Application of Coal-Derived Char in Cement-Stabilized Sodium Bentonite-Sand Soil for Subgrade Applications

Coal-derived char is a carbon-rich and porous material. Recent studies have shown that coal char addition exhibits good potential for improving the geotechnical properties of cement-stabilized natural clay sand soil. However, the effect of coal char addition on cement-stabilized problematic subgrade soil with expansive clay remains unclear. This study focused on soil composed of 75 wt.% sand and 25 wt.% sodium bentonite clay, incorporating coal char (up to 10 wt.% of cement) and cement (1 wt.% of soil). The objective is to evaluate the impact of char addition on cement-stabilized soil containing an amount of expansive clay, with a focus on its potential use in subgrade applications. The Atterberg limits, Harvard miniature compaction, water content, and unconfined compressive strength (UCS) were performed. It was found that the cement addition increased the optimum water content (OWC) by 1%, which further increased by 0.8% with the addition of 10% char, and the maximum dry density decreased following the opposite trend of OWC. The UCS increased with increasing curing time for untreated (due to increased bonding time) and stabilized soils. The 10% char addition showed a higher UCS at 7 days as compared to the cement-stabilized soil. Furthermore, the char addition showed 1.25% more water content at 28 days as compared to the cement-stabilized soil because of the high water-holding capacity in porous char, which would be beneficial for hydration and pozzolanic reactions. In summary, the char addition can improve the engineering properties of subgrade soils containing expansive clay, thereby expanding its potential applications in the cement-treated subgrade.