A novel study on polyhydroxyalkanoate biopolymer as an eco-friendly binder for granite residual soil

Aiming to address the issue of sustainable ground improvement, a natural biopolymer, polyhydroxyalkanoate (PHA), was used to treat granite residual soil (GRS). Different dosages of PHA (0.2–1%) as well as a combination of PHA with xanthan gum (XG, 1.5%) were investigated under varying curing periods. To highlight the broader applicability of the technique, the influence of CO₂ exposure and heavy metal contaminants (Zn and Cu) on the strength characteristics of biopolymer-treated GRS was also examined. Results showed that the unconfined compressive strength of untreated GRS increased from 427 to 536 kPa with 1% PHA and further to 827 kPa with the combined treatment of 0.8% PHA + 1.5% XG. An optimal curing period of 28 days yielded substantially higher strength compared with 7 and 14 days. Moreover, the presence of CO₂ markedly enhanced soil strength, from 200 kPa in untreated samples to 1600 kPa in PHA–XG-treated soils, while exerting negligible influence on strain behavior. Microstructural analysis exhibited a significant reduction in voids in the treated soil, primarily due to the enhanced interlocking of flaky soil particles. This was further supported by the observed decrease in specific surface area (from 7.2 to 5.8 m²/g) and pore volume (from 0.046 to 0.036 cm³/g) after biopolymer treatment. Owing to its natural origin, high biocompatibility, and environmentally benign characteristics, this study demonstrates the strong potential of PHA as a sustainable and low-carbon stabilizer for ground improvement.