Effect of Mortar Surplus Coefficient on the Mix Design and Performance of Granite Tunnel Slag Aggregate Concrete

To facilitate the graded resource utilization of granite tunnel slag (GTS), the mix proportions of GTS aggregate concrete were optimized using the mortar surplus coefficient (MSC) as a mix-design parameter. Concrete mixtures with different MSC values were designed by adjusting the coarse-aggregate gradation and sand ratio. The relationships between MSC and workability, mechanical properties, early-age shrinkage, and cracking resistance were systematically investigated, with the observed performance variations interpreted as the coupled responses of MSC and the corresponding mix-design variables. The results show that relatively high aggregate packing densities were achieved when the mass ratios of 5–10 mm, 10–20 mm, and 16–31.5 mm coarse aggregates were 4:3:3 or 2:6:2. Concrete flowability generally increased with MSC, while early-age shrinkage showed an overall increasing tendency across the investigated mixtures. Within the investigated mixtures, an indicative MSC interval of 1.79–1.90 corresponded to a favorable balance between workability, compactness, and compressive strength. In addition, JP-262 (MSC = 1.82) exhibited improved cracking resistance compared to REF-172 (MSC = 1.67). However, at MSC = 2.01, the largest early-age shrinkage was observed, indicating that excessive mortar content may adversely affect volumetric stability. An MSC-based mix-proportion optimization method for GTS aggregate concrete is proposed, providing a theoretical basis and technical support for optimizing the overall performance of granite tunnel slag aggregate concrete and promoting the efficient resource utilization of tunnel slag.