Feasibility of Using Foamed Lightweight Soils with Phosphogypsum as Partial Substitute for Cement in Subgrade Engineering: Multiscale Properties and Sustainability Analysis

Phosphogypsum (PG), as industrial solid waste, can partially replace cement in foamed lightweight soil (FLS) to promote environmentally sustainable development. However, to verify this statement, a comprehensive assessment of the amended FLS is necessary to evaluate its engineering performance, durability, and sustainability prior to PG use. This study aims to evaluate the multiscale characteristics of conventional FLS and PG-modified FLS (PG-FLS). The macroscopic engineering performance, encompassing fluidity, water absorption, and compressive strength, was investigated. Furthermore, durability was assessed under various conditions, such as dry–wet cycling, freeze–thaw cycling, sulfate erosion, and outdoor exposure. In addition, microscopic observations were conducted to examine pore morphology and hydration products to better understand the mechanisms that contribute to the modified macroscopic performance. Finally, a thorough sustainability analysis was carried out to enable a comprehensive comparison. This study confirmed that the physical and mechanical properties of PG-FLS satisfied the application requirements of subgrade engineering. Specifically, its compressive strength ranged from 0.78 to 1.3 MPa (>0.3 MPa), and the fluidity ranged from 160 to 186 mm (160–200 mm). It also exhibited superior durability in sulfate erosion and outdoor long-term exposure environments, showing a durability coefficient surpassing 100%. The hydration products of eroded PG-FLS contain more primary ettringite than conventional FLS, thereby delaying the development of cracks and improving the corrosion resistance by filling in pores with ettringite crystals. Because PG-FLS meets the requirements of engineering applications and construction costs vary significantly with site-specific design requirements, sustainability analyses were performed for production costs, CO2 emissions, and energy consumption. The PG-FLS showed superior sustainability. Therefore, PG-FLS is a promising eco-friendly option for subgrade engineering because it promotes waste utilization and assists in carbon neutrality.