The temporal evolution of an amorphous silica, the diatomit used as ch
arge in the cement, in an alcali-reaction at different temperatures wa
s studied in the liquid phase with a ''bilan de masse'' and in the pri
mary and neoformation solid phases by electronic microscopy, X-ray dif
fraction and N.M.R resonance of Si-29 NMR spectroscopy. The diatomit i
s transformed from the beginning of the reaction and the neoformation
solid phase is calcium silicates hydrates with a chain structure showi
ng two kinds of Q(2) entities.