Abstract: Developing cement-replacing road binders by utilizing the latent cementitious activity of red mud is an effective approach for its large-scale and high-value utilization. In this study, Bayer red mud (RM) and S95-grade ground granulated blast-furnace slag (GGBFS) were used as the main raw materials, with solid sodium silicate as the cementing agent (CA), to prepare a red mud–slag-based cementitious material and its stabilized crushed stone. The effects of RM proportion and CA dosage on compressive strength and reaction characteristics were investigated. The RM proportion was varied from 30% to 70%, while the CA dosage was varied from 3% to 7%. Compressive strengths were measured at 3 and 28 d, and 28 d specimens were characterized by FTIR, XRD, and TG–DTG analyses. Based on these results, stabilized crushed stone was prepared using RM:GGBFS = 50:50 and a CA dosage of 5%, with cementitious-material dosages of 4%–8%. Its unconfined compressive strength, indirect tensile strength, and freeze–thaw resistance were evaluated, and the aggregate–paste interface was examined by SEM/EDS. Finally, a 120 m trial section was constructed on the Zhangdian East connection line of the Linzi–Linyi Expressway for field verification.The results showed that when the RM proportion exceeded 50%, the reduction in compressive strength became more pronounced. Increasing the CA dosage from 3% to 5% raised the 3 and 28 d compressive strengths from 9.6 and 20.5 MPa to 18.9 and 33.2 MPa, respectively, whereas further increases in CA dosage produced limited additional strength gains. FTIR and XRD results indicated that a CA dosage of 5% promoted Si–O–T network reconstruction and the formation of low-crystallinity C-(A)-S-H-type and hydrotalcite-like phases, while RM proportions above 50% restricted the continued formation of effective cementitious phases. TG–DTG results showed that total mass loss did not vary consistently with compressive strength, indicating that strength was governed mainly by the composition and structure of effective cementitious phases rather than by the total amount of hydrated products. For stabilized crushed stone, the most pronounced performance improvement occurred when the cementitious-material dosage increased from 4% to 5%. At a dosage of 5%, the 7 d unconfined compressive strength and indirect tensile strength were 5.2 and 0.46 MPa, respectively, exceeding the project control values of 5.0 and 0.40 MPa. After 28 d of pre-curing followed by 20 freeze–thaw cycles, the residual strength ratio was 85%, higher than the project control value of 80%. SEM/EDS observations showed that the cementitious paste filled interparticle voids and surface depressions, bridged adjacent particles, and formed a closely bonded aggregate–paste interface. Considering strength, freeze–thaw resistance, material consumption, and project requirements, a cementitious-material dosage of 5% was recommended for field application. The field trial further verified the engineering applicability of the recommended mixture. The compaction degrees of the subbase and two base layers were 101.8%, 102.5%, and 102.8%, respectively. The average 7 d core strengths of the base and subbase were 7.8 and 7.3 MPa, exceeding the corresponding project requirements of 5.0 and 4.0 MPa. After 3 d of water immersion, the average core strengths were 6.9 and 5.9 MPa, corresponding to strength retention ratios of 88.5% and 80.8%, respectively. The recommended mixture was compatible with conventional mixing, paving, and compaction procedures and satisfied the early load-bearing and short-term water-stability requirements of the trial section. This study provides a basis for the development of red mud–slag-based road cementitious materials and their application in road base and subbase construction.