Strength Development of PPC Concrete with Rice Husk Ash: Optimal Replacement Levels for Sustainable Construction

The construction industry is a major contributor to greenhouse gas emissions, with cement production responsible for 8–10% of global CO 2 output. This study investigates the use of rice husk ash (RHA) as a partial cement replacement in Portland Pozzolana Cement (PPC) concrete, which already contains fly ash. Five replacement levels (5%, 7.5%, 10%, 12.5%, and 15% by weight of PPC) were tested for compressive and flexural strength (modulus of rupture, MOR) at 3, 7, 28, and 56 days. An M20-grade mix, designed in accordance with Saudi Building Code (SBC) provisions, was adopted to ensure practical applicability. Results showed that moderate RHA contents (5–7.5%) enhanced strength, with maximum compressive (37.62 MPa) and flexural (5.47 MPa) strengths recorded at 7.5% RHA after 56 days, representing 3.5% and 9.6% improvements over the control, respectively. All RHA mixes exhibited reduced early-age strength due to delayed pozzolanic activity, whereas higher replacements (≥12.5%) caused strength loss from excessive cement dilution. The novelty of this study lies in demonstrating the synergistic effect of RHA with PPC—an underexplored dual-pozzolan system—and identifying optimal replacement levels for sustainable construction. The findings highlight RHA as a viable supplementary cementitious material that reduces clinker use and carbon emissions while maintaining or improving long-term concrete performance.