Le mécanisme de l'hydratation du silicate tricalcique

During the 1st few hrs. the hydration of 3CaO.SiO2 (I) is autocatalytic. The mechanism of this acceleration was studied by examg. the effect of a material (amorphous SiO2) which modifies the concns. in the soln. without directly affecting the surface of the I. Amorphous SiO2 accelerates the reaction both in suspensions and in pastes. In suspensions a remarkable development of the concns. in the aq. phase is observed: the concns. of Ca++ and OH- are at 1st lower but then become higher in the presence than in the absence of the amorphous SiO2; that of the silicate ions proceeds in the opposite direction. These data are in agreement with the following mechanism: on the 1st contact with water the surface of the I is covered with a film made up of a hydrate which is metastable in relation to conversion into another hydrate covering the surface to a lesser degree. The conversion of the 1st hydrate into the 2nd is accelerated: (a) by redn. of the [Ca++] and [OH] in the liq. phase; (b) by the presence of nuclei of the 2nd hydrate. The film of the 1st hydrate is not visible by electronic microscopy; externally the 2nd hydrate resembles CaO.SiO2.H2O (I) and is in its turn transferred into a hydrate resembling CaO.SiO2.H2O (II). [on SciFinder (R)]