The products of hydration of C*l3*sS have been examined by vibrational infrared spectroscopy. A conclusion has been drawn from these investigations, that the process of hydration of C*l3*sS occurs in a well-ordered manner. As the diffusion of calcium ions and protons increases, apart from calcium hydroxide the orthosilicate hydrates more and more lean in calcium are formed, so that appears a phase, spectroscpically corresponding with hillebrandite.