aluminate in sample (b) compared to (a) indicates the retardation effect of gypsum on the hydration of strontium aluminate cement. Moreover, both samples still showed the diffraction features of tri-strontium aluminate after 50 h of hydration process, while this compound was not recognized in pure sample of strontium aluminate cement without gypsum

ion formed during the hydration is used to form ettringite

Expansive Strontium Aluminate Cements 229


that slows down the hydration of strontium aluminate cement.

5.1). That indicates that [Al(OH)4]

(Fig.6). Both samples are slightly carbonated.

The composition of hydrated cement stone was further investigated by thermal analysis, Xray diffraction analysis, infrared spectroscopy and scanning electron microscopy in order to explain the effect of gypsum on the course of hydration and to evaluate the changes in the equilibrium composition of hydration products of SrAC.

**Figure 4.** DTAof hardened cement stone prepared from the mixture containing 2 % (a) and 5 % (b) of gypsum.

The TG-DTA results are shown in Fig.4. They reveal higher weight change in the first step of thermogravimetric curve for the sample with 5 % of gypsum. The intensity of the effect related to the dehydration of gel phase and of main product of hydration (Sr3AH3) decreases (Chapter 5.1). That indicates that [Al(OH)4] ion formed during the hydration is used to form ettringite that slows down the hydration of strontium aluminate cement.

of solvent and drying upon silica-gel in desiccator. The storage of sample in the desiccator also

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

The composition of hydrated cement stone was further investigated by thermal analysis, Xray diffraction analysis, infrared spectroscopy and scanning electron microscopy in order to explain the effect of gypsum on the course of hydration and to evaluate the changes in the

**Figure 4.** DTAof hardened cement stone prepared from the mixture containing 2 % (a) and 5 % (b) of gypsum.

The TG-DTA results are shown in Fig.4. They reveal higher weight change in the first step of thermogravimetric curve for the sample with 5 % of gypsum. The intensity of the effect related

restricts the course of carbonation [409,744,745].

equilibrium composition of hydration products of SrAC.

**Figure 5.** X-ray analysis of cement stone after 50 h of hydration: sample with 2 % (a) and 5 % (b) of gypsum.

The results of X-ray diffraction analysis of cement stone after 50 hours of hydration at the temperature of 25°C is shown in Fig.5. Higher intensity of diffraction lines of unreacted strontium aluminate in sample (b) compared to (a) indicates the retardation effect of gypsum on the hydration of strontium aluminate cement. Moreover, both samples still showed the diffraction features of tri-strontium aluminate after 50 h of hydration process, while this compound was not recognized in pure sample of strontium aluminate cement without gypsum (Fig.6). Both samples are slightly carbonated.

**Figure 6.** Infrared spectrum of hydrated cement stone of sample containing 2 % (a) and 5 % (b) of gypsum.

Infrared spectrum (Fig.6) shows the features typical for hydrated strontium aluminate cement (tri-strontium aluminate hexahydrate and strontium carbonate) which are described in Chapter 5.1. Fundamental stretching modes of SO4 2- ions are located on the left shoulder of multiple band below 1300 cm-1 (1250 – 950 and 700 – 580 cm-1).

X-ray diffraction analysis does not provide the evidence about the formation of ettringite in the sample prepared from the mixture of strontium aluminate cement with 2 % of gypsum. On the other hand, SEM images (Fig.7) show rare and small clumps of needle-like crystals which occur between the plates of the main product of hydration, therefore a very small

Expansive Strontium Aluminate Cements 231

amount of strontium sulfate or ettringite is probably formed.

**Figure 8.** Strontium aluminate cement with 5 wt.% of gypsum.

The bands of CO3 2-anion with Cs symmetry occur in the spectra of both samples due to the carbonation to SrCO3: the band of antisymmetric stretching *ν*<sup>3</sup> (1453 cm-1), symmetric stretching *ν*1 (1071cm-1) 1 , out of plane bending *ν*2 (857 cm-1), in plane *ν*<sup>4</sup> (699 cm-1) and the combination band *ν*1+*ν*4 at 1771 cm-1 [170,746-748]. The bands of hydration products are described in Chapter 5.1 (please see the discussion to Fig.4).

**Figure 7.** Strontium aluminate cement with 2 wt.% of gypsum.

<sup>1</sup> Vibration mode is IR active due to the aragonite structure of SrCO3 (Please see Chapter 2).

X-ray diffraction analysis does not provide the evidence about the formation of ettringite in the sample prepared from the mixture of strontium aluminate cement with 2 % of gypsum. On the other hand, SEM images (Fig.7) show rare and small clumps of needle-like crystals which occur between the plates of the main product of hydration, therefore a very small amount of strontium sulfate or ettringite is probably formed.

Chapter 5.1. Fundamental stretching modes of SO4

**Figure 7.** Strontium aluminate cement with 2 wt.% of gypsum.

1 Vibration mode is IR active due to the aragonite structure of SrCO3 (Please see Chapter 2).

The bands of CO3

1

5.1 (please see the discussion to Fig.4).

*ν*1 (1071cm-1)

multiple band below 1300 cm-1 (1250 – 950 and 700 – 580 cm-1).

2- ions are located on the left shoulder of

2-anion with Cs symmetry occur in the spectra of both samples due to the

, out of plane bending *ν*2 (857 cm-1), in plane *ν*<sup>4</sup> (699 cm-1) and the combination

carbonation to SrCO3: the band of antisymmetric stretching *ν*<sup>3</sup> (1453 cm-1), symmetric stretching

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

band *ν*1+*ν*4 at 1771 cm-1 [170,746-748]. The bands of hydration products are described in Chapter

**Figure 8.** Strontium aluminate cement with 5 wt.% of gypsum.

The sample prepared with 5 % of gypsum shows the diffraction lines of both, strontium sulfate and ettringite. Higher content of sulfate bearing phase, compared to cement with 2 % of gypsum, is also well visible from scanning electron microscopy images, where large amount of needle-like crystals grow on the surface of clusters consisting of partially hydrated grains of strontium aluminate cement.

**Chapter 9**

**Advanced Techniques for Preparation of Strontium**

The solid state synthesis described in Chapter 4.3 is the traditional way for the synthesis of strontium aluminate phase. Small amounts of fluxing agents (Fig.1), such as B2O3 (H3BO3) or

There are also advanced techniques of the synthesis of strontium aluminate phases including the sol-gel process, the precipitation from solution, the pyrolysis and combustion techniques, the microwave synthesis, the mechano-chemical route, etc. It must be pointed, that the techniques mentioned in this chapter are the sophisticated synthesis routes which are tailored mainly for the processing of nano-scale powders, doped with rare earth elements (REE ions such as Ce4+, Eu2+, Dy3+…), in order to prepare the precursors for strontium aluminate based phosphors for long-persistent luminescence materials, electronic and structural applications (Chapter 10.1). Therefore, the suitability of the techniques mentioned above for the synthesis of strontium aluminate clinker is limited by their complexity, time-consuming preparation

**The Sol-gel process** is a method based on the turn of the "*free dispersion system*" with particles of colloidal size (**sol**) into "*bound dispersion system"* (**gel**) with the particles cross-linked into the three-dimensional network. In principle there are two ways of the formation of gel:

**1.** Linking of growing particles into chains, connection of chains into branched chains and then into domains of three-dimensional networks (**microgel**). As the regions of formed

> © 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.

**Aluminate Phases**

LiF can be used [749,810].

**1. Techniques using powder precursors**

**Figure 1.** Flow chart of the preparation of REE doped SrAl2O4 phosphor powders [749].

process, labor productivity and by the cost of applied chemicals.

The comparison of Fig.7 and Fig.8 as well as the results of calorimetry (Fig.3) indicate the change in hydration mechanism from the crystallization to the diffusion. There is a way to prepare expansive strontium aluminate cement, but the control of setting time of the paste is necessary in order to reach required rigidity, which enables to handle the expansion stress. The positive effect was observed for calcined clay (Chapter 7.2).
