ORIGINAL RESEARCH article
Front. Mater.
Sec. Mechanics of Materials
Effect of phosphorus and fluorine impurities on the properties and hydration mechanism of α-hemihydrate gypsum composite cementitious materials
Provisionally accepted
- 1Guizhou University, Guiyang, China
- 2School of Chemistry and Chemical Engineering, Guizhou University, Guiyang, China
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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, Na2SiF6, H3PO4, and Ca3(PO4)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, Na2SiF6, and Ca3(PO4)2 significantly shortened the setting time and improved early compressive strength, whereas H3PO4 exhibited a pronounced retarding effect. Compressive strength initially increased and then decreased with NaF content, peaking at 0.4%, while a dosage of 1% Na2SiF6 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 H3PO4 facilitates the formation of hydroxyapatite, which contributes to improved structural stability and densification, partially mitigating the inhibitory effect on hydration.
Keywords: hydration, microstructure, Soluble impurities, strength, α-hemihydrate Phosphogypsum
Received: 15 Nov 2025; Accepted: 05 Feb 2026.
Copyright: © 2026 Li, Li, Yang and Cao. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
* Correspondence: Jianxin Cao
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