Thermal conductivity characteristics and predictive modeling of rubber sand composite geomaterials under multiple influencing factors

Waste tire rubber has low density, high elasticity, and excellent fatigue resistance. When mixed with sand, it forms a sustainable composite geomaterial in which thermal conductivity (TC) governs thermal performance. This study systematically investigated the TC of rubber-sand composite geomaterials (RSCG) with varying rubber particle sizes, rubber contents, water contents, and dry densities using the transient thermal probe method. Scanning electron microscopy (SEM) was also used to examine heat-transfer pathways and thermal-resistance mechanisms. Results show that TC decreases rapidly at low rubber contents and then more gradually with further rubber addition, whereas TC increases monotonically with dry density. Water content generally increases TC, but its effect weakens at higher rubber contents. Fine rubber particles (40 mesh) more effectively fill pores and reduce direct sand-sand contacts, causing a pronounced TC decrease at relatively low rubber contents. In contrast, coarse rubber particles (20 mesh) form more heterogeneous pore structures, allowing water-induced TC enhancement to remain pronounced over a broader rubber-content range. Where an identifiable turning point was observed, the critical water contents for both particle-size systems were mainly 8%-9%. A Wiener-bounds-based four-phase TC model considering rubber, river sand, water, and air was developed and showed significantly better predictive accuracy than the Kersten and Gangadhara models. The findings support TC prediction, mixture optimization, and engineering applications of RSCG.