Gel formation and thermal stability in geopolymer paste incorporating electric arc furnace slag

Ground granulated blast furnace slag (GGBS) has been widely used to develop geopolymers due to its high calcium and amorphous silica content. In this study, electric arc furnace (EAF) slag (10–30 wt%) was incorporated with GGBS to examine gel formation, thermal behaviour, and compressive strength of geopolymer paste. The addition of EAF slag influenced geopolymerization, forming strong Si–O–T linkages and a dense aluminosilicate network. The calcium-rich environment produced by both slags facilitated the formation of a hybrid (N,C)-A-S-H gel. Incorporating 20 wt% EAF slag yielded the highest compressive strength, improved microstructure, and enhanced thermal stability with reduced mass loss at elevated temperatures. • Phase change in EAF slag-based geopolymer paste was identified. • Up to 20 % EAF slag enhances overall geopolymer paste performance. • FTIR peak shifts confirmed geopolymerization in EAF slag paste. • Excessive EAF slag (>30 %) reduced strength due to unreacted Ca-rich content.