A thermodynamic model of dissolution and precipitation of calcium silicate hydrates

A thermodynamic incongruent dissolution/precipitation model of calcium silicate hydrate (C-S-H) is proposed, assuming a binary nonideal solid solution of Ca(OH)2 and SiO2. Using this model, both dissolution and precipitation of the C-S-H phase, with a continuous change in the Ca / Si ratio of the solid phase, can be predicted. The notable features of the model are its good continuity and simplicity so that calculation can be easily compiled in a calculation code. A series of experiments were carried out. C-S-H precipitates were prepared using two techniques: precipitation by contacting Ca(OH)2 solution with C-S-H gel and hydrolysis in a mixture of Ca and Si solutions. The equilibria in these experiments were predicted well by the proposed model. A calculation using the model also predicted well the dissolution of ordinary Portland cement hydrate with water exchange.