Study on the regulation of modified phosphogypsum's mechanical properties and volume stability by alkaline modifiers

Phosphogypsum (PG) is a predominant solid waste generated in phosphoric acid production, which brings about critical environmental issues due to long-term land disposal and secondary pollution. To advance the resource utilization of PG, this study evaluated its feasibility as an alternative to natural gravel in construction composites. However, the acidic impurities inherent in PG induce cracking and insufficient strength, imposing stringent limitations on its practical application. To address these bottlenecks, this paper systematically investigates the enhancement effect of alkaline modifiers (soda residue and calcium hydroxide (Ca(OH) 2 )) on the properties of phosphogypsum under the premise of flowability and setting time, and employing XRD, TGA, and SEM analyses, the mechanical attributes of the phophogypsum were systematically assessed, and its hydration process was elucidated. Experimental results demonstrate that alkaline modifiers significantly enhance the mechanical properties of phosphogypsum: specimens modified with Ca(OH)₂ exhibited 59.1%, 69.5%, and 102.2% higher compressive strength at 3, 7, and 28 days respectively, compared to unmodified phosphogypsum BN — where BN denotes field-collected phosphogypsum B dried at 120°C. Meanwhile, alkaline modification effectively suppresses the long-term expansion of PG, leading to a pronounced promotion in volume stability. A strong correlation between setting time and shrinkage behavior was also established, where shorter setting times coincide with lower shrinkage magnitudes and better dimensional stability. This study proposes a reliable technical approach for the efficient valorization of phosphogypsum, providing significant implications for promoting the green treatment and recycling of industrial solid waste.