The collapse of surface goaf beneath highways can result in instability and damage to roadbeds. However, filling the goaf areas with foam concrete can significantly enhance the stability of the roadbeds while considerably reducing the costs of filling materials. This study analyzes the effects on destructive characteristics, mechanical properties, stress–strain curve features, and relevant metrics, while also observing the microstructure of basalt fiber-calcined gangue-silty clay foam concrete (BF-CCG-SCFC). The results indicate that the water–binder ratio significantly influences the cubic compressive strength, split tensile strength, and fluidity of BF-CCG-SCFC. Silty clay reduces the cubic compressive strength, split tensile strength, and fluidity of BF-CCG-SCFC. Conversely, an appropriate amount of calcined gangue and basalt fiber significantly increases the cubic compressive strength and split tensile strength, while decreasing fluidity. To satisfy the strength and fluidity requirements of the filler material in hollow areas, the optimal water–binder ratio for BF-CCG-SCFC is 0.55, the ideal mixing ratio of calcined gangue to silty clay is 2:2, and the basalt fiber content should be 1%. The study examines the influence of varying water–binder ratios, the combined proportions of calcined gangue and silty clay, and different basalt fiber contents on the elastic modulus, peak stress, and peak strain of BF-CCG-SCFC. Additionally, the water–binder ratio influences the matrix strength through the non-hydration reactions of doped particles, while gangue and clay induce a “gradient hydration effect” during the hydration process. The incorporation of basalt fibers enhances the mechanical interlocking between the fibers and the matrix.

To address the environmental issues arising from the large-scale stockpiling of steel slag and to explore its efficient utilisation in road sub-bases, this study investigated the effects of cement dosage and steel slag content on the mechanical properties of cement-stabilised steel slag mixtures. Through unconfined compressive strength tests, compressive modulus of elasticity tests and splitting tensile strength tests, the study revealed the mechanisms by which cement dosage and steel slag content influence microstructure. The results indicate that as the cement content increases, the unconfined compressive strength, compressive modulus of elasticity and splitting tensile strength all show an upward trend, although the rate of increase gradually decreases. With increasing steel slag content, the unconfined compressive strength and splitting tensile strength first increase and then decrease slightly, whilst the compressive modulus of elasticity continues to rise. When 60% steel slag was incorporated, the 28-day unconfined compressive strength and splitting tensile strength reached their peak values, representing increases of 22.17% and 72.7% respectively compared to the control group. Further examination of the microstructure revealed that increasing the cement content and steel slag content enhances structural density and reduces surface porosity; however, excessive cement content and steel slag content have an adverse effect on mechanical properties. Consequently, the synergistic effect of an appropriate amount of steel slag and cement can significantly improve the mechanical properties and microstructure of the mixture. These findings are of great significance in promoting the green utilisation of solid waste materials, such as steel slag, in road engineering.