Valorization of phosphogypsum into sustainable binder in grouting engineering: Effect of polyacrylamide on slurry stability, leaching behaviour, hydration and performance

Abstract

A newly discovered phosphogypsum-based excess-sulphate slag cement (PESSC), characterised by its multicomponent composition and diverse physical properties, exhibits sluggish crystallisation and hydration, resulting in inadequate slurry stability and potential toxic leaching. A water-soluble polyacrylamide polymer (PAM) effectively retained water, and adjusted the flocculent structure of PESSC, while influencing the hydration process due to its hydrolysis. Comprehensively, this study explored the slurry stability and macroscopic properties of PESSC with five levels of PAM content, assessing its practical engineering feasibility and providing rationales for selected concentrations. A significant increase in plastic viscosity and thixotropy of PESSC slurry was detected after the addition of PAM through the formation of a three-dimensional cementitious network which inhibited the water transfer and gypsum dissolution. Correspondingly, the bleeding of modified PESSC was inhibited accompanied by the advance of maximum bleeding ratio, showing greater slurry stability. However, PAM hydrolysis had an adverse influence on the initial hydration of fresh PESSC paste as the weakening of pH value development and product precipitation, but decreased the leaching toxicity. With further hydration, Ca²⁺ began to link with both the end and side groups of PAM and gradually established a connection between hydrates and PAM, enhancing the mechanical strengths. PESSC/PAM composite exhibited an impressive increase in hydration degree, where adding PAM increased both the MCL and Al/Si of crosslinked C-(A)-S-H gels. Modifying PESSC with PAM within 0.5 ‰ is deemed to meet performance and environmental requirements in engineering.