Gelation of strongly basic silico-alkaline solutions was promoted by appropriate additions of calcium ions. The structure of the aggregates formed in the precursor sols and the resulting gels were studied, within a wide length scale, using small-angle X-ray, small-angle neutron, and elastic light scattering. The study of the kinetics of aggregation was performed in situ. The experimental results demonstrate that gels are composed of aggregates exhibiting a fractal structure, large particles formed in the solutions just after calcium addition and, in some cases, small primary particles remaining in the solution phase. The structural features of the gels are strongly dependent on the concentration of calcium ions. Reaction limited aggregation and diffusion-limited aggregation of primary silicate species are the predominant mechanisms of aggregation and gel formation in solutions with low and high calcium concentration, respectively.