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.