Monocalcium aluminate

Sugama and Allan [5] used calcium aluminates (tricalcium aluminate, C -A, monocalcium aluminate, C A, or calcium dialuminate, C A2) as the cation donors and reacted them with an ammonium polyphosphate fertilizer solution and formed quicksetting cements. The purpose of this study was to develop cements that are not affected by the CO2 environment and are useful as downhole cements in geothermal wells (see Chapter 15). The composition of the fertilizer was 11.1 wt% N as ammonia, 37.0 wt% P2O3, 50.79 wt% water, and the rest trace elements. Differential scanning calorimetry (DSC) showed that the reaction rates of the three minerals are in the decreasing order ... 

Figure 5.32. A. Al NMR spectra of (top) unhydrated alumina cement (principally monocalcium aluminate), and (bottom) product of full hydration with demineralised water at a cement water mass ratio of 1 1. Asterisks indicate spinning side bands. B. Change in the percentage of four-coordinated Al in alumina cement during hydration, as a function of time estimated by Al MAS NMR. Open symbols (a) hydration with demineralised water. Filled symbols (b) hydration with 0.5 mass percent Li2C03 solution. After Luong et al. (1989), by permission of the American Ceramic Society.

The mechanism of CAand CJ2A2 hydration consists in the congruent dissolution, nucleation of hydrates and further crystals growth. The hydration process can be easily followed taking as an example the reaction of monocalcium aluminate with water, without admixture of CJ2A2 or other phases with C/A>1. Due to the low nucleation rate in this condition, in the liquid phase a high concentration of calcium and aluminium ions is maintained for a long time (Fig. 9.2). 

The calcium aluminate cement (CAC) was developed as a solution to the sulphates attack in OPC, and was patented in France in 1908. The CACs are cements consisting predominantly of hydraulic calcium aluminates mainly monocalcium aluminate, CA, but also contains minor amounts of Ci24y, C 2 and C AF. 

The volume of calcium aluminate hydrates formed in the hydration of CA is smaller than the sum of the volumes of the original CA and the water participating in the reaction. This chemical shrinkage for the hydration of monocalcium aluminate amounts to 16 vol.% in the formation of CAHj and to 25 vol.% if CjAHg and AHj are formed as products of hydration. 

Monocalcium aluminate (CA) is the main phase of calcimn aluminate cements, and the one mainly responsible for their properties. This phase is present in calcium aluminate cements in amounts from 40% upwards. It may incorporated variable, but usually small, amounts of FejOj and Si02, especially in cements with lower AI2O3 contents (Rayment and Majumdar, 1994). is also present in most commercial cements. Calcium... 

The hydration process in OPC+CAC blends may also be accelerated by adding small amoimts of a lithium salt, such as Li2C03, to the mix. The hthium salt, which accelerates the hydration of the monocalcium aluminate phase very effeetively, is also highly effective in mixes in which the calcium aluminate cement is the minor component. 

Monocalcium aluminate (CA) hydrates rather rapidly initially, but still a significant fraction of it reacts shortly after setting that is, at a time that is favorable for the expansive process. The reaction ceases within a few days. 

Monocalcium aluminate, a major component of high alumina cement, hydrates to metastable products CAHjo, C4AHj3, and C2AHg which eventually convert to the stable cubic aluminum hydrate, CjAH. In the presence of 2-4% SMF, SNF, or modified lignosulfonate, the DTA results have shown that the degree of conversion of CAHjo and C2AHg phases to the cubic phase is marginally retarded. 

Ramachandran, V. S., and Feldman, R. F., Hydration Characteristics of Monocalcium Aluminate at a Low Water/Solid Ratio, Cement Conor. Res., 3 729-750 (1973)... 

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