The ball-mill hydration of tricalcium silicate at room temperature

The hydration of tricalcium silicate, Ca₃SiO₅, in a small steel ball mill at room temperature was investigated. The stoichiometry of the reaction is represented by 2Ca₃SiO₅ + 6H₂O = Ca₃Si₂O₇·3H₂O + 3Ca(OH)₂ The initial products of the reaction are calcium hydroxide (both crystalline and amorphous) and a calcium silicate hydrate, designated as “hydrate III.” The latter shows marked similarities to the calcium silicate hydrate, tobermorite, produced by other methods of hydration of Ca₃SiO₅. The similarities include CaO/SiO₂ ratio, specific surface area, water content, heat of solution, heat of hydration, and surface energy (enthalpy).Hydrate III differs from tobermorite in that it is unstable; it slowly converts to another calcium silicate hydrate, afwillite. The chemical formula of afwillite is Ca₃Si₂O₇·3H₂O, the same as that of hydrate III and tobermorite, but afwillite has a different crystal structure, and it is a well-crystallized substance. Hydrate III appears to be a form of tobermorite with a structure so disordered that it exhibits only a single X-ray diffraction line, that corresponding to the strongest line of tobermorite.The kinetics of the hydration reaction and of the conversion of hydrate III to afwillite were examined. A possible mechanism for the former was proposed.