A Comprehensive Study on Optimizing Activator Composition for Enhanced Strength and Micro-Structure in High-Strength Alkali-Activated Slag Binders

The production of Portland cement involves exploiting natural reserves of limestone and coal and emitting approximately one metric tonne of carbon dioxide for every metric tonne of cement produced. This article investigates the optimisation of alkaline activators for enhanced strength and micro-structure in high-strength alkali-activated slag binders/mortars to explore alternative construction materials that are environmentally sustainable, require less energy consumption, and offer greater cost-effectiveness. The study reveals that the AAS binder and cement exhibit comparable hydration characteristics. However, the AAS binder demonstrates a shorter dormancy period, an earlier and heightened second exothermic peak, lower cumulative heat and inadequate Ca(OH)2 crystallisation. Furthermore, the AAS binder has a lower Ca/Si ratio and a higher Al/Si ratio in the gels. The pore structure of hardened AAS binder consistently shows higher compressive strength and denser, more stable micro-structure than cement paste, particularly in the early stages. Alkali-activated slag (AAS) mortar exhibits a 52-89% increase in compressive strength compared to cement mortar, suggesting the potential of AAS binder to replace cement. The compressive strength of AASM-4 mortar mix with a Na2O concentration of 7%, an activation modulus of 1.2, and a water-to-binder ratio of 0.4 is 89.4 MPa after 28 days of air curing. Furthermore, all mortar mixes featuring Na2O concentrations between 4 and 7% achieved over 70% of their 28-day compressive strength within seven days. In conclusion, the research posits that the AAS binder has the potential to replace cement as a building material in specific engineering applications, mainly when high strength is a requirement.