Expansive soil modified by iron tailing sand and calcium carbide slag as subgrade material

Expansive soil encounters large changes in strength and volume with the water content alteration for rich in hydrophilic montmorillonite minerals. It can be physically/chemically modified to suppress the water sensitivity by mixing with recycled industrial by-products (iron tailing sand and calcium carbide slag in this case) as the backfilling material of subgrades for the engineering practices. The specimens were prepared with 30% iron tailing sand and 6–12% calcium carbide slag. Atterberg consistency limit, the free swelling index, loaded swelling rate, unconfined compressive strength, water stability, and microstructure were determined at the preset curing times. The results showed that after modification, the free swelling index decreased to below 30%, satisfying the requirements for the expressway subgrade. The loaded swelling rate was negative under loading and soaking condition, indicating compressibility. The compressibility was evident from the peak value with a calcium carbide slag content of 10%, and a similar trend was observed for the evolution of unconfined compressive strength. With the increase in calcium carbide slag content, the water stability coefficient of modified soil also increases. The pore size distribution was of typical bi-modal characteristics, and the modification just dominated the large porosity. The microstructure was the densest, and the fewest inter-aggregate pore volume was observed at the calcium carbide slag optimal content, 10%.