Sustainable Utilization of Hazardous Fly Ash and Silica Fume with Chemical Activators for Early Strength Enhancement of High-Volume Fly Ash Concrete

Hazardous waste management is a critical environmental challenge, with sustainable solutions gaining prominence in the construction sector to reduce ecological impact. This study systematically investigates the effects of fly ash (FA), silica fume (SF), and chemical activators on enhancing the mechanical performance and sustainability of concrete. Eighteen concrete mixes were designed with a constant cementitious content of 400 kg/m3 and a 0.43 water–binder ratio. The experimental program consisted of three stages: (1) evaluating FA as a partial cement replacement at 35%, 50%, and 70% to assess its workability and strength effects; (2) incorporating SF at 5%, 10%, and 15% in combination with FA to study synergistic improvements in matrix densification; and (3) applying chemical activators—Na2SO4 and NaOH—at 5% of binder content to optimize early-age strength for FA-SF mixes. A total of 324 specimens were tested for slump, compressive strength (fc), and splitting tensile strength (ft) at 3, 7, and 28 days. Results indicate that high-volume fly ash (i.e., 50% and 70%) delayed early-age strength development, but the incorporation of SF and chemical activation significantly improved strength. SF additions enhanced both fc and ft, with 10% SF providing optimal performance. Further, microstructural analysis using scanning electron microscopy and energy-dispersive X-ray spectroscopy confirmed that chemical activation, particularly with Na2SO4, reduced porosity and enhanced C-S-H gel formation, contributing to strength improvements. Additionally, novel predictive models for fc and ft were developed, integrating the effects of curing age, FA, SF content, and chemical activation. These models demonstrated excellent predictive accuracy, offering practical tools for performance prediction and mix design optimization. Furthermore, a comprehensive cost analysis of all the mixes showed that the optimized mix with 50% FA, 10% SF, and 5% Na2SO4 was 4% more cost-effective than the control mix. These findings provide actionable strategies for engineers and researchers to optimize sustainable concrete mix designs, promote hazardous waste utilization, and advance circular economy objectives in construction.