Monocalcium aluminate hydrate

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 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. 

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)... 

Ettringite cements contain an aluminate donor such as monocalcium almninate, tricalcium aluminate, tetracalcium trialmninate sulfate, or tetracalcium aluminate ferrite, together with calcium sulfate dihydrate and in some instances also calcium hydroxide. In the hydration of such mixes ettringite is formed as the main or sole reaction product. If allowed to react umestricted, the hydrating paste exhibits a significant expansion however, hardened pastes with strengths comparable to those of other cements may be produced if the hydration is allowed to take place under mechanically restricted conditions. This may occur in completely closed steel molds, by which measure undesired expansion of the paste m be effectively prevented (Odler and Yan, 1994). 

Braniski, A., 1965. Infrared spectrophotometric study of hydrated monocalcium, monobarium aluminates, and of calcium, strontium, and barium silicate cements. Zement-Kalk-Gips. 54 449. 

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