Influence of starting material on the early age hydration kinetics, microstructure and composition of binding gel in alkali activated binder systems

The influence of starting material on the hydration kinetics and composition of binding gel in alkali activated binder systems was evaluated. The starting materials used were ground granulated blast furnace slag, class C fly ash and class F fly ash. All starting materials were activated using alkaline solution with a SiO2/Na2O ratio of 1.5. The hydration kinetics were monitored using in situ isothermal conduction calorimetry and the chemical compositions of the binder gels were determined by energy dispersive X-ray spectroscopy. In the fly ash systems, the calorimetric curves had only one peak, which occurred in the first 30 min of reaction, and lacked an induction period. Two peaks were distinguishable in slag systems, though the induction period was much shorter than that of a typical OPC system. The gel composition ratios, including Ca/Si, Na/Si, Na/Al and Al/Si, were different in each of the systems and are discussed in detail.