Blended and Multicomponent Cements Based on Strontium Aluminate

1. Blended cements

The term blended cement (multicomponent cement) is usually used to denote inorganic binder that contains a mineral additions in combination with Portland cement and often also with calcium sulfate. Blended cements usually contain single blending component as an addition, in the case that two or more blending component are applied, the term multicomponent cement is used. A variety of industrial by-products and natural materials quantify as mineral additions, and may be used as constituents of inorganic binders of cementitious systems. These materials may possess their own cementitious properties, or they may be latent hydraulic, pozzolanic, or even non-reactive [12,387,599-601].

1. Hydraulically reactive materials: these materials exhibit the cementing properties when mixed with water, without the presence of any other constituent acting as activator. Some fluidized-bed ashes or industrial slags are the examples of such materials [602-604].

2. Latent hydraulic materials: are also able to react hydraulically, but only in the presence of at least small amounts of suitable activator (Ca(OH)₂, NaOH or CaSO₄). They are usually glassy or amorphous and contain significant amount of CaO, SiO₂ and Al₂O₃. Blast furnace slag (Fig.1(c)) is the most widely used latent hydraulic material [505,506].

3. Pozzolanic materials or pozzolans: when mixed only with water, they do not exhibit cementing properties. These materials can react with calcium hydroxide in the presence of water at ambient temperatures, to form calcium silicate/aluminate hydrates. Variety of materials of natural or artificial origin, such as fly ash, microsilica (Fig.1(a)), calcined clays (b) and shales, diatomaceous earth, etc.have these properties [3,607-610].

4. Non-reactive constituents of inorganic cements: do not react chemically, but modify the rheology of fresh paste and some properties of hardened material.

Figure 1.

SEM image of microsilica (a), calcined kaolin (b) and blast furnace slag (c).

ASTM C2 19 defines blended hydraulic cement as hydraulic cement consisting of two or more inorganic constituents (at least one of which is not Portland cement or Portland cement clinker) which separately or in combination contribute to the strength gaining properties of cement (made with or without other blendings constituents, processing additions and functional additionsadded during grinding) and are added by intergrinding or blending.

The types of blended cements covered by this specification are divided as follows [611]:

Portland blast-furnace slag cement;

Portland-pozzolan cement;

Slag cement;

Pozzolan-modified Portland cement;

Slag modified Portland cement.

1.1. Blends of strontium aluminate and aluminate cement

The effect of calcium aluminate onto the hydration of strontium aluminate was investigated by the calorimetric experiment (Fig.2) using the blend of both constituents. Isothermal calorimetric measurement shows that the main hydration effect of calcium aluminate cement

Information about the hydration of AC cement can be found in Chapter 5 and 6.

is delayed in time, the peak height decreases while the peak width increases with increasing content of strontium aluminate in the mixture. The effect of fast evolution of heat after mixing strontium aluminate cement with water (Chapter 5.2) increases with the content of it the sample. After the first 95 hours of hydration the hydration curves of all samples drop down to the baseline, but the cements still produces the heat flow of 0.09 (Secar 51), 0.11 (20:1), 0.09 (10:1), 0,20 (5:1), 0.08 J⋅g^-1 (1:1).

Figure 2.

Heat flow (a) and heat of hydration (b) for the blend of strontium aluminate and calcium aluminate cement.

Therefore, increasing amount of SrAC in CAC increases instantaneous heat released immediately after mixing with water and also increases the length of induction period and the width of the main hydration effect.

Hexagonal CAH₁₀ hydrate as the main product of hydration was recognized by powder XRD analysis of the product of hydration (Fig.3). Furthermore, the diffraction lines of the rest of unhydrated calcium aluminate (CaAl₂O₄) and minor constituents of applied AC such as tetragonal gehlenite (Ca₂Al₂SiO₇) and calcium titanium iron oxide (Ca₃TiFe₂O₈), appeared.

Figure 3.

X-ray diffraction analysis of cement stone of SrAC and CAC blend in the mass ratio of 10:1.

The scanning electron microscopy of hydrated cement stone (Fig.4) shows the formation of well crystallized hexagonal product of CAH₁₀ (Table 6.1). The hydrates of acicular crystal character composed of radiating mass of slender, needle-like crystals with the hexagonal base were formed. These crystals are the main product of hydration in multicomponent AC cement as well (Chapter 7.4).

Figure 4.

SEM image of hydrated cement stone of AC and SrA blend in the mass ratio of 10:1.

Figure 5.

Thermal analysis (a) and infrared spectroscopy (b) of hydrated cement stone.

Thermal analysis of hydrated sample (Fig.5(a)) shows huge endothermic peak at 120 °C and smaller endothermic peat at 250 °C. The first effect is caused by the loss of molecular water of CAH₁₀. Since much water is bound very weakly, the water loss starts with drying. The peak intensity and its shape are then affected by initial drying of the specimen as well. The dehydration of crystalline CAH₁₀ takes place in two steps. Firstly, interlaminar water is released and subsequent loss of water leads to the collapse of the structure. In the case that water cannot evaporate easily, the thermal decomposition of monocalcium aluminate hydrate leads to the tricalcium aluminate hexahydrate (C₃AH₆, hydrogarnet) and cubic phase of gibbsite (AH₃):

$$3CA{H}_{10}\to C_{3}A{H}_6+2A{H}_3+18H$$E1

The second small peak is attributed to the dehydration of AH₃, C₃AH₆ [7,12,269,612] and Sr₃AH₆ (please see Chapter 6.5). The dehydration of hydrogarnet leads to the mayenite (dodeca-calcium heptaaluminate, C₁₂A₇) and Ca(OH)₂ [613]:

$$7C_{3}A{H}_6\to C_{12}{A}_7+9CH+33H$$E2

The thermal decomposition of calcium hydroxide takes place within the temperature range from 350 to 450 °C [613]. As the consequence of gradual dehydration of C₁₂A₇ (mayenite contains 1.3 % of water which corresponds to the formula C₁₂A₇H [12,613]), the mass of sample decreases to the temperature of 900 °C. The effect on the right shoulder of DTA and DTG peak that appears near to the temperature of 170 °C indicates the presence of C₂AH₈ in the hydrated sample [483].

Infrared spectrum of hydrated cement stone is shown in Fig.5(b). In general the strong and broad band observed around 3400–3000 cm^-1 corresponds to the OH stretching vibration of adsorbed water and water in formed hydrates. The maximum located at 3505 cm^-1 belongs to the hydroxyl stretching vibration of hexagonal CAH₁₀. Water bending mode is placed at the wavenumber of about 1650 cm^-1. The band ν₃(CO₃^2-) around 1410 cm^-1 indicates the presence of both, calcium and strontium carbonate. The ν₂(CO₃^2-) is very weak, but still recognizable part of the complex region below 1200 cm^-1. These features are located at 878 (CaCO₃) and 855 cm^-1 (SrCO₃). The band at 1008 cm^-1 was assigned to the stretching of Al-O bond.

The low-frequency rotational modes of hydroxyl groups (=Al-OH) of C₂AH₈ (920 and 705 cm^-1) and CAH₁₀ (970 and 795 cm^-1) confirm the presence the both hydrates in the sample which were implied from the results of thermal analysis. The doublet band, that reaches the maximum at the wavenumber of 570 and 525 cm^-1 is placed in the spectral region of characteristic frequency of the Al-O-Al stretching of AlO₆ octahedra [483,614].

1.2. Blends of strontium aluminate and Portland cement

Calorimetric data for the hydration of blend of strontium aluminate and Portland cement is shown in Fig.6.

Figure 6.

Heat flow (a) and heat of hydration (b) for the first 50 h of hydration of the mixture of strontium aluminate cement with Portland cement.

The heat on hydration decreases with increasing content of Portland cement in the blend. The heat flow released after mixing with water decreases more rapidly for the mixtures with higher content of Portland cement. Moreover, the comparison of calorimetric data measured for pure PC with the results of blended cement shows, that there areno measured features related to the original Portland cement.

Figure 7.

Influence of PC on hydration heat released by SrAC during the first 48 h of hydration.

The hydration heat released during the first 48 hours of hydration shows the highest value for strontium aluminate clinker which decreases with decreasing content of strontium aluminate clinker according to the exponential law (Fig.7). After the first 48 hour of hydration allcurves already drop down to the baseline, but the cements still produce heat flows of 0.28 (SrAC), 0.28 (3:1), 0.20 (1:1), 0.18 (1:3) and 0.50 mW⋅g^-1 (PC). Thus the value of heat flow decreases with the content of Portland cement in the blend.

Observed decrease of heat released during the hydration of blends with increasing content of the Portland cement should be explained by the layer of hydration product of PC and ettringite formed on the surface of SrAC grains. Higher content of Portland cement also means increasing amount of gypsum in the sample and hence higher amount of ettringite, which can be formed. That often pronounces the possibility to use strontium aluminate cement as the expansion or shrinkage compensation additive as is discussed in Chapter 8.

After the calorimetric experiment, the hydration was stopped by the same way as was described in Chapter 8.3 in order to carry out the XRD, TG-DTA and IR analyses of hydrated cement stone.The SEM analysis of cement stone after the first 48 h of hydration shows the effect of Portland cement on the morphology of hydrated cement stone. The most significant features are higher amount of formed ettringite in the blends with higher content of Portland cement, i.e. higher amount of gypsum (Fig.10 and Fig.11), while only rare AFt crystals can be found in SEM images of the sample with only 25 % of Portland cement (Fig.8).

Figure 8.

The hydrated cement stone prepared from the mixture of SrAC and PC (3:1).

Figure 9.

X-ray diffraction analysis of the blend of strontium aluminate cement with Portland cement.

Large amount of formed ettringite in the blend with the highest content of Portland cement was also recognized in XRD pattern of hydrated cement stone (Fig.9). It is an interesting fact, that the features of ettringite not only disappear from the diffraction pattern, but are substituted by growing diffraction lines (18.35, 17.43, 15.43, 25.04 and 22.34° on 2Θ scale) of new phase. It could be supposed, that is formed with increasing content of strontium aluminate cement in the blend.

Figure 10.

The hydrated cement stone prepared from the mixture of SrAC and PC (1:1).

Figure 11.

The hydrated cement stone prepared from the mixture of SrAC and PC (1:3).

Figure 12.

Thermal analysis (a-c) and infrared spectroscopy (d) of hydrated SrA – PC blend.

Infrared spectrum of hydrated cement stone shows that the intensity of band related to the stretching of OH groups of tri-strontium aluminate hexahydrate decreases with decreasing content of strontium aluminate clinker in the blend. On the contrary the features of hydration products of Portland cement such as the amounts of portlandite, ettringite and CSH gel increase. These results are in agreement with X-ray diffraction analysis (Fig.9), electron microscopy (Figs.8-11) as well as thermal analysis (Fig.12).

Increasing intensity of endothermic effect related to the thermal decomposition of hydrogarnet (Sr₃AH₆, please compare with Fig.5(a) in Chapter 5) at 275 °C and decreasing intensity of the first endothermic peak and its shift to lower temperatures are one of the most expressive features of increasing content of hydration products of PC. The blend of strontium aluminate and Portland cement in the mass ratio of 1:1 shows the highest carbonation as well as portlandite.

The infrared spectrum and thermal analysis show the features of both, calcium carbonate and strontium carbonate. The intensity of DTG peak of calcium (680°C) and strontium carbonate (855 °C) reflects the composition of the blend. The solid state synthesis of strontium aluminate (Eq.43 in Chapter 4) turns the effect on DTA to exothermic, while sole thermal decomposition of calcite is naturally the endothermic process.

2. Multicomponent cements

The addition of strontium aluminate cement into calcium aluminate cement increases the time of hydration period and slows-down the rate of heat release (Chapter 7.1.1). It may be useful in order to avoid possible overheating of large constructions caused by setting calcium aluminate cement. The possibility of the preparation of multicomponent cements with admixtures (Fig.1) of pozzolanic materials (microsilica (MS) and metakaolinite (MK)) and latent hydraulic materials such as blast furnace slag (BS) are discussed in this chapter.

Figure 13.

Heat flow (a) and heat of hydration (b) released from the multicomponent cement during the first 128 h of hydration from the blend of CAC and SrC (1:10) with microsilica (MS), metakaolinite (MK) and blast furnace slag (BS).

Figure 14.

SEM images of hydrated cement stones consisting of CAC+SrA blend (10:1) with 10 % of microsilica (MS), metakaolinite (MK) and blast furnace slag (BS).

Isothermal calorimetry performed with the blends of strontium-, calcium aluminate cement containing the materials mentioned above is shown in Fig.13. The main hydration effect of the mixture SrAC and CAC without mineral admixture (NA) started after 50 hours from mixing with water, the heat flow from hydrated sample reached the maximum after 78 hours and was asymptotically falling to the baseline till the 110th hour of hydration. The heat flow from hydrated sample was 0.11 mW g^-1 after the 115th hour of hydration.

The highest heat flow was measured for microsilica. Microsilica also significantly accelerated the occurrence of the main hydration effect, which began at 7.5 h after mixing with water. The main hydration effect reached the maximum at the time of 15 h and was falling down to the baseline till the 40th hour of hydration. The heat flow from hydrated sample was 0.04 mW g^-1 after the 115th hour of hydration.

Metakaolinite also increased the heat flow release from the sample and the maximum rate of hydration (64 h). Although the main hydration effect started almost at the same temperature as for the mixture without mineral admixture, it was dropping down to the baseline much faster. Total hydration heat was almost the same for all multicomponent cements (Fig.13 (b)), however, metakaolinite accelerated the hydration of the SrAC-CAC blend. The heat flow from hydrated sample was 0.02 mW g^-1 after the 115th hour of hydration.

The main hydration effect of multicomponent cement with blast furnace slag arises 60 hours after mixing with water. The maximum of the heat flow peak was slightly higher, but it was delayed in comparison with the blend without mineral admixture. The peak was dropping to the baseline till the time of 115 hours after mixing with water. The heat flow from hydrated sample was of 0.27 mW g^-1 after the 115th hour of hydration.

The effect of mineral admixture on the hydration of multicomponent cements should be expressed as follows:

• Heat flow peak height: MS >> MK >> BS > NA.

• Length of induction period: BS > NA > MK >> MS.

Electron microscopy of hydrated cement stones shows the formation of CSH₁₀, the shape of which is well developed in the sample containing blast furnace slag (BS) and in the blend of calcium-and strontium aluminate cement without mineral admixture (NA).

3. Effect of substitutions on the properties of strontium aluminate cements

The ionic substitutions in the structure of strontium aluminate as the main hydraulic phase of SrAC can induce some modifications in the structure, reactions and properties. As was mentioned in Chapter 1, Sr can partially replace Ca in numerous metal oxides. Therefore, it is often possible to synthesize single-phase mixed oxides, which change the reactivity with water and the setting behaviour of clinker phases or can be used for the preparation of multicomponent cements (described in Chapter 7.2).

There is a complex distribution of Ca and Sr over available sites in Ca_3xSr_xAl₂O₆ (where 0 ≤ x≤ 3), but this distribution is not present in hydrated materials where only single Ca/Sr site exists. Therefore, the reaction proceeds by the dissolution of oxide and by the precipitation of corresponding hydroxide at crucial concentration. Since the solubilities of Ca and Sr compounds are likely to be different (in general Ca salts are more soluble), the precipitation of Sr hydroxide will commence prior to that of Ca hydroxide. In the other words, the hydration reaction is not a topotactic reaction. Alternatively, since Ca and Sr occupy different positions in the Ca_3xSr_xAl₂O₆ lattice, it is possible that the reaction at Sr-rich phase is favored, preferentially leaching this phase into the solution and favoring the precipitation of Sr-rich hydroxide [8,615].

The investigation of deuterated samples shows that the hydration of mixed calcium-strontium aluminates of the Ca_3xSr_xAl₂O₆ type produces a mixture of isostructural (space group of) cubic hydrogarnet phases. The hydrogarnet structure consists of four asymmetric units with the disordered Ca/Sr cations occupying one site within a distorted cube. These cations are displaced so that there are four short and four somewhat longer M-O distances. Al has approximately octahedral environment. In general, the Al-O distance (and the lattice size) increases slightly as the Sr content increases. The difference is however relatively small, the distance increases from 1.910 to 1.945 Å [8].

Figure 15.

Structure of AlO₆ octahedra and dodecahedra according to Prodjosantoso [8].

The formation of two or three new phases from the initial single-phase oxides, suggests that these new phases may be a miscibility gap in the Ca₃Al₂(O₄H₄)₃-Sr₃Al₂(O₄H₄)₃ series. Once formed, mixed metal hydroxides seem to be stable and there is no suggestion of Ca²⁺/Sr²⁺ions exchange [8,416,615].

Strontium can also replace calcium in the tetra-calcium aluminoferrite (Ca₄Fe₂Al₂O₁₀, C₄AF), which is one of the five major components (~10 %) of Ordinary Portland Cement (OPC) and Calcium Aluminate Cement (CAC). The compound is commonly named according to naturally occurring mineral brownmillerite of similar composition or as ferrite. However, a continuous solid solution Ca₂Al_xFe_x-1O₅ exists, where 0 ≤ ≤ 0.7 and both Al³⁺and Fe³⁺cations occupy the tetrahedral and octahedral sites. Therefore C₄AF is only a point in this series of solid solution for x=0.5. The end member C₂A, where x=1, can be prepared only at increased pressures of 2500 MPa. The series x < 0.7 is not quite isostructural, and the space group changes from to near to x > 0.33 [7,12,616-618].

Ordered vacancy perovskite-related type materials such as brownmillerites can be expressed by a general formula A₂B₂O₅ or A₂BB´X₅ (Fig.16(a)), where B=transition metal cations with octahedral co-ordination and B´=transition metal cations with tetrahedral coordination. The materials adopting the brownmillerite structure consist of a single B cations or of two different B cations, which are commonly disordered over both octahedral and tetrahedral sites or may be ordered according to the preference of transition metal cations for a particular co-ordination environment [619]. The oxygen nonstoichiometry of brownmillerite also changes with the oxygen partial pressure and the temperature [618,620-622].

Figure 16.

The conventional chain-ordered Ibm2 structure (a) and the Pbma supercell (b) of Sr₂Fe₂O₅ [622].

The brownmillerite structure can be explained by removing ½ of the oxygen atoms from the layer of the cubic perovskite-type structure, consisting of octahedra, creating vacancies along the [110] direction. Corner-sharing BO₆ octahedra alternates with layers of B´O₄ tetrahedra arranged in chains parallel to (100) direction. The crystal structure can be described as a sequence of B´–BB´–B–B´–B parallel to (010) direction. Cations A are required for the charge compensation and they occupy voids between BO^6-and B´O^4-polyhedra. Cations A are coordinated by 8 oxygen neighbours forming a distorted bicapped trigonal prism. The presence of ‘channels’ of oxygen vacancies between these chains allows some brownmillerites and their derivatives to display ionic conductivities comparable to the stabilized zirconia systems [618,622,623].

The displacement of tetrahedral B and O ions from their ideal positions upon the formation of brownmillerite from the parent perovskite gives rise to two opposite orientations for the tetrahedral chains, which can be assigned ‘left’ (L) and ‘right’ (R) [621,624].

The symmetry of brownmillerite is thus determined by the distribution of L-and R-chains in the structure, with three commonly recognized configurations [622]:

1. Pure L (or R), resulting in the Ibm2 space group (Fig.16(a));

2. Complete L/R disorder (Icmm);

3. Strict alternation of L and R layers along b direction (Pcmn).

Figure 17.

Cooperative twist yields to left or right-handed chains [624].

The brownmillerite structure type of mixed ionic/electronic conductor Sr₂Fe₂O₅ has been well-established since 1970s [625]. Although the diffraction studies of Sr₂Fe₂O₅ have clearly established a body-centered unit cell, the assignment of either the Icmm or models to describe the tetrahedral chain arrangement has varied among experiments [626]. Electron diffraction (ED) images revealed the evidence for short-range Sr₂Fe₂O₅ superstructures defined by complex ordering patterns of L and R chains (Fig.17) both within and between the tetrahedral layers [627]. Single crystal neutron diffraction revealed a supercell (Fig.16(b)) with a doubled axis (space group), compared to the conventional model containing disordered left-and right-handed tetrahedral chains. Therefore, the model conventionally used to describe the local chain order of Sr₂Fe₂O₅ is incorrect [622].

Transition brownmillerite phases Ca_1-xSr_xFe_2.5+δ where 0.3 ≤ x ≤ 0.7 are studied predominantly due to their promising applications in electrochemical devices at high temperature such as ceramic membranes for oxygen separation, electrodes of solid oxide fuel cells (SOFCs), electro- and photocatalysts, battery electrodes and sensor materials, etc. [620,622,628,629,630]. The transition brownmillerite phases with the composition of La_1-xSr_xCo_0.8Fe_0.2O_3-δ can be prepared under low partial pressures of oxygen, where the value of > 0.7 [629].

The LaACuGaO₅, where A=Ca, Sr, brownmillerite system consists of alternating layers of corner-sharing CuO₆ octahedra and GaO₄ tetrahedra stacked along the long axis, with La and A cations statistically occupying the 10-fold cavities within the structure [619,631,632]. Anion-vacancy ordered ‘triple layer’ structure, related to the brownmillerite structure by the insertion of an additional layer of octahedra has the composition LaCa₂Fe₂GaO₈ [633].

Mn-containing brownmillerites (A₂MnBO_5+δ, where A=Ca, Sr and B=Ga, Al) may be perspective compounds as low-field CMR materials. The system Sr₂MnGaO_5+δ (Fig.18) consists of alternating MnO₂ and GaO_1+δ layers (–MnO₂–AO–GaO–AO–MnO₂–AO–GaO–MnO₂–) [619,634-638].

Figure 18.

Crystal structure of Sr₂MnGaO₅ brownmillerites with Ibm2 (a) and Pnma (b) space symmetry. Cations are drawn as spheres [638].

Perovskite oxide with the general formula of ABO₃ is a kind of frequently encountered structure in inorganic chemistry. Its cubic crystal structure is composed of a three-dimensional framework of corner-sharing BO₆ octahedra. SrFeO_x (SFO), where 2.5 ≤ x ≤ 3.0, were extensively studied as ferromagnet and antiferromagnet and due to their photocatalytic activity [620,639-641].

Brownmillerite-and perovskite-type compounds exhibit also some serious disadvantages including the thermodynamic instability under large oxygen chemical potential gradients, poor thermomechanical properties associated with very high chemical and thermal expansion and the reactivity with CO₂ [642].The relation between the orthorhombic brownmillerite and the cubic perovskite-type structure is given as: a ~ √a_per, b ~ 4a_per, c ~ √a_per, where a_per is the cell dimension of the cubic perovskite [618].

The protonic conduction was discovered for the brownmillerite-type Ba₂In₂O₅ in 1995 by Zhang and Smyth [643] that is related to the formation of Ba₂In₂O₅ H₂O hydrate in a humidified atmosphere. Protons bound to oxygen in hydrate cross-links between oxygen [644-646]. In order to improve the proton conductivity it is necessary to stabilize the hydrate compound at high temperature (573 K). The dehydration temperature increases∼50 K due to the substitution of In by Sc, Lu and Y. Since the substitution of In by M³⁺was not possible for x > 0.3 in Ba₂(In_1-xM_x³⁺)₂O₅ H₂O, further increase in the thermal stability of the hydrate compound by the substitution of In cannot be expected. Tetragonal Ba₂Sc₂O₅ and cubic BaScO_3-δ also form hydrates Ba₂Sc₂O₅⋅0.60H₂O and BaScO_3-δ⋅0.37H₂O, but the dehydration temperatures are even lower than that for Ba₂In₂O₅ [647,648].

3.1. Substitution of Al₂O₃ by Cr₂O₃

Cr₂O₃ and Al₂O₃ are sesquioxides having the same corundum crystal structure, which is approximately hexagonal, involving close-packed oxide ions with Cr³⁺and Al³⁺ions occupying two thirds of available octahedral sites [649,650]. The solid solution ((Al_1-xCr_x)₂O₃) obeying Vegard’s law

Empirical rule that enables to calculate the crystal lattice parameter of solid solution or alloy from the concentration of constituents.

[651] is formed over the entire range of compositions

An immiscibility gap is present below 950°C, where two crystalline phases exist (alumina-rich and chromia-rich solid solutions), but the actual decomposition of homogeneous solid solution into two phases proceeds very slowly [651].

in Al₂O₃ – Cr₂O₃ system. With regard to the positive influence of chromia (Cr₂O₃) on the sintration as well as physical and refractory properties of Al₂O₃ ceramics [652-656], the effect of substitution of alumina by chromia in the strontium aluminate cement was investigated.Cr-doped structures have been widely studied as ceramic pigments [649] and catalyzers [657-659].

Cr₂O₃ is not stable and changes to CrO₃ in an atmosphere in which the oxygen partial pressure is relatively high such as air. CrO₃ exhibits high vapor pressure, so it is difficult to obtain a dense sintered body of Cr₂O₃ in an air atmosphere. In order to obtain a dense sintered body of Cr₂O₃, the sintering should be performed in an atmosphere with low oxygen partial pressure in which Cr₂O₃ remains stable [653].

Figure 19.

Thermal analysis of clinker prepared with 5 % (a) and 10 % Cr₂O₃ (b).

The toxicity and the transport behaviour of Cr depend strongly on its valence. The most stable oxidation states in the environment are hexavalent and trivalent. Cr(VI) exists primarily in the form of HCrO₄^-(bichromate) and CrO₄^2-(chromate), which are strong oxidants and can cause kidney tubule necrosis and, by inhalation, lung cancer. Cr(VI) compounds are typically soluble in groundwater, and thus mobile and bioaccessible. Where the intermediates to low redox potentials exist, Cr(VI) can be reduced to Cr(III) which usually forms insoluble oxides and oxyhydroxides being less bioaccessible [660].

Thermal analysis of raw materials with the substitution of 5 and 10 % Al₂O₃ by Cr₂O₃ (Fig.19) shows increased intensity and peak temperature (from 735 to 785 °C) of endothermic process, which is probably related to the formation of chromia spinel phase. To compare with raw material without chromia (Fig.23 in Chapter 4), there is sharp transformation effect of α-SrCO₃ to β-SrCO₃ (please compare with Fig.12 in Chapter 4). It is possible to use the thermolysis of mixed alums in order to prepare mesoporous chromia-alumina [661] as pure and high reactive precursor for the synthesis. That may lead to the assumption that the addition of Cr₂O₃ into the raw meal suppresses the decomposition of strontium carbonate and leads to less reactive clinker containing the sintering additive (similarly to the effect described in Chapter 6.1.4).

Figure 20.

Scanning electron microscopy of clinker prepared with 5 % (a) and 10 % Cr₂O₃ (b).

The addition of Cr₂O₃ in to the raw meal leads to the formation of tetragonal strontium oxide chromate (3SrO⋅3Al₂O₃⋅SrCrO₄) [662] and strontium hexaaluminate causing the detriment of strontium aluminate as the main clinker phase (Fig.21). This endothermic process should be described by the following reaction:

$$\begin{array}{l}8SrC{O}_3+6A{l}_2{O}_3+C{r}_2{O}_3+3/2O_2(g)\to \\ 2(3SrO\times 3A{l}_2{O}_3\times SrCr{O}_4)+8C{O}_2(g)\end{array}$$E3

The lack of SrO supports the formation of strontium hexaaluminate.

That behaviour also enables to explain the missing effect related to the formation of strontium aluminate, which is formed at higher temperature probably via the diffusion of Al³⁺ions through Cr₂O₃-Al₂O₃ layer of formed solid solution. The opposite model, in which the spinel layer would be formed on Cr₂O₃ particles cannot explain the suppression of the formation of strontium aluminate revealed from the TG-DTA experiment.

3SrO⋅3Al₂O₃⋅SrCrO₄ is a pigment with hexavalent chromium and provides prepared clinker with yellow-greenish color. Since the transformation temperatures and the temperatures of endothermic effect are the same for both samples, increasing content of Cr₂O₃ seems to have no influence on the thermal decomposition of strontium carbonate (after the formation of layer on the surface of SrCO₃ grains the increasing abundance of Cr₂O₃ doesn’t have any effect).

Figure 21.

XRD analysis of strontium aluminate clinker doped by chromia.

The raw meal was treated to the temperature of 1500 °C under static air and ground after cooling. Scanning electron microscopy of ground clinker shows the formation of globular aggregates of rounded particles with the surface showing intersections of spherical shapes, which are better developed in the sample prepared with 10 % of Cr₂O₃ (Fig.20). Ground particles show brittle fracture through the glassy-like phase of clinker particles.

The diffraction lines of strontium oxide chromate ground clinker prepared with 10 % of chromia reaches even higher intensity than strontium aluminate. The amount of formed SrA₆ phase seems to be decreasing with increasing content of Cr₂O₃ in the raw meal. Considerable amount of formed phases without hydraulic activity leads to the conclusion that the raw meal containing chromia should be prepared in slightly reducing atmosphere.

Figure 22.

Heat flow (a) and heat of hydration (b) for the samples doped with chromia.

Figure 23.

XRD analysis of strontium aluminate clinker after hydration.

The influence of the substitution of Al₂O₃ by Cr₂O₃ on the properties of strontium aluminate cement was investigated by isothermal calorimetric measurement of hydration at 25 °C (Fig.22).

Figure 24.

SEM images of hydrated cement stone for the sample prepared with 5% of Cr₂O₃.

The results show that the introduction of Cr₂O₃ into the structure of SrA significantly reduces the hydraulic activity and leads to the appearance of induction period. That behaviour is probably caused by the formation of chromium spinel (please see Chapter 10.12).

Figure 25.

SEM images of hydrated cement stone for the sample prepared with 10 % of Cr₂O₃.

This behaviour results from decreasing content of strontium aluminate in the clinker with increased amount of added chromia. That means that newly formed phase of strontium aluminium oxide chromate has no hydraulic activity. X-ray diffraction analysis of hydrated cement stone supports the conclusion that strontium oxide chromate does not show significant hydraulic activity, if any. The main product of hydration remains tri-strontium aluminate hexahydrate as for the clinker prepared without chromia and the intensity of diffraction lines of strontium oxide chromate is not changed.

Electron microscopy shows that the morphology of hydration product is different from strontium aluminate cement without the addition of Cr₂O₃ into the raw meal (Fig.6 in Chapter 5). Moreover, the external appearance of hydrates changes with the amount of added chromia. Long columnar crystals, typical for the sample prepared with 5 % of Cr₂O₃ (Fig.24) are not present in the hydrated cement stone prepared from ground clinker with 10 % Cr₂O₃ (Fig.25). With increasing content of Cr₂O₃ the size of spheres was also significantly reduced.

3.2. Substitution of Al₂O₃ by Fe₂O₃, B₂O₃ and Y₂O₃

Strontium aluminate cements with the substitution of 5 % of Al₂O₃ by Fe₂O₃, B₂O₃, Y₂O₃, V₂O₅, ZrO₂, MnO₂ and ZnO were prepared in order to evaluate the influence of these compounds on the behaviour during hydration. The samples were pelletized and thermally treated to the temperature of 1500 °C (1 hour). The thermal analysis of raw materials upon thermal treatment is shown in Fig.26.

Figure 26.

Thermal analysis of raw meals heated at the rate of 10 °C min^-1: DTA (a) and TG (b).

All investigated samples show almost the same mass loss of 8 %. The major part of mass loss is caused by the thermal decomposition of strontium carbonate that takes place within the temperature range from 700 to 1 000 °C.

Dopant element	Mechanism		R^2	Ea	A
g(α) = kt	Abr.	[kJ⋅mol^-1]	[s^-1]
Fe	-ln (1-α)^3	A3	0.9925	583.06	3.04·10^23
B	1-(2α/3)-(1-α)^2/3	D4	0.9996	379.56	3.79·10^12
V	-ln (1-α)^3	A3	0.9911	664.46	7.07·10^26
Y	A3	0.9941	613.93	1.54·10^24
Zr	A3	0.9976	611.70	5.28·10^24
Mn	1-(1-α)^2/3	F1/3	0.9977	169.61	8.13·10^3
Zn	-ln (1-α)^3	A3	0.9922	613.93	3.74·10^24

Table 1.

Influence of dopants on the thermal decomposition of strontium carbonate.

There is a significant difference compared to the sample prepared with the addition of chromia (Chapter 7.3.1) since the intensive exothermic peak related to the synthesis of strontium aluminate arises for all samples. On the contrary, this peak is smaller for the sample prepared with B₂O₃.

The model fitting method (Chapter 1.6.1) applied to the TG results provides the kinetic data listed in Table 1. The kinetics of thermal decomposition of strontium carbonate is changed by the presence of dopants (please consult with the results in Chapter 4.2). It can be seen that Fe, V, Y, Zr and Mn have similar effect on the mechanism (random nucleation) and kinetics of thermal decomposition of SrCO₃. The addition of B₂O₃ changes the mechanism to the non-steady state 3D diffusion and MnO₂ leads to the process driven by the rate of chemical reaction of one-third order.

Figure 27.

X-ray analysis of strontium aluminate clinker prepared with dopants before hydration.

The admixtures lead to the decreasing intensity of strontium aluminate diffractions (Fig.27(a)) in the clinker after the thermal treatment. Based on this effect, they can be ordered as follows:

$$B_2{O}_3>ZnO>Mn{O}_2>Zr{O}_2>F{e}_2{O}_3>Y_2{O}_3>V_2{O}_5.$$

The shift of the most intensive diffraction lines of SrAl₂O₄([-211], [220] and [211], Fig.27) for the samples prepared with ZnO and Fe₂O₃ indicate the substitution of Zn²⁺↔ Sr²⁺and Fe³⁺↔ Al³⁺, respectively. While, the clinker prepared with ZnO shows well developed SrAH₆ crystals in hydrated cement stone (Fig.29), the clinker doped with Fe₂O₃ exhibits the highest heat released upon hydration (Fig.28).

Observed influence of B₂O₃ on the synthesis and hydration (Fig.28) of strontium aluminate clinker is caused by the ability to incorporate BO₄ unit into the framework of SrAl₂O₄ instead of AlO₄ tetrahedra [663]. (BO)₄ tetrahedra are of more ionic nature than (AlO)₄ due to smaller size (r_B3+< r_Al3+) and higher electronegativity (X_B=2 and X_Al=1.6, Pauling´s scale) of boron [377,664]. B₂O₃ is also known as a glass former, which has low melting point around 460 ◦C. It is regarded as an excellent flux to facilitate the material diffusion (pleasse see Table 1). Thus, B₂O₃ is usually added in the preparations of SrAl₂O₄ in order to reach lower forming temperature [663].

Figure 28.

Hydration of clinker prepared with dopants.

The hydration of prepared samples was investigated by isothermal calorimetry at the temperature of 25 for the first 72 h of hydration (Fig.28). Since there is no induction period and large amount of heat is released immediately after mixing with water, the course of hydration of clinkers doped with ZnO and MnO₂ is similar to that of pure strontium aluminate. The heat flow then asymptotically decreases to the baseline. According to the decreasing heat of wetting, the samples can be ordered as follows:

$$\mathrm{ZnO}>>Mn{O}_2>B_2{O}_3>F{e}_2{O}_3\approx Y_2{O}_3>Zr{O}_2>>V_2{O}_5$$

Other clinkers show the induction periods the lengths of which decrease with following dopants applied:

$$V_2{O}_5>>Zr{O}_2>F{e}_2{O}_3>Y_2{O}_3>>B_2{O}_3.$$

According to the time of maximum of main hydration peak and its height, the samples should be ordered as follows:

• Time: V₂O₅>>ZrO₂ ≈ Y₂O₃ ≈ B₂O₃> Fe₂O₃.

• Intensity: Y₂O₃ ≈ ZrO₂ ≈ Fe₂O₃> V₂O₃> B₂O₃.

Electron microscopy of hydrated cement stone (Fig.29) shows the formation of globular (grape-like) aggregates. But also the formation long columns of layered aggregates (a, b and c), long needle-like crystals (b) and well developed SrAH₆ crystal (g, h) can be observed. The stacked columnar structure of gibbsite aggregate indicates the option to use doped strontium aluminate cement for the preparation of iron-and aluminium based double layered hydroxides (LDH [665-672]).

Figure 30(a) describes typical Raman spectrum of pure monoclinic SrAl₂O₄, where the main peak at 467 cm^-1 belongs to bending of O-Al-O in [AlO₄] tetrahedra [795,1009]. The partial substitutions of Al₂O₃ and SrCO₃, as the reactants in solid state reaction with different metal oxides lead to theformation of new phases as is indicated by the shifts of the main peak of [AlO₄] tetrahedral (Fig.30(b)).

The significant shifting was observed for the structure doped with MnO₂ and should be described as the distortion of monoclinic lattice structure. The experimentally determined influence of various metal oxides in lattice shift is ordered as follows:

$${\mathrm{MnO}}_2>ZnO>B_2{O}_3>Y_2{O}_3>Zr{O}_2>pureSrAphase>V_2{O}_5>CaO.$$

Increasing content of boron in prepared clinker leads to the formation of large hexagonal plates during hydration (Fig.31). According to EDX (Energy Dispersion Spectroscopy), the composition of these crystals corresponds to Sr₂AH_x. The formation of well developed crystals of Sr-analogue of C₂AH₈was observed only for the samples doped by high amount of boron.

Figure 29.

Electron microscopy of hydrated cement stone of clinker prepared with 5 % of B₂O₃(a), Fe₂O₃ (b) Y₂O₃(c), V₂O₃ (d), ZrO₂ (e), MnO (f) and ZnO (g) with well developed crystal of Sr₃AH₆ (h).

Figure 30.

Raman spectra of SrAl₂O₄ phase (a) and doped Sr Al O₄ structures (b) (where=Ca, Zn, Mn and=B, Fe, V, Y, Zr in substitution of 5 wt.%).

Figure 31.

Large hexagonal plates of Sr₂AH_xof hydrated cement stone with 10 % of boron.

3.3. Substitution of SrO by CaO

The substitution of Sr²⁺in the structure of strontium aluminate by Ca²⁺ (1, 5 and 10 %) introduced as CaCO₃ into the raw meal leads to the substitutions in the Sr₃A during thermal processing of clinker. The content of calcium in formed solid solution of (Sr_3-xCa_x)A (where=0.34 (1%), 1.02 (5%) and 1.12 (10%)) increases with the extent of substitution, whereas the content of the (Sr_3-xCa_x)A phase decreases (Fig.32(a)). In other words, the introduction of CaCO₃ into the raw meal suppresses the formation of tristrontium aluminate phase during the thermal processing of clinker.

Figure 32.

XRD analysis of strontium aluminate clinker doped by calcium.

The main diffraction lines of strontium aluminate (-211, 220 and 211) are shifted to the higher position on the °2Θ scale with increasing amount of Sr²⁺cations substituted by Ca²⁺ (Fig.32(b)). Therefore the introduction of smaller ions (Table 1 in Chapter 8) of higher electronegativity leads to the decrease of volume of basic cell unit of strontium aluminate. That leads to lower reactivity with water (Fig.33, please refer to the discussion in Chapter 5.6 related to the size of opened channels in the structure of SrA and their influence on the reactivity with water).

Figure 33.

Heat flow (a) and heat of hydration (b) for the samples doped with calcium.

The hydration of prepared samples was investigated by isothermal calorimetry at the temperature of 25°C for the first 72 h of hydration (Fig.33). The decrease of hydration heat (b) for the sample prepared with 5 and 10 % of calcium can be observed as the consequence of higher thermodynamic stability (please see the discussion in Chapter 1.2.4) as well as decreasing amount of formed (Sr_3-xCa_x)A solid solution.

Figure 34.

Electron microscopy of hydrated cement stone of clinker prepared with 1, 5 and 10% of Sr²⁺substituted by Ca²⁺.

The course of hydration of the sample prepared with 1% of calcium supports the results of X-ray diffraction analysis (Fig.32) indicating that the substitution in the Sr₃A structure proceeds prior to the substitution in the structure of SrA. That can be also explained via the mechanism of formation of SrA phase (please reffer to Fig.24 and the corresponding discussion in Chapter 4). The highest content of formed solid solution of (Sr_2.66Ca_0.34)A

Calcium has approximately two-fold dissolution heat than strontium.

(Fig.32a) which has lower reactivity with water than SrA provides the explanation for the heat flow released from the sample upon hydration (please compare the heat released from SrA and Sr₃A, Fig.12 in Chapter 5).

Figure 35.

WDX analysis of hydrogarnet crystal formed during the hydration of strontium aluminate clinker prepared with 10 % of Ca²⁺.

The maximum of main hydration effect, which appearsat 1.5 h for the sample prepared with 10% of calcium indicates, that the hydration properties are more similar to those of CAC as the content of Ca²⁺ions in formed solid solution increases.

The hydrogarnet phase (Sr_3-xCa_x)AH₆ and AH₃ are recognized as the main products of hydration of prepared clinker. Electron microscopy (Fig.34) shows the formation of well developed crystals of hydrogarnet phase in the samples prepared with 5 and 10% of Sr²⁺ substituted by Ca²⁺. Combining these results with previously described influence of Ca²⁺ ions on the formation of Sr₃A phase, the crucial role of lack of tristrontium aluminate and lower reactivity of Ca-substituted SrA in the nucleation and growth of large hydrogarnet crystals at the beginning of hydration process can be supposed.

Fig.35shows the composition of hydration product of strontium aluminate clinker prepared with 10% of Ca²⁺. The (Sr+Ca):Al atomic ratio ~3:2 corresponds to the formation of Ca-substituted tristrontium aluminate hexahydrate with the composition given by the formula (Sr_2.7Ca_0.3)AH₆. Therefore, Ca²⁺ ions substitute Sr²⁺ in the hydrogarnet phase in the ratio corresponding to the amount of cations substituted in the original clinker (10 %).