Abstract
Rice husk ash (RHA) is a widely available agricultural waste with potential as a precursor for nano-silica, offering a supplementary cementitious material for concrete production. This study investigated the effect of nano-silica synthesised from RHA, calcined at 600 °C, as a partial replacement for Portland Limestone Cement (PLC) on the workability, compressive strength, and mass gain of concrete at 0%, 4%, and 8% replacement levels. The constituent fine and coarse aggregates were characterised for moisture content, specific gravity, and particle-size distribution before concrete production, and confirmed suitable for normal-weight concrete. Workability decreased progressively with increasing nano-silica content, with slump values dropping from 60 mm for the control mix to 45 mm and 35 mm at 4% and 8% replacement, respectively, although values remained within a workable range. The curing-period mass increase - measured here as the percentage increase in specimen mass between demoulding and the end of curing, and reported as a study-specific indicator distinct from standardised water absorption - was lowest at 4% replacement (0.66%), against 0.92% for the control and 0.71% at 8% replacement. The 8% replacement achieved the highest 28-day compressive strength, at 21.34 MPa, compared with 18.99 MPa at 4% replacement and 15.55 MPa for the control, associated with the combined filler, nucleation, and pozzolanic effects of nano-silica. Within the replacement levels investigated, 4% replacement provided the best overall balance of workability, compressive strength, and mass gain, while 8% replacement is better suited to applications where maximum 28-day compressive strength is the governing requirement. The findings demonstrate the potential of RHA-derived nano-silica as a supplementary cementitious material for PLC concrete, while providing a route for agricultural-waste valorization.