Phosphogypsum (PG) and fly ash (FA), waste products of industrial processes, serve as potential alternative materials for geotechnical applications. However, the major challenges with these geomaterials are the strength loss under adverse environmental conditions and the presence of heavy metals and sulfates, particularly in raw PG. Although there are studies on the strength evaluation of PG-FA mixtures, the studies on durability and sulfate management are limited. Thus, this study investigates PG-FA mixes activated with 2% of lime and 1-molar sodium hydroxide (1M-NaOH), focusing on their strength, durability, and environmental performance for road/rail subgrade and subbase layers. The mixes, comprising 38% to 70% PG and 30% to 60% FA, achieved unconfined compressive strength between 0.75 and 1.5 MPa and California bearing ratios exceeding 7%, thus satisfying Indian Road Congress specifications. Notably, all PG-FA mixes with 2% lime along with direct exposure curing, and the 70% PG-30% FA mix activated with 1M NaOH (70PG30FA1MNaOH) retained strength above 1.5 MPa and exhibited weight loss below 14% after 12 wetting–drying cycles, demonstrating good durability. Leaching tests confirmed that most of the reported trace elements in the PG-based mixes complied with the European Commission (EC) inert or nonhazardous waste categories defined by the EC landfill acceptance limits; however, antimony, fluoride ions, and sulfate ions slightly exceeded inert waste thresholds but remained within nonhazardous waste limits. The inclusion of lime and NaOH effectively reduced soluble sulfate concentrations below the 3,000 mg/L threshold set by AASHTO standard, thereby minimizing the risk of sulfate-induced expansion. Mineralogical and microstructural analyses confirmed the formation of various cementitious compounds contributing to improved mechanical and durability performance. These findings highlight the potential of PG-based mixes as durable, environmentally compliant materials for conventional subgrade and subbase applications, supporting industrial waste valorization.
Thakur et al. (Tue,) studied this question.