# Utilization of Phosphogypsum and Sediment in Subgrade Material for Pavement Construction
## 1 Introduction  
Industrialization has undoubtedly driven significant economic growth globally, but it has also resulted in the generation of large quantities of waste that pose serious environmental risks [1]. Managing this waste presents a formidable challenge, particularly within critical industries such as the food sector. Fertilizer production plays a crucial role in maintaining agricultural productivity and ensuring food security. Fertilizers replenish the nutrients that crops deplete from the soil, and without them, agricultural yields would be severely impacted. The fertilizer production process begins with the extraction of phosphate rock (PR), which is then converted into phosphoric acid. Typically, about 85% of PR is processed using either wet or thermal methods. This results in the generation of approximately 4-5 tons of phosphogypsum (PG) as a by-product for every ton of phosphoric acid produced. The phosphoric acid is then used to produce fertilizers, which account for up to 95% of the total phosphate rock output [2; 3]. The characteristics of PG, including its chemical composition and mineralogical properties, are influenced by factors such as the type of phosphate rock used, the production process, the efficiency of the industrial process, and the chemical quality of reagents [4]. The nature of the phosphate ore--whether magmatic or sedimentary--affects the impurities found in the PG [5; 6]. Another pressing environmental issue is the pollution of sediments with heavy metals, a problem that is increasingly prevalent [7; 8]. These metals, due to their persistence, toxicity, and potential for bioaccumulation, pose long-term threats to ecosystems and human health [9; 10; 11; 12; 13]. Sediment removal from watercourses is often part of routine maintenance to mitigate flooding risks and maintain waterway functionality. However, it is also undertaken when significant aquatic degradation occurs, which could have adverse ecological effects [14; 15]. Heavy metals in aquatic systems are primarily bound to suspended particles and sediment, with only a small fraction existing as free ions in the water. These metals can become immobilized through processes such as coagulation, flocculation, or adsorption onto sediment surfaces and may form insoluble compounds like metal sulfides [16]. When restoring aquatic environments, sediment removal often necessitates remediation, especially if the sediment is heavily polluted [17]. To address these challenges, stabilization and solidification (S/S) technologies have emerged as effective methods for immobilizing hazardous constituents in contaminated materials [18; 19]. These methods involve mixing binder materials (e.g., Portland cement, lime, kaolin) with contaminated materials to reduce environmental risks associated with the mobility of harmful components. Through S/S, toxic constituents of waste are physically and chemically immobilized, thus minimizing their environmental threat and ensuring compliance with regulatory standards. Stabilized wastes, when adhering to regulations, may be suitable for controlled utilization in landfills or as construction materials for applications such as road subgrade and backfill. In recent years, various approaches have explored the recycling of PG in construction and building materials. PG has been used as a substitute for natural gypsum in producing plaster, wallboard, and cement [20]. The EU Renewable Energy Directive has highlightedthe importance of PG due to the shortage of natural and flue gas desulfurization (FGD) gypsum [21]. Although PG is more accessible and cost-effective than natural gypsum, and its use can help reduce the environmental impact of gypsum mining, several challenges persist. These include the presence of soluble phosphorus, fluorine, and organic compounds in PG, which may hinder its reuse in construction materials. The unsolved problems in this field include the environmental and logistical challenges of managing polluted sediments and the limited reuse options for PG due to its chemical composition.