Influence of colloidal nanosilica on hydration kinetics and properties of CaO/CaSO4-activated slag binder

To solve the energy consumption and CO2 emission during cement production, the new binders must be developed as an alternative to cement. CaO/CaSO4-activated slag binder is an eco-friendly and safe cementitious material; however, its low strength during initial stages limits its applications. In this study, colloidal nanosilica (CNS) was employed as an additive to improve the strength of CaO/CaSO4-activated slag binder, and the effects of CNS on the workability, hydration kinetics, hydration products (type, quantity, and polymerization degree), and binder microstructure were thoroughly investigated. A moderate CNS content, through its nucleation effect, significantly increased the hydration rate of the nucleation and crystal growth (NG), phase boundary interaction (I) and diffusion (D) processes, which generated large quantities of calcium aluminosilicate hydrate (C-A-S-H) gel in the initial hydration stage. Meanwhile, the addition of CNS improved the polymerization degree of C-A-S-H gel. This amorphous reactant well-filled the pore space between slag particles and yielded a compact microstructure, consequently enhancing the binder strength. Considering the reduction in fluidity and the increase in production cost, the CNS mass fraction was controlled as ∼3%, and the binder reached the satisfactory strengths of 3.87, 24.47, 31.43, and 41.78 MPa at 1, 3, 7, and 28 d, respectively.