Sorbents prepared by hydration of fly ash and Ca(OH)(2) are widely used in flue gas desulfurization process. In this work, kinetic data for this hydration reaction in the conditions established ill previous works were obtained by determining the conversion of Ca(OH)2 with the time. The purpose was to find the kinetic model for this pozzolanic reaction and to include the influence of the preparation variables. According to bibliography, the dissolution of silica from the fly ash in the presence of hydroxyl ions from Ca(OH)(2) was considered to be the rate-limiting step, so a liquid-solid reaction takes place. Results showed that the reaction rate was not affected by the stirring rate. Results obtained at different reaction conditions fit rather well (standard deviation equal or lower than 8%) to the equation corresponding to the shrinking core model with diffusion control. Liquid to solid ratio showed no influence on the reaction rate. Temperature was fitted to the Arrhenius equation, with activation energy of 24 Kcal./mol. A linear fit of the fly ash/Ca(OH)(2) ratio in the mathematical expression and their parameters were also found.