Reaction kinetic studies of Ca3SiO5 formation from sol-gel-derived precursors were carried out in situ using dynamic X-ray diffraction. Infrared spectra data, TEM experimental results, and X-ray diffraction patterns, all confirmed that CaO and Ca2SiO4 are the only compounds formed from a stoichiometric gel prior to Ca3SiO5 formation (<1250 degrees C). The kinetic data for Ca3SiO5 formation are consistent with an Avrami nucleation and growth mechanism but with a sudden drop in the time exponent at higher conversion. The sharp drop of the exponent is attributed to the impingement of grain growth in the longitudinal direction and this is confirmed by SEM observations of the fully converted gels. The presence of 10% steam did not change the mechanism nor the activation energy of the reaction but significantly enhanced the nucleation and grain growth rate at both stages of the reaction.