Abstract: Cement production is associated with substantial carbon emissions, releasing approximately 0.95 tons of CO₂ for every ton of clinker produced. The cement industry accounts for 8% of global CO₂ emissions, ranking as the third-largest source after the power and steel industries, and contributing significantly to ecological imbalance and global environmental challenges. Concurrently, the cumulative stockpile of industrial solid waste has exceeded 60 billion tons, occupying approximately 2 million hectares of land and raising substantial ecological concerns. To address these dual challenges, this study proposes the partial or complete replacement of cement with typical alkaline industrial solid waste (AISW) for producing AISW-based lightweight soil (AISW–LS). Leveraging the inherent advantages of LS—including low density, adjustable density and strength, excellent workability, low cost, post-curing self-supporting behavior, simple construction, broad applicability, and good durability—the preparation process, physical and mechanical properties, durability, and microstructural characteristics of AISW–LS were systematically investigated. The results demonstrate that AISW serves as an adequate cement substitute. The resulting AISW–LS exhibits excellent flowability (133.3 mm–256.7 mm), reduced water absorption (8.2%–60.94%), low thermal conductivity (0.117–0.223 W·(m·K)^–1), and high durability (durability coefficient of 0.54–3.23). By modulating the dosage of red mud, carbide slag, and soda residue, or varying the wet density, the 28-d compressive strength of AISW–LS can exceed 1.0 MPa, meeting the engineering requirements for subgrade filling. Within the AISW–LS system, AISW particles are cemented by hydration products to form a skeletal structure, while fine AISW particles fill the interstitial voids, increasing compactness and thereby enhancing compressive strength. Furthermore, the incorporation of AISW raises the pH of the mixed slurry, thereby promoting the efficiency and degree of ordinary portland cement (OPC) hydration and acting as an alkali activator to stimulate the latent reactivity of materials such as ground granulated blast-furnace slag. Regarding foam stability, AISW blocks liquid flow channels, effectively slowing foam drainage and increasing the distance between adjacent bubbles, thereby preventing coalescence and enhancing stability. Specifically, the addition of red mud increases the yield stress of the precursor paste; this higher yield stress reflects stronger intermolecular interactions, which reduce fluid mobility within pores and further enhance foam stability. Leaching tests confirm that heavy metal concentrations in AISW–LS comply with environmental standards, indicating negligible environmental risk when used as subgrade fill. Consequently, AISW–LS offers significant economic and ecological benefits by transforming industrial waste from an environmental burden into a high-value resource. A carbon footprint analysis reveals that CO₂ emissions for lightweight soil are distributed across four stages: raw material production and transportation, and LS preparation and casting. Among these, raw material production is the primary contributor, accounting for 95.04%–98.47% of total emissions. Based on an analysis of the Liansu Expressway project, the CO₂ emissions for 1.0 m³ of soda residue-based lightweight soil (SR–LS) and OPC–LS were 100.75–113.91 kg and 255.25–323.38 kg, respectively. Thus, the CO₂ emissions of SR–LS were significantly lower, representing only 35.22%–39.47% of those generated by OPC–LS. Overall, this study provides a practical pathway for the high-value utilization of AISW and low-carbon development of construction materials, offering substantial potential for engineering applications.