[Objective] Extensive red sandstone spoil generated from highway tunnel projects in central-southern China poses significant disposal challenges due to its high water content, susceptibility to slaking and disintegration, low strength, and high compressibility—characteristics that fail to meet subgrade material standards. To realise the goal of “treating waste with waste and turning waste into treasure”, we use quicklime, calcined coal gangue powder, fly ash and cinder powder as multi-source solid-waste curing agents to explore the optimal ratio and mechanism for co-curing red sandstone spoil, providing a basis for resource utilisation. [Methods] Red sandstone spoil was served as the base material, quicklime, calcined coal gangue powder, grade-III fly ash and cinder powder were selected as curing agents, with their chemical compositions determined by X-ray fluorescence spectroscopy. Single-mix tests were conducted to investigate the effect of each curing agent alone on the strength of the red sandstone spoil and to determine appropriate dosage ranges. Then orthogonal experimental design was employed, using 7-day unconfined compressive strength (UCS) as the index to obtain the optimal ratio. Based on the optimal mix ratio, the red sandstone spoil was modified with curing agent dosages of 8%,10%,12%,and 14%,and cured for 3,7,14,and 28 days respectively. Unconfined compressive strength (UCS) was tested. Water stability coefficients were measured through water immersion tests. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were conducted to examine changes in microstructure and mineral phases before and after curing, revealing the stabilization mechanism. [Results] Optimal mix proportions and dosages:the optimal mass ratio was quicklime∶fly ash∶calcined coal gangue∶cinder powder=4∶8∶8∶7. The strength showed an initial increase followed by a decline with increasing total dosage, with 12% total dosage yielding the highest strength. Macro-scale performance improvement: at 12% dosage, 7-day UCS reached 1.659 MPa (16 times that of raw spoil) and 28-day UCS 2.255 MPa. Water stability was significantly improved, with the coefficient reaching 59.37% at 12% and 65.90% at 14%, with strength increasing over time, particularly rapidly between 3-14 days. Microscopic mechanism: XRD analysis showed that the contents of orthoclase and albite decreased in the improved red sandstone spoil, while the diffraction peaks of quartz and calcite were enhanced, indicating that ion exchange and pozzolanic reactions occurred, generating cementitious products such as calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H). SEM observation revealed that the soil particles were loose and porous before improvement, while after improvement, the cementitious products bonded the particles to form a dense skeletal structure, which became more complete with curing age. [Conclusions] The optimal co-curing ratio is quicklime∶fly ash∶calcined coal gangue∶cinder powder=4∶8∶8∶7 at 12% dosage, delivering the best mechanical properties and water stability. The mechanism is a lime-centred synergistic reaction: lime hydration provides an alkaline environment and Ca²⁺, triggering ion exchange (soil particles changing from potassium/sodium type to calcium type) and pozzolanic reactions (forming cementitious products), while solid waste particles fill pores to reinforce the soil skeleton. This offers an economical and eco-friendly route for red sandstone spoil utilisation, with both engineering and ecological value.