Synergistic utilization of ternary solid wastes for sustainable red sandstone subgrades: A simplex-centroid design approach.

The engineering application of crushed red sandstone, a potential subgrade filler in the mountainous areas of southern Anhui, China, is often hindered by its poor water stability and mechanical vulnerability. Upcycling multi-source solid wastes into binders offers a resource-efficient, sustainable, and effective solution for the improvement of crushed red sandstone. In this study, a ternary binder composed of calcium carbide slag (CCS), rice husk ash (RHA), and fly ash (FA) was developed for modifying crushed red sandstone, and its mixing proportions were systematically optimized using a simplex-centroid design. Macroscopic engineering tests and microstructural analyses were conducted to elucidate the performance evolution and underlying mechanisms. The results show that compared to the sole use of CCS, the synergistic incorporation of RHA and FA significantly enhanced the mechanical performance. Specifically, higher RHA contents prominently improved the structural cohesion, whereas a balanced ternary combination maximized the unconfined compressive strength (UCS) and California Bearing Ratio (CBR). By superimposing the contour plots of multiple performance indicators, an optimal multi-objective mix proportion range was successfully identified. Mechanistically, this modification is driven by a profound Ca-Si-Al chemical synergy: the highly alkaline environment created by CCS vigorously triggers the dissolution of highly reactive amorphous silica from RHA and glassy aluminosilicates from FA. This dynamic balance drives robust pozzolanic reactions to precipitate copious C-(A)-S-H gels. Coupled with cation exchange, physical filling, and carbonation, these multi-phase cementitious products effectively bridge soil particles, fill interparticle voids, and refine the pore structure, ultimately transforming the loose granular matrix into a heavily interlocked, dense load-bearing skeleton. This study provides a rational mixture-design strategy for converting multi-source solid wastes into a clinker-free binder and offers scientific guidance for the sustainable utilization of red sandstone as subgrade filler.