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RESEARCH PRODUCT

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

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Abstract Alkaline silicate solutions were prepared with varying SiO 2 –Na 2 O ratios, Rm=[SiO 2 ]/[Na 2 O]=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 ratio Rm=2 using 29 Si nuclear magnetic resonance ( 29 Si NMR). In the case of the sample with Rm=3, for which no NMR data was available, we used small-angle X-ray scattering (SAXS) experiments to get this information. The SAXS 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 scattering (SANS) and elastic light scattering (ELS). For gelling solutions, during aggregation of the initial particles in the sol and for the resulting gels, the scattering data were fitted using a fractal model for the structure including a correlation cut-off distance and a contribution of remaining individual sol particles. Experimental and calculated scattering curves are in agreement for all studied samples. The results obtained may help to understand the mechanism of formation of silico-calco-alkaline gels in concrete chemistry.