Synthesis and Reaction Mechanism of an Alkali-Activated Metakaolin-Slag Composite System at Room Temperature

Abstract

Synthesis of geopolymer using metakaolin (MK) usually requires a relatively high curing temperature, which limits the application of MK-based geopolymers in the practice of civil engineering. With the goal of developing a cementitious composite that can be cured at room temperature, in the present study, ground granulated blast furnace slag powder was incorporated into MK for the synthesis of a geopolymer. With the aid of compressive strength tests, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM), the influence of slag’s impact on the consistency, gelling time, and mechanical properties of the gelling system, as well as the reaction mechanism of the composite system, were investigated. The results showed that the incorporation of slag into MK improved the consistency of the slurry and shortened the setting time. A high-strength paste was synthesized successfully using the MK-slag composite system under ambient temperature curing. Results of XRD and FTIR analyses indicated that MK geopolymerization and slag hydration occurred simultaneously in the MK-slag composite system via alkali activation. The structure of hardened paste consists of both C-S-H-type and N-A-S-H-type gels. The activator module and the MK/slag mass ratio were the main factors affecting the strength of the synthesized products: when the slag replacement ratio was no more than 40%, the strength of the reaction products decreased with an increase of the activator module; when the slag replacement ratio reached or exceeded 60%, the strength of the reaction products increased with the increasing activator module.