Effect of phosphorus and fluorine impurities on the properties and hydration mechanism of α-hemihydrate gypsum composite cementitious materials

The performance of α-hemihydrate phosphogypsum (α-HPG) as a cementitious material is significantly affected by surface-bound phosphorus and fluorine impurities. To identify the key impurities responsible for performance degradation, X-ray photoelectron spectroscopy (XPS) was employed to analyze the surface chemical composition of α-HPG, revealing the presence of NaF, Na 2 SiF 6 , H 3 PO 4 , and Ca 3 (PO 4 ) 2 . Based on these findings, intrinsic impurities were effectively removed via recrystallization, and the purified α-HPG was used to prepare α-HPG-based composite cementitious materials (α-HPGM). Subsequently, the effects of different types and dosages of these impurities on the setting behavior, compressive strength, softening coefficient, and hydration mechanism of α-HPGM were systematically investigated. Results indicate that recrystallization markedly enhances the inherent reactivity of α-HPG. The addition of NaF, Na 2 SiF 6 , and Ca 3 (PO 4 ) 2 significantly shortened the setting time and improved early compressive strength, whereas H 3 PO 4 exhibited a pronounced retarding effect. Compressive strength initially increased and then decreased with NaF content, peaking at 0.4%, while a dosage of 1% Na 2 SiF 6 yielded the highest long-term strength at 28 days (23.97 MPa). Microstructural analysis confirmed that fluorine-containing impurities promote the formation and distribution of ettringite and C-(A)-S-H gels, thereby enhancing structural compactness and mechanical performance. In contrast, phosphorus-containing impurities inhibited hydration reactions, leading to increased porosity and reduced strength. Notably, in the highly alkaline environment provided by added quicklime, an appropriate amount of H 3 PO 4 facilitates the formation of hydroxyapatite, which contributes to improved structural stability and densification, partially mitigating the inhibitory effect on hydration.