The structural properties of soda-silicate sols and of the lime-soda-silicate gel formation

Alkaline silicate solutions were prepared with varying. SiO2-Na2O ratios, R m = [SiO2]/[Na2O] = 2 or 3, with a constant silica concentration equal to 1 .5 M. The structure and the number fraction of different silicate molecular species present in the solutions were classically determined for molar rat io Rm = 2 using Si-29 nuclear magnetic resonance (Si-29 NMR). In the case o f the sample with Rm = 3, for which no NMR data was available, we used smal l-angle X-ray scattering (SAXS) experiments to get this information. The SA XS model was previously tested with Rm = 2 silicate solution. When Ca++ is added to alkaline silicate solutions, a gel is obtained under well-defined conditions. The gel formation was studied by SAXS, small-angle neutron scat tering (SANS) and elastic light scattering (ELS). For gelling solutions, du ring aggregation of the initial particles in the sol and for the resulting gels, the scattering data were fitted using a fractal model for the structu re including a correlation cut-off distance and a contribution of remaining individual sol particles. Experimental and calculated scattering curves ar e in agreement for all studied samples. The results obtained may help to un derstand the mechanism of formation of silico-calco-alkaline gels in concre te chemistry. (C) 1999 Elsevier Science B.V. All rights reserved.