Calcium aluminate CAS

A shorthand notation is often nsed to designate the oxides (e.g., C for CaO and A for AI2O3 see Table 5.7), and it is also used to designate the compounds formed between the components during heating, such as calcium aluminate (CA) and tricalcium silicate (C3S) ... 

Calcium Aluminates. CAS 065997-16-2], Four in number. Iiave been prepared by high-temperature methods and identified. 3CaO- AI>Oi. at l,535aC. decomposes with partial fusion 5CaO- ALOj. nip 1,455 C, CaO AbOt. tnp 1.590°C. 3CaO- AI Oi. mp 1.720 C. 

Figure 4.14 The wollastonite-calcium aluminate (Ca-Si03 i aAl204) phase diagram showing the intermediate phase gehlenite, Ca2Al2Si07...

A study of the infrared spectra of a number of calcium aluminates (CA, CA2, C12A7) and catalysts and adsorbents based on them, with different S (30-400 m2.g-l) showed that the bands in the range 400-900 cm-l are due to the... 

In the presence of water, superficial pitting corrosion can occur. As shown by Bailey in 1920 [5] and confirmed by Takatani [6], corrosion will cease soon due to the formation of an insoluble surface film of calcium aluminate Ca(A102)2 by the reaction of aluminate ions AIO2 that are present at pH values above 12, with Ca ions originating from the dissociation of Ca(OH)2-... 

Figure 2.22 Hydration kinetics of various blends between calcium aluminate cement and hemihydrate. The numbers given refer to the ratio between calcium aluminate (CA) and hemihydrate (HH). The samples were hydrated at 20°C using a water-solid ratio of 0.40. (From BizzozeroJ., Hydration and dimensional stability of calcium aluminate cement based systems , PhD Thesis no. 6336, Ecole Polytechnique Federale de Lausanne, Switzerland, 2014.)...

Figure 4.1 XRD scans (IO°-45° 20 range) of typical industrially produced cements white Portland cement (WPC), plain portland cement (PC), calcium aluminate cement (CAC) and calcium sulfoaluminate cement (CSA). The diffraction peaks of the main phases are indicated alite (CjS M3), belite (P-CjS), aluminate (CjA), ferrite (C4AF), calcium aluminate (CA), ye elimite (Yee), anhydrite (Anh), gypsum (Gyp), gehlenite (Geh), mayenite (May) and magnetite (Mag).

Reacts with calcium and magnesium hydrides in tetrahydrofuran forming tetrahydro aluminates, Ca(AlH4)2 reacts with hydrides of alkali metals in ether forming aluminum hydride ... 

Several other anhydrous calcium aluminosilicates are known, including grossular or garnet (C3AS3), which is a high-pressure phase, various dehydration products of zeolites, and various products formed metastably by crystallization from melts or glasses. Most are too acid in composition to be of clear relevance to cement chemistry, but some of the devitrification products, especially those with compositions near to CA and structures similar to those of nepheline (Na3KAl4Si40i6) or kalsilite (KAlSiOj (Y4), are of possible interest in relation to the formation of calcium aluminate cements. 

As noted in Section 3.3.1, the decarbonation of calcite is greatly favoured by intimate mixing with quartz or clay minerals. Under these conditions, much CO2 is lost before any free lime can be detected. The formation of C2S as an early product is well established, but the situation with the calcium aluminate phases is more complex, no one phase being dominant as an initial product (W11). The aluminate or aluminosilicate phases most often reported as early or intermediate products in laboratory experiments with pure chemicals or raw meals have been CA, CjjA, and, less frequently, gehlenite (W12,D6-D8,W13,L8,L9,I6,C9,C10,R10). CjA can form by 850°C (L9,C10) but seems usually to be a later product. Ferrite phase forms readily, and is initially of low Al/Fe ratio (D7,R10). 

The C4A and CjA hydrates are most conveniently prepared by adding CaO or saturated CH solution to a supersaturated calcium aluminate solution obtained by shaking CA or white calcium aluminate cement (Section 10.1.1) with water. Such solutions typically contain up to about 1.2gCa01 and 1.9 g AljOj 1 , these concentrations depending on the shaking time, temperature, proportioning and particle size of the starting... 

Mineral additions may be broadly categorized as pozzolanic materials or latent hydraulic cements. Neither type reacts significantly with water at ordinary temperatures in the absence of other substances. Pozzolanic materials are high in Si02 and often also in AI2O3, and low in CaO they are sufficiently reactive that mixtures of them with water and CaO produce C-S-H at ordinary temperatures and thereby act as hydraulic cements. If they contain AI2O3, calcium aluminate or aluminate silicate hydrates are also formed. Because they are low in CaO, this component must be supplied in stoichiometric quantity. In a composite cement, it is provided by the Portland cement through decreased formation of CH and decreased Ca/Si... 

The primary reaction of any pozzolanic material is an attack on the SiOj or AljOj-SiOj framework by OH ions. It may be supposed that the OH ions attach themselves to silicon and other network-forming atoms, with consequent breaking of bonds between the latter and oxygen atoms. After this has occurred several times, the silicate or other oxy anion is detached from the framework. It may either remain in situ or pass into the solution. The charges of those that remain are balanced, partly by H, and partly by metal cations. Since a cement pore solution is essentially one of potassium and sodium hydroxides, the immediate product is likely to be an amorphous material with and Na as the dominant cations, but the more abundant supply of Ca and the lower solubility of C-S-H and hydrated calcium aluminate or silicoaluminate phases will ensure that this is only an intermediate product. Its presence is indicated by the relatively high potassium contents observed in or near to the reacting pfa particles. 

Table 10.1 gives typical chemical compositions of commercially produced calcium aluminate cements. The essential compound in all of them, because it develops the main hydraulic activity and is consequently responsible for the strength development, is monocaicium aluminate, CA. In white calcium aluminate cements, it can occur with various combinations of the other binary phases of the CaO-Al20j system and corundum. The crystal structures of these compounds and the phase equilibria relating to their formation were considered in Chapter 2. In the sintering process by which these cements are made, the reaction conditions are of the utmost importance, as... 

Calcium aluminate cements harden rapidly as soon as the massive precipitation of hydrates begins. This may be attributed to the fact that, unlike those of Portland cement, the major hydration products are crystalline. Relatively high proportions of water are taken up in the hydration reactions, the theoretical w/c ratios needed for complete hydration of CA being 1.14, 0.63 and 0.46 for the formation of CAHj, CjAHg -F AHj and CjAH -F 2AHj, respectively. For this reason, and also because of the rapid heat... 

Pastes inpregnated with PMMA or sulphur are still sufficiently permeable to water that expansion occurs on long exposure (F46). In polymer-impregnated (S108) and MDF (R64) cement pastes, there is evidence of interaction between Ca ions and carboxylate and possibly other groups of the polymer. In MDF pastes made with calcium aluminate cement, the polymer (PVA) was found to inhibit the normal hydration reactions of the cement, but to react with Ca and AH to give an ionically cross-linked polymer and calcium acetate. TEM showed the material to be essentially a dispersion of grains of clinker or hydration products in a continuous polymer matrix. 

Since calcium oxide is more than sparsely soluble and its reaction with phosphoric acid or a soluble phosphate is highly exothermic, researchers have used less soluble salts of calcium to react with the phosphates and form a phosphate ceramic [4-12]. In the acidic medium of the phosphate solutions, the salts of calcium dissolve slowly and release Ca (aq) into the solution, which subsequently reacts with phosphate anions and forms calcium phosphates. The best calcium minerals for forming CBPCs are combination of oxides of calcium and insoluble oxides such as silica or alumina, e.g., calcium silicate (CaSi03) and calcium aluminate (CaAl204), or even a phosphate of calcium such as tetracalcium phosphate (Ca4(P04)2 0). These minerals are reacted with acid phosphate salts to form phosphate cements. 

With the procedure given above, one may also calculate the pH dependence of (Ca (aq)) from dissolution of calcium aluminates. The results of such calculations are summarized in Table 13.1. Figure 13.2 shows the solubility diagram for (Ca (aq)) as a function of pH for both silicates and aluminates drawn from the equations given in Table 13.1. 

The authors [23] demonstrated that a substantial effect could be achieved by adding a mixture of separately activated Al(OH)j and Ca(OH)2, taken at a molar ratio of 1 1, to ordinary cement instead of calcium aluminates. It was also found that the mechanical activation of only AI(OH)j was enough, since Ca(OH)2 exhibits substantial solubility in water. 

Portland cement and high-alumina cements contain, in addition to calcium silicate phases, calcium monoaluminate, CaAl204 (or CA in cement chemist s shorthand, where C = CaO and A = AI2O3). The Al NMR spectra of this compound, in which the Al is exclusively in tetrahedral coordination, and a number of other calcium alu-minates have been determined (Muller et al. 1986), and more recently, using satellite transition spectroscopy (SATRAS) which has allowed the multiple tetrahedral sites in the various calcium aluminates to be distinguished (Skibsted et al. 1993). The NMR parameters for the synthetic aluminates and a number of their hydration products are shown in Table 5.4. 

The calcium aluminates, especially CaAl204 (CA) and Ca3Al20e (C3A) react readily with water, contributing to the hydraulic activity of the cement. The Al in the hydrated phases is exclusively in six-fold coordination, making Al NMR a convenient method for monitoring the progress of hydration of both the pure aluminate phases and alumina cements (Figure 5.32A). This technique has been used to study the... 

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