Effect of phosphate impurities on the properties of β-hemihydrate gypsum pastes plasticized by polycarboxylate superplasticizer

This study aims to investigate the effect of HPO 4 2- /PO 4 3- on the working performance and mechanical strength of beta-hemihydrate gypsum pastes containing polycarboxylate superplasticizer (PCE) and reveal its underlying mechanism. The results showed that HPO 4 2- /PO 4 3- has a detrimental effect on the fluidity, compressive strength, and softening coefficient of beta-hemihydrate gypsum pastes. However, in the presence of PCE, all properties were improved compared to the blank group when the HPO 4 2- /PO 4 3- content was less than 0.3 %. The fluidity and compressive strength of the pastes can be increased by up to 95 % and 32 %, respectively. It was found that HPO 4 2- /PO 4 3- reacted with Ca 2 + to form insoluble calcium phosphate salt (ICPS), and the adsorption of PCE on the surface of ICPS increased the total adsorption amount, thereby enhancing the dispersion ability of PCE and increasing the fluidity of fresh slurry. In addition, the adsorption behavior of PCE on the surface of ICPS weakened its inhibitory effect on the hydration of hemihydrate gypsum, which was confirmed by TG-DSC. The increased hydration products and reduced water requirements significantly improved mechanical properties. These findings enhance the understanding of the interaction between phosphorus impurities and PCE, which can guide the improvement of the performance of beta-HPG and promote the resource utilization of phosphogypsum.