Effect of superplasticizers on the properties of II-anhydrite calcined from phosphogypsum

Calcining phosphogypsum to produce II-anhydrite represents a highly promising route for its resource utilization; however, the effects of superplasticizers on anhydrite systems—particularly the interactions between superplasticizers and anhydrite—remain unclear, despite their extensive investigation in cement and hemihydrate gypsum systems. This study aims to evaluate the superplasticizers, namely polycarboxylate acid (PC) superplasticizer, sulfonated naphthalene formaldehyde (SNF), and sulfonated melamine formaldehyde (SMF), on such properties of anhydrite prepared from phosphogypsum as workability, hydration process, physical and mechanical properties, microstructure, and adsorption behaviour, while also first clarifying the correlation between superplasticizers adsorption characteristics and anhydrite hydration regulation. The results show that PC, SNF, and SMF significantly improved the fluidity, and contributed to 213.52 %, 185.75 % and 448.05 % enhancement in the compressive strength at the optimum content of 0.15 %,1.5 %, and 1.5 %, respectively, which are determined by the minimum water requirement for normal consistency. PC retarded the hydration reaction of anhydrite and decreased the hydration degree, leading to the formation of short columnar hydration products with obvious pores, while SMF accelerated this hydration and enhanced the hydration degree, resulting in a dense microstructure with much lower porosity. PC undergoes chemical adsorption on the anhydrite surface, whereas SNF and SMF are primarily physically adsorbed. This work further establishes the quantitative relationship between superplasticizers dispersion performance and anhydrite macro/micro properties, and proposes dosage and performance-adaptive superplasticizers selection strategies for anhydrite systems.