Mechanical and microstructural properties of spent coffee ground improved soda saline-alkali soil: A zero-cost improvement material

The adverse properties of soda saline-alkali soil often result in subgrade damage and slope instability. This study proposes the utilization of the zero-cost and environmentally friendly spent coffee ground (SCG) to improve the engineering properties of soda saline-alkali soil. The mechanical, physicochemical, and microstructural properties of SCG-improved soil were investigated through a series of tests. Results indicated that with increasing SCG content, the unconfined compressive strength, shear strength, and electrical resistivity increased, and the coefficient of compressibility, pH, cation exchange capacity, exchangeable sodium ion percentage, absolute value of zeta potential, and porosity decreased. The mineral composition was not changed with the addition of SCG. The pH reduction was due to the acidic functional groups in palmitic acid, stearic acid, and chlorogenic acid from SCG. K⁺ and Ca²⁺ in SCG exchanged with Na⁺ in pore solution, reducing the thickness of diffuse double layer and increasing the attraction between clay particles. Moreover, the palmitic acid, stearic acid, and their derivatives wrapped the soil particles to form larger agglomerated particles, which enhanced the cohesion and internal friction angle. This study demonstrates that SCG can be used to improve the engineering properties of soda saline-alkali soil, providing preliminary insights for green soil improvement.