Effect of Admixtures in Blast Furnace Slag-fly Ash Based Alkali-activated Paste

Portland cement can be replaced with alkali-activated binders (AABs), a sustainable material. They make use of industrial byproducts rich in aluminosilicates to produce hardened binders under alkaline conditions. The effective utilization of alkali-activated binders in each particular place is greatly influenced by the ease in accessibility of suitable precursors and activators. Slag-based AAB can make a strong and durable mix under ambient curing. But its poor workability and fast setting characteristics limit its scope of wide applications. The purpose of this investigation is to determine the effect of admixtures on the setting time of Alkali-activated Paste with blends of slag and fly ash (BFS-FA AAP). A combination of red gypsum and phosphoric acid is the admixture considered for the study. Setting time of BFS-FA AAP activated with sodium hydroxide and sodium silicate is studied considering different control factors, including Na₂O/b ratio, BFS/binder and activator modulus (SiO₂/ Na₂O). The phosphoric acid, when added at 0.45 M along with red gypsum at 2.5% of binder content retarded the setting time of AAP effectively.