Abstract
This study evaluates the influence of dry rice husk (RH) on the early-age hydration kinetics of Portland cement. Temperature evolution monitoring was used to analyze the effects of RH content (5–15% by mass of cement) and pre-conditioning regimes (2 h water soaking and hot-water treatment at 100 °C) on maximum temperature rise (ΔTmax), time to peak hydration (tmax), reaction-rate indicator (R), suppression coefficient (Ks), and retardation index (Ri) relative to a control cement paste. The results show that raw RH significantly modifies hydration behavior by reducing heat evolution and delaying the acceleration stage. Increasing RH content progressively decreased ΔTmax and prolonged tmax. At 15% RH after thermal pre-conditioning, ΔTmax decreased by approximately 77% compared to the control, while the retardation index reached Ri = 5.99, indicating a substantial delay in peak hydration. Scanning electron microscopy (SEM) revealed that increasing RH content leads to a more heterogeneous and porous microstructure of the hardened cement paste, with hydration products forming around rice husk particles. These microstructural features correspond with the observed suppression of hydration kinetics. The results indicate that untreated rice husk acts as an active kinetic modifier rather than an inert filler in cement systems due to its high water absorption capacity and interaction with the pore solution chemistry.