Abstract: Applying eco-friendly xanthan gum biopolymer (XG) in geotechnical engineering significantly enhances soil strength and boosts resistance against erosion from wind and water. The presence of pyruvate groups and glycosidic bonds in XG molecules leads to hydrolysis under acidic conditions, reducing the viscosity of XG hydrogels that bind soil particles. This study explores the effectiveness of using a combination of alkaline fly ash (FA) and XG to treat acid-contaminated soils through Atterberg limits, compaction, and unconfined compressive strength tests. The results demonstrate that FA contributes to increased plasticity limit, optimum moisture content, and maximum dry density, significantly enhancing the soil reinforcement capability of XG in acidic environments. The strength of acid-contaminated soils treated with this combined approach surpasses that of uncontaminated soils treated with the same approach. Moreover, FA mitigates the acidic soil conditions, preventing XG hydrolysis and enhancing soil treatment efficiency. Our findings recommend using a mixture of FA (9%) and XG (3%) for treating acid-contaminated soils, achieving a 310% strength increase compared to untreated soils. For uncontaminated soils, using XG alone (4%) results in a 90% strength improvement over untreated soils.

Key words: acid-contaminated soils, xanthan gum, fly ash, Atterberg limit, compaction, unconfined compressive strength