AWWA WQTC63967

The most important ingredients of cement powder are dicalcium and tricalcium silicate. When the cement powder is mixed with water, these ingredients hydrolyze to form a calcium silicate hydrate (C-S-H) and calcium hydroxide. An example of such a reaction is the formation of 1.4 nm tobermorite through the hydrolysis of dicalcium silicate. As cement paste hydrates and hardens, it forms a heterogeneous structure of amorphous C-S-H with saturated calcium hydroxide in its pores. The C-S-H in the hydrated cement can be viewed as a metastable form of calcium silicate hydrate. Although more than 30 crystalline C-S-H phases are known, adequate thermodynamic data are not available on these. In this discussion simple calcium silicate, or Wollastonite (CaSiO₃), is proposed as a model for which data are available and which might be used to develop a saturation index. The thermodynamic properties of Wollastonite are known and can be found in compilations like those in Stumm and Morgan (1996). From these it is possible to determine the solubility product of Wollastonite as a function of temperature (see the appendix of this discussion for a summary of this derivation). At 25ºC the solubility product, Kso, is 1013.05. Using that solubility product, a saturation index is proposed for judging the solubility of cement-based surfaces exposed to water. Includes 13 references, tables, figures, appendix.