Sustainable Stabilization of Soil–RAP Mixtures Using Xanthan Gum Biopolymer

Xanthan gum (XG) is a biopolymer primarily composed of polysaccharides that is increasingly employed to stabilize problematic soils. Although promising results have been obtained in clayey soils, its effect on other geomaterials remains underexplored. This study investigates the impact of XG on the mechanical strength (q u ), stiffness (Go), and microstructure of compacted mixtures of soil and reclaimed asphalt pavement (RAP). A two-part mixing method was adopted: Initially, the XG was mixed with water to form a hydrosolution before mixing in the soil and subsequently combined with the soil–RAP mixture. Xanthan gum was incorporated at dosages of 0.5%, 1.0%, 1.5%, and 2.0% relative to the dry soil weight, while RAP contents were varied at 10%, 20%, and 30% on a dry soil basis. The compaction density was adjusted between 17 and 18 kN/m³, with an optimum moisture content of 18% as determined by the Proctor test. Specimens were cured in a humid chamber for 14 and 28 days. The experimental methodology included unconfined compression tests, ultrasonic pulse velocity measurements, and characterization using scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM–EDS). The findings show that the mechanical strength of the soil–RAP mixture increased with the incorporation of up to 1% XG, which was identified as the optimal dosage. The strength values declined at higher dosages (1.5% and 2.0%). Moreover, the highest strength and stiffness were achieved with a 10% RAP content, while mixtures containing 20% and 30% RAP exhibited reduced performance. Microstructural analysis revealed that at 1% XG, there was a pronounced interaction between the XG and the soil–RAP matrix; however, as the RAP content increased, the larger voids present led only to a moderate interaction between the materials. Additionally, a correlation between the stiffness parameter (Go) and the unconfined compressive strength (q u ) was established, showing that the Go/q u ratio was dependent on the percentage of XG yet remained independent of curing time—a finding that contrasts with previous correlations for this type of soil that were unaffected by other factors.