Mechanical and Microstructural Performance of Fly Ash-Based Geopolymer Mortar Activated by Silica Fume-Derived Sodium Silicate

The construction industry faces growing pressure to adopt sustainable materials due to the high CO2 emissions associated with ordinary Portland cement (OPC) production. Geopolymers synthesized from industrial by-products such as fly ash offer a promising low-carbon alternative. However, the extensive use of commercial sodium silicate (SSC) as an activator remains constrained by its high cost and energy-intensive manufacturing. This study investigates a silica fume-derived sodium silicate alternative (SSA) combined with NaOH as a more sustainable activator for fly ash-based geopolymer mortar. Mortars were prepared with alkali activator-to-precursor (AA/P) ratios of 0.7 and 0.5 and cured at 65 °C and 80 °C. SSA-based mixes exhibited comparable flowability to SSC-based mortars, with slightly longer setting times making them favorable for placement. Mechanical tests showed the superior performance of SSA systems, with AS0.7-65 achieving the highest compressive strength and AS0.7-80 demonstrating greater flexural and tensile strength. Microstructural analyses (SEM, EDX, ATR-FTIR) revealed denser matrices and enhanced sodium aluminosilicate hydrate (N-A-S-H) and calcium-rich N(C)-A-S-H gel formation. Economic assessment indicated approximately 30% cost reduction and a modest (~2%) decrease in CO2 emissions. These findings highlight SSA as a technically viable and sustainable activator for next-generation geopolymer construction.