Calcium aluminate cement composition

Mineral composition of calcium aluminate cements can be assessed based on the three component Ca0-Al203-Si02 system and the following phases can occur CA, CA2, C,2A, C2S, C2AS (Fig. 9.1) [5]. However, the mineral composition is variable and depends primarily on the chemical composition, in which iron plays the main role and the atmosphere oxidizing (rotary kilns) or reducing one (blast furnaces, L furnaces), thus the Fe +/Fe + ratio. It should be also remembered that the calcium aluminate cements composition is rather closer to the C-A-F system. 

Due to the absence of Ca(OH)2 in the calcium aluminate cement paste a higher durabiUty of glass fibers was expected in fiber reinforced composites. Majumdar [26] proved that the calcium aluminate cement composites had much better mechanical properties, but only the case of alkali resistant glass. 

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. 

CjAHg is the only stable ternary phase in the CaO-AUOj H,0 system at ordinary temperatures, but neither it nor any other hydrogarnet phase is formed as a major hydration product of typical, modern Portland cements under those conditions. Minor quantities are formed from some composite cements and, in a poorly crystalline state, from Portland cements. Larger quantities were given by some older Portland cements, and are also among the normal hydration products of autoclaved cement-based materials. CjAHg is formed in the conversion reaction of hydrated calcium aluminate cements (Section 10.1). 

Table 10.1 Typical compositions of calcium aluminate cements (weight percentages)...

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... 

Deterioration may occur through a variety of chemical or physical processes. The rest of this chapter deals with these, with emphasis on chemical processes in which the cement paste is involved. For convenience, the various forms of attack are considered separately, but examination of deteriorated concretes often shows that more than one form of attack has been operative. This is often because one form of attack renders the concrete more susceptible to damage by another. Portland and composite cement concretes are considered calcium aluminate cements were discussed in Section 10.1.11. 

Aluminate compositions include calcium aluminate cements, which have high chemical resistance, especially to sulfate, and is also used in refractory applications where ordinary Portland cements would be unsuitable. These same cements are used in bioceramic applications. The bioceramic applications reflect both the high mechanical strength of the calcium aluminate cements and also the biocompatibility of Ca-bearing phases, which bond well with, for example, bone. 

Ellis, H. Fire barrier coating composition containing magnesium oxychlorides and high alumina calcium aluminate cements or magnesium oxysulphate, US Patent 4,572,862. 

Generally, concrete used for water treatment plant pipes, water tanks, and filters is made using (ordinary) Portland cement, whereas cement-mortar linings can be made of diflerent types of cements, such as Portland cements, blast furnace cement or the non-Portland calcium aluminate cement. Each type of cement has a typical composition. Table 4.14 gives the typical composition of the primary cements used for manufacturing pipe. 

In calcium aluminate clinkers the lower or higher Cj2A, amounts are present, which causes set acceleration of cement paste. Sometimes in the sintered clinkers even the CjA phase can be present, due to the local inhomogeneity of the raw mixture, in which the larger limestone grains can occur. Calleja [16] proposed the simplified formulae for the calculation of calcium aluminate cement phase composition. 

There is a substantial difference between the phase composition of Portland and calcium aluminate cement. In the first one the phase composition does not change significantly at the variable content of minor components. 

A important experimental support of in situ hypothesis was presented by Cottin [76]. The mixture of calcium aluminate cement and gypsum is hydrating with too low water content. The change of volume calculated from the phase composition of the paste corresponds very well to the measured value. 

Table 10.2 summarizes the composition and main characteristics of a series of commercial calcium aluminate cements that roughly represent the spectrum of products currently available. Both their oxide composition and their phase composition vary over a wide range, affecting the properties of the product. Calcium aluminate cements with lower AljOj contents are those most widely used, whereas high-Al203 products are reserved for special applications. 

Table 10.2 Composition and properties of commercial calcium aluminate cements.

In the later stages the hydration progresses similarly to that of pure Portland cement in OPC-rich mixes and also to that of pure calcium aluminate cement in CAC-rich mixes. In mixes of intermediate composition stratlingite (gehlenite hydrate, C2SAHg) may also be formed. 

If OPC+CAC mixes ate used as fast-setting cements, compositions in which Portland cement is the main constituent ate preferred. Ratios of 75-90% of Portland and 10-25% of calcium aluminate cement may be considered typical. 

High alumina cements (also called calcium aluminate cements) are obtained from bauxite AI2O3 fused together with limestone at a temperature of 1,600 °C into clinker, which is then ground into powder. The chemical composition of high alumina cement is given below after Neville (1997) and Bukowski (1963) ... 

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