Investigating the effect of alkali-activated slag on the strength of clay soil

Ordinary Portland cement and lime are two traditional admixtures that account for 6% of all CO2 emissions, thus making them the leading causes of global warming. Alkali-activated slag types of cement are new environmentally friendly cement types that could improve soil quality. This study presented a mathematical approach to optimize a sodium silicate activator for alkali-activated slag (AAS) binder by utilizing the full factorial method. Three factors (i.e., percentage of slag to dry weight of soil, sodium oxide concentration as a percentage of slag weight (Na2O%), and silicate modulus ratio (SiO2/Na2O), each at three levels), were evaluated according to the unconfined compressive strength and direct shear after seven and 28 days of curing. The results showed that utilizing 10% slag, a modulus ratio of 0.8, and a Na2O% of 6% increased the uniaxial strength of treated samples to more than twice their original values after 7 days of curing. The direct shear strength tests revealed that the slag content significantly affected the strength of alkali-activated soil. Adding 8% slag, SiO2/Na2O of 0.8, and Na2O% of 6% increased natural clay’s cohesiveness from 21 to 51 kPa and the friction angle from 28 to 29°.