Study on mechanical properties and expansion inhibition of high-volume modified phosphogypsum road base materials under high humidity environment

Phosphogypsum tends to undergo significant expansion strain in high-humidity environments, which negatively impacts its application in road engineering. In this study, phosphogypsum-based road materials were modified by incorporating various dosages of sodium hexametaphosphate and polycarboxylate water reducer. The mechanical properties, expansion strain, and microstructure of the materials were evaluated under high humidity conditions through both macroscopic and microscopic characterizations. The results indicate that: (1) As the dosage of sodium hexametaphosphate increases, the strength initially rises and then decreases. The highest strength was observed with 0.75% sodium hexametaphosphate, achieving 5.91 MPa at 7 days and 8.98 MPa at 28 days. (2) With increasing dosages of polycarboxylate water reducer, the strength progressively improves. When 3% polycarboxylate water reducer is added, the strengths at 7 days and 28 days reach 4.34 MPa and 4.89 MPa, respectively. (3) Under high-humidity conditions, the addition of sodium hexametaphosphate and polycarboxylate superplasticizer to the specimens promoted the formation of stable complexes, effectively reducing the concentration of free Ca²⁺ ions. This process inhibited the formation of ettringite, thereby significantly suppressing the expansion strain of the specimens. As a result, the stability of phosphogypsum-based materials in high-humidity environments was notably improved. These findings provide a novel theoretical framework for the resource utilization of phosphogypsum solid waste and offer new perspectives on its application in road construction. With advancing technology and further research, the potential applications of phosphogypsum are expected to expand, unlocking even greater possibilities.