Geotechnical and microstructural properties of EICP-treated coal char-modified expansive soils

Abstract

Enzyme-induced carbonate precipitation (EICP) is an innovative soil stabilization method, though its performance can be limited by insufficient nucleation sites and uneven calcite distribution. This study investigates the use of coal-derived char as an additive in EICP-treated sodium bentonite-sand mixtures to improve their mechanical and physicochemical properties. Soybean-derived urease was used to catalyze carbonate precipitation in mixtures containing 0 to 20% coal char by dry weight of the soil. Performance was evaluated using bulk density, pH, thermal conductivity, calcium carbonate content (Ccc), unconfined compressive strength (UCS), California bearing ratio (CBR) with soaked swelling tests, unconsolidated undrained triaxial compression, XRD, and SEM analyses. EICP treatment with coal char improved soil performance. UCS increased by up to 35%, with 10% char yielding optimal strength across curing periods. Calcium carbonate content (Ccc ) reached 2.79% at 28 days with 10% char, compared to 0.18% in controls. Bulk density increased by 15–22%, swelling decreased, and CBR values improved. Thermal conductivity declined to 0.35 W/mK at 20% char. SEM observations are consistent with calcite precipitation on and within char pore surfaces, enhancing nucleation and matrix densification. The combined EICP-coal char system offers a promising method for stabilizing expansive soils while valorizing coal by-products.