Role of slag in microstructural development and hardening of fly ash-slag geopolymer

This paper presents the microstructural development and hardening rate of fly ash-slag geopolymers. The solid precursors are treated with a potassium hydroxide-silicate activator of modulus 1.25. Ultrasonic wave reflection and Proctor penetration methods are used to determine the rate of hardening. Microstructural development is investigated using SEM-EDS, and the reaction rate is monitored using semi-adiabatic calorimetry. Both UWR and Proctor penetration methods capture changes in the hardening rate due to changes in the reaction mechanism. An increase in the rate of hardening is observed with the addition of slag. It is concluded that the calcium that dissolves from slag significantly influences both early and late age properties, and the availability of free calcium ions seems to prolong fly ash dissolution and enhance geopolymer gel formation. It is proposed that the hardening process is initiated by the precipitation of C-A-S-H and that rapid hardening continues due to accelerated geopolymerization. (C) 2012 Elsevier Ltd. All rights reserved.