Rice Husk Ash for Sustainable Ground Improvement: A Comprehensive Geotechnical Review

Abstract

Rice husk ash (RHA), an abundant agricultural by-product containing about 85–95% predominantly amorphous silica when produced by controlled combustion at approximately 500–700 °C, has been extensively investigated as a low-carbon additive for ground improvement. This paper systematically reviews the experimental evidence published between 1987 and 2026 on the production and characterization of the ash, the mechanisms of soil–RHA interaction, and the quantitative effects on index, compaction, swelling, strength, permeability, and durability properties, for RHA used alone, combined with calcium-bearing activators (lime, cement, calcium carbide residue), and in alkali-activated geopolymer systems. The collected results form a consistent quantitative picture. Plasticity and swelling decrease systematically with RHA content: the differential free swell of a black cotton soil dropped from 73% to 14% with 10% calcium carbide residue and 15% RHA. Maximum dry density decreases while optimum moisture content increases, a compaction trade-off attributable to the low specific gravity and porous morphology of the ash. With an appropriate activator and adequate curing, unconfined compressive strength increases severalfold (from 142 to 817 kPa at 12% RHA in an expansive subgrade, and by 74–192% at 28 days), soaked CBR rises from 2.41% to 17.29%, and lean field mixes such as 3% cement + 1% RHA achieve a base-course CBR of 165%. In soft and organic soils treated by deep or mass in-situ mixing, RHA acts instead as a partial cement replacement: up to 15–35% of the binder can advantageously be RHA, the strength of cement-admixed soft clay increased by more than 100% at 28 days, and stabilized peat reached a fivefold strength gain over the untreated state. Reported optimum dosages cluster at about 4–12% RHA by dry weight of soil, reflecting the stoichiometric balance between reactive silica and available calcium or alkali; beyond the optimum, a dilution effect causes strength to stall or decline. Geopolymer RHA systems exhibit superior durability, surviving 12 wetting–drying cycles with mass losses of only 10.5–24.4%, whereas silica-deficient mixtures disintegrate. The discussion interprets these results mechanistically, derives their design implications, quantifies their significance for reducing binder-related CO₂ emissions, and identifies the standardization, field-validation, and life-cycle research still required.