New polysilicate for high early strength and durability of concrete

In the present paper a new accelerating additive for cement mixtures, based on polymeric metal silicate hydrates, is described. The new additive is highly effective in promoting the nucleation and growth of silicate phases of portland cement. The additive contributes in developing high early strength and in improving the microstructure of hydrated cement paste and represents a new development in the field of concrete admixtures, based on the application of nanotechnology. The new additive consists of polysilicate nanoparticles, characterized by low Ca/Si ratio and high polymerization degree, acting as a catalyst for the crystallization of C-S-H. The polymeric nature of the new silicate was demonstrated by Gel permeation Chromatography (GPC). Hydration kinetics of cements pastes containing different dosages of the new admixture were studied by XRPD and compared to equivalent admixture-free cement pastes. The data confirmed that the new silicate hydrate increases the C3S dissolution and the portlandite formation rates and reduces the induction period of cement hydration. Concrete tests confirmed a beneficial effect of the admixture on early strength development. The precipitation of C-S-H induced by the new polymeric silicate hydrates in the capillary porosity refines the microstructure of the cement paste and also improves the water permeability and durability of concrete structures. The development of accelerating additives based on polymeric metal transition silicate hydrates offers several advantages over conventional accelerators. In the precast industry, the strong acceleration of hydration rate achievable with this additive could reduce or even eliminate the steam curing, with considerable energy savings and consequent reduction of CO2 emissions. In the ready-mixed concrete, the accelerated strength development permits an increase of productivity and a reduction of production cycles. With respect to durability, the refinement of the microstructure achievable with the seeding additives improves water-tightness and steel-reinforcement corrosion protection.