Comparative Assessment of Commercial and Waste-Derived Sodium Silicate Activators in PFA-GGBS Alkali-Activated Concrete

The growing demand for Portland cement has intensified concerns over construction-related CO2 emissions, driving interest in low-carbon alternatives such as alkali-activated concrete (AAC). Although AAC can substantially reduce embodied carbon, the production of commercial sodium silicate (SS), a widely used activator, remains a significant environmental burden. This study evaluates the feasibility of a waste-derived sodium silicate activator produced from silica fume (SF) and sodium hydroxide (NaOH) and compares its performance with that of a conventional commercial activator in blended f fuel ash (PFA) and ground granulated blast furnace slag (GGBS) systems. Fresh and hardened properties were assessed across a range of activator-to-binder (A/B) and SS/SH ratios, considering workability, setting time, and compressive strength requirements for structural concrete applications. Increasing the A/B ratio from 0.16 to 0.20 improved compressive strength and accelerated setting, while higher SS/SH ratios enhanced early- and later-age strength through increased soluble silica availability and gel formation. Although higher activator contents reduced workability due to increased viscosity, both commercial and SF-derived activators achieved comparable mechanical performance. The high amorphous silica content of SF was found to promote dense C-(A)-S-H gel development, while 10 M NaOH and a 24 h ambient-temperature extraction process provided sufficient silica dissolution without additional heating. These findings demonstrate the potential of SF-derived sodium silicate as a sustainable activator for producing low-carbon AAC with performance comparable to conventional systems.