Screening the Effects of Design Parameters on the Indirect Tensile Strength of Rice Husk Ash–Based Geopolymer-Stabilized Deltaic Clay Soil; A Quarter Fractional Factorial Design Approach

This study evaluates the indirect tensile strength (ITS) of rice husk ash (RHA)-based geopolymer-stabilized deltaic clay soil, characterized by high plasticity (liquid limit 76.5%, plasticity index 35.3%) and low bearing capacity (CBR 3.99%). Using a quarter fractional factorial design with 32 runs, seven key mix parameters were screened: alkaline activator-to-RHA ratio (0.20–0.40), sodium silicate-to-sodium hydroxide ratio (1–3), sodium hydroxide concentration (8–14 M), curing period (4–72 hours), curing temperature (40–120°C), water-to-solid ratio (20–25%), and compaction delay (0–180 minutes). After 28 days curing, ITS ranged from 0.49 to 0.66 MPa, indicating substantial improvement over untreated soil. Effect analysis revealed compaction delay had a significant negative impact on ITS (effect = –0.0869, t = –9.379), while sodium silicate-to-sodium hydroxide ratio (effect = 0.0220, t = 2.381) and sodium hydroxide concentration (effect = 0.0210, t = 2.237) positively influenced strength. Among interactions, only the alkaline activator-to-RHA ratio combined with sodium silicate-to-sodium hydroxide ratio was significant (effect = 0.0230, t = 2.453), highlighting the critical synergy between precursor content and activator composition. These findings underscore the importance of optimizing compaction delay, activator composition, and precursor ratio to enhance geopolymerization and tensile strength through sodium aluminosilicate hydrate (N-A-S-H) gel formation. This research addresses a crucial gap in tensile strength characterization of geopolymer-treated deltaic clays and supports sustainable agro-industrial waste valorization for geotechnical applications.