“Processing”Search All-EnPress Publisher LLC.

This review provides a comprehensive, processing-centric synthesis of four decades (1986–2025) of rice husk ash (RHA) research, linking combustion chemistry, purification, mechanical activation, and fabrication routes to microstructural evolution and material performance in metal matrix composites (MMCs), cementitious systems, and geopolymers. A consistent finding is an optimal reinforcement window of 4–10 wt.% RHA, achieving significant improvements in hardness (32–55%), tensile strength (28–38%), wear resistance (40–60%), compressive strength (10–25%), elastic modulus (10–30%), and damping capacity (18–35%), while avoiding particle clustering and porosity at higher loadings. Fabrication routes including stir casting (scalable, 2–10 wt. % RHA), powder metallurgy/spark plasma sintering (superior densification, 0–20 wt. % RHA), and friction stir processing (40–50% surface hardness increase) are critically evaluated. Hybrid reinforcements—RHA-SiC (55–75% hardness), RHA-B₄C (40–60% wear resistance), RHA-CNT/graphene (25–40% fracture toughness), and RHA-graphite (10–25% friction reduction)—overcome monolithic RHA brittleness. In cementitious applications, 10–20% RHA replacement of Portland cement enhances long-term strength, reduces chloride penetration by 30–50%, and achieves 15–30% theoretical CO₂ reduction. Emerging AI/ML approaches (ANN, SVR, random forest, XGBoost-SHAP) achieve predictive accuracy of R ≈ 0.94–0.99 for mechanical and tribological properties. Critical barriers to industrial deployment include lack of standardized RHA characterization, limited pilot-scale validation, and scarce life-cycle assessment data. A prioritized roadmap addresses standardization, interface engineering databases, pilot demonstrators, AI-ready data infrastructure, and alignment with circular economy principles and ASTM/ACI standards.