DIFFUSION-LIMITED AND REACTION-LIMITED AGGREGATION OF AQUEOUS SILICATE SOLUTIONS

Two different aggregation processes (diffusion- and reaction-limited cluster-cluster aggregation) are observed in aqueous silicate solutions which are acidified. The type of aggregation depends on total silica concentration of the starting silicate water glass and the presence of oligomeric silicate anions in the starting silicate solution. If Q4(3)-silicate anions are present in solution and the water glass is low in concentration (almost-equal-to 0.2 wt%), fast, diffusion-limited cluster-cluster aggregation (DL-CCA) is observed, whereas if no oligomers are present, slow, reaction limited cluster-cluster aggregation (RL-CCA) is observed. Diffusion-limited cluster-cluster aggregation is induced by fast reaction of reactive oligomers with monomers and other oligomers. The low silica concentration induces diffusion to become rate determining in the aggregation process. The fractal dimensionality of gels formed by RL-CCA does not depend on total silica concentration, whereas the size of the aggregates strongly depends on total silica concentration.