Stabilization and strengthening mechanisms of red sandstone cement-stabilized macadam using solid waste

To overcome the performance deterioration of (CSM) caused by the easy disintegration of red sandstone, this study used red sandstone as the aggregate and employed a composite binder consisting of cement, ground granulated blast-furnace slag (GGBFS), and phosphogypsum (PG) to prepare CSM base course. The hydration process, phase assemblage, mechanical performance, and interfacial characteristics were systematically investigated to evaluate the effectiveness of the cement–GGBFS–PG composite cementitious system (CGBP system) in mitigating the deterioration of red sandstone-based CSM. The results indicate that (1) the CGBP system can offset the strength loss induced by the red sandstone aggregate, increasing the cement-normalized unconfined compressive strength (UCS) to 60.6%; (2) in the early curing period, the UCS of specimens using red sandstone as the aggregate was lower than that of RS-100; however, with increasing curing age, the synergistic effect of GGBFS pozzolanic reactivity and PG grew more pronounced, leading to higher strengths for RSG-70 and RA-65 after 56 days; (3) appropriate dosages of PG and GGBFS facilitate the formation of C-(A)-S-H and ettringite, which enhances mechanical properties, whereas excessive amounts cause overproduction of ettringite, inducing volume expansion and reducing strength; (4) microstructural observations confirm that hydration products effectively fill pores and grow more uniformly within the aggregate–matrix interfacial region, resulting in a markedly improved interfacial transition zone.