Insights into the mechanisms of nucleation and growth of C–S–H on fillers

A complete understanding of the mechanisms upon which a filler acts in a cement-based material, e.g. as a C–S–H nucleation and/or growth-inducing factor, is of high importance. Although various studies report on accelerated cement hydration in the presence of fillers, the reason behind these observations is not completely understood yet. This work contributes to this subject, by providing an experimental evidence on the (electro) chemical aspects of the filler surface modification in the model solution, simulating the pore solution of cement paste. The nature of the various interactions with regard to the affinity of a filler surface towards C–S–H nucleation and growth was discussed in detail in this work with regard to zeta potential measurements of micronized sand and limestone particles in the model solutions. These results are further supported by microscopic observations of morphology and distribution of hydration products on the filler surfaces, together with considerations on thermodynamic principles in view of hydration products formation and distribution. The C–S–H nucleation and growth appeared to be due to the interactions between a filler surface and calcium ions in the pore solution. These interactions were determined by the chemical nature of the filler surface. The interaction mechanisms were found to be governed by relatively weak electrostatic forces in the case of micronized sand. This was reflected by a non-significant adsorption of calcium ions on the filler surface, resulting in non-uniformly distributed and less stable C–S–H nuclei. In contrast, the nucleation and growth of C–S–H on limestone particles were predominantly determined by donor–acceptor mechanisms, following moderate acid–base interactions. Consequently, a strong chemical bonding of calcium ions to a limestone surface resulted in a large amount of uniformly distributed C–S–H nuclei.