Xanthan gum (XG) is an eco-friendly biopolymer with potential applications in soil amendment. However, in acidic soil environments, the pyruvate groups and glycosidic bonds in XG molecules tend to hydrolyze, thereby weakening the efficiency of soil improvement. In this study, the feasibility of utilizing alkaline Class-F fly ash (FA) to assist XG in reinforcing acidic soils was evaluated through proctor compaction, unconfined compression, and one-dimensional consolidation. With the decrease of pore fluid pH values, the beneficial effect of FA on the reinforcing efficiency of XG seemed to grow. As a result, the soil with the strongest acidity (pH = 3) had even higher strength and lower compressibility than the neutral soil that was treated under the same condition. The improvement in both mechanical strength and compressibility of acidic soils might be caused by the crossing-linking of XG molecular strands and the mitigated hydrolysis of XG hydrogels due to the presence of FA. Based on the findings, it is suggested to use FA in combination with XG for treating the acidic soils and use XG alone for treating the neutral soils. The research outcomes will promote the reuse of solid waste Class-F FA in sustainable geotechnical engineering practices, e.g., biopolymer-based soil amendment in acidic soils.
