Performance assessment of homebrewed sodium silicate, and its comparison with commercial sodium silicate in one-part geopolymer mortar: Fresh, hardened, and microstructural properties

Abstract

The high cost, carbon footprint, and embodied energy of commercial sodium silicates have prompted the search for sustainable alternatives for geopolymer synthesis. This study introduces a novel thermochemical route for producing sodium silicates from silica-rich materials tailored for one-part geopolymers. Sodium silicates were synthesized from rice husk ash (RHA, high silica), eco-processed pozzolan (EPP, low silica), and illitic clay (IC, intermediate silica) combined with sodium hydroxide powder. These homebrewed activators were used to produce one-part geopolymer mortars from a binary blend of ground granulated blast-furnace slag (GGBS) and palm oil fuel ash (POFA) using an alkaline activator-to-binder ratio of 0.18, providing a low-carbon and user-friendly alternative to two-part geopolymers. The fresh and hardened properties, including flow diameter, fresh/hardened density, compressive and flexural strengths, water absorption, porosity, and ultrasonic pulse velocity (UPV), were evaluated. Mortars activated with homebrewed sodium silicates were compared with those activated with commercial sodium silicate under ambient curing conditions. The sodium silicates were characterized using pelletized X-ray fluorescence (XRF), while X-ray diffraction (XRD) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX) were used for mineralogical and microstructural analyses. RHA-derived activators achieved the highest 28-day compressive (49.95 MPa) and flexural (6.16 MPa) strengths, surpassing the commercial reference (43.55 MPa; 5.20 MPa), while the EPP-based mortar recorded the lowest strength (37.00 MPa). Flowability was greatest for the commercial mix (215 mm) and lowest for IC-based mortar (161.5 mm). SEM/EDX confirmed the formation of semi-amorphous C–N-A-S-H and minor C–S–H gel. All mortars exceeded 37 MPa, thereby meeting the structural grade requirements.