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A model for dissolution of CaO-SiO2-H2O (C-S-H) gel is proposed by considering a Margules type of non-ideal mixture of binary solid solutions at Ca/Si (C/S)<1. The Guggenheim and Prausnitz equations are used to represent the activities of model solids as a function of mole fractions. The Gibbs-Duhem equation, together with the activities of model solids, is applied to express the conditional solubility products of model solids as a function of C/S ratio. The determination of Guggenheim’s empirical parameters is performed with the geochemical code PHREEQE on experimental data. The dissolution model is developed by considering the effect of disorder in the structure of C-S-H gel at C/S<1. The solubility results predicted by the proposed model are comparable with experimental data. The study predicts some structural information of C-S-H gel at C/S<1.

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We have studied the interaction phenomena for the substitutions of Al(OH) − 4 and SO 2− 4 with both paired and bridging Si tetrahedra likely to be occurred in C-S-H gel coexisting with ettringite system. Gaussian 94 program has been used to obtain the total energy of the molecule in the optimized structure employing HF/3-21G * level of theory. It has been investigated that the net charge on the substitution site causes the modification of the original structure during the substitution. The substitutions of Al(OH)