Coal Char-Cement Stabilization of Common Clay Soils

Abstract Coal char addition to the physical, chemical, mechanical, mineralogical, and microstructural properties of cement-stabilized kaolin, calcium bentonite, and sodium bentonite was investigated using char contents up to 30% (by mass of cement). This study employed unconfined compression tests, triaxial compression tests, one-dimensional consolidation, thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy to evaluate coal char’s influence on cement hydration and clay-cement interactions. Results showed that coal char increased the unconfined compressive strength of stabilized kaolin and sodium bentonite by an average of 6.2% and 32%, respectively, through enhanced hydration and pore refinement. Calcium bentonite exhibited an 8% strength gain after 7 days but a 10.2% reduction after 56 days, likely because early calcium aluminate silicate hydrate formation, reducing pore solution alkalinity and limiting hydration. Adding 10% coal char reduced the compression index and coefficient of volume change for all soils, with sodium bentonite showing the greatest improvement. Under triaxial compression, shear strength increased by 110.8% for kaolin and 38.6% for calcium bentonite, while sodium bentonite exhibited a 3% reduction due to crack development. These findings demonstrated coal char’s potential to enhance cement stabilization, depending on clay mineralogy.