Influence of H3PO4 and H2PO4- on the performance of PCE in hemihydrate gypsum pastes

Phosphorus impurities are the main factor causing the performance deterioration of beta-hemihydrate phospho-gypsum (beta-HPG), and polycarboxylate superplasticizer (PCE) is a common admixture to improve beta-HPG properties. Revealing the interaction mechanism between phosphorus impurities and PCE is significant for efficiently utilizing beta-HPG. This study systematically investigated the influence of H3PO4 and H2PO4- on the performance of PCE in hemihydrate gypsum pastes. The results indicate that H3PO4 and H2PO4 weakened the dispersion ability of PCE, prolonged the setting time of hemihydrate gypsum pastes, and deteriorated the mechanical properties. H3PO4 had the most significant adverse effect on the performance of PCE and the mechanical properties of hemihydrate gypsum, while H2PO4- had the most noticeable impact on the setting time. The conductivity, adsorption amount, and X-Ray photoelectron spectroscopy results showed that H3PO4 and H2PO4- formed an adsorption layer on the surface of hemihydrate gypsum through the interaction with Ca2+. Then, the adsorption layer interfered with the adsorption behavior of PCE, resulting in a decrease in its adsorption amount, which was responsible for the weak dispersion ability. In addition, the hydration process of hemihydrate gypsum was hindered due to the formation of the adsorption layer, leading to prolonged setting time and a low degree of hydration, contributing to the deterioration of mechanical properties.