The long-term strength development of high-sulfur belite-calcium sulfoaluminate (high-sulfur BCSA) cement remains constrained by the low hydration reactivity of β-dicalcium silicate (β-C₂S) and the difficulty in obtaining its high-temperature active polymorphs. Ionic doping, which activates and stabilizes highly cementitious dicalcium silicate/calcium sulfoaluminate (C₄A₃$) crystalline phases while inducing lattice distortion, has been proven as an effective approach to enhance mineral reactivity.

With the depletion of high-grade limestone resources, the utilization of low-grade magnesian limestone and phosphorus-containing solid wastes, such as phosphogypsum and phosphorus slag, for producing high-sulfur BCSA has become an inevitable trend. This leads to the unavoidable coexistence of Mg²⁺/P⁵⁺ in the clinker mineral system. However, existing studies have primarily focused on the doping effects of individual ions, while the synergistic influence and underlying mechanisms of magnesium/phosphorus components in the quaternary clinker system (C₂S-C₄A₃$-C₄AF-CaSO₄) on sintering behavior and performance remain poorly understood.

In this study, MgCO₃ and Ca₃(PO₄)₂ were employed as the sources of MgO and P₂O₅ doping, respectively, to systematically investigate the effects of Mg/P co-doping on the phase composition, crystal structure evolution, and mechanical properties of high-sulfur BCSA clinker, along with the associated mechanisms. The findings not only provide a theoretical foundation for optimizing the strength development of high-sulfur BCSA cement but also establish a technical basis for the large-scale, high-value utilization of Mg/P-bearing industrial solid wastes.

The designed mineral composition of high-sulfur BCSA clinker was as follows (mass fraction): 50 % C₂S, 30% C₄A₃$, 7% C₄AF, and 13% f-CaSO₄. The raw meal samples were prepared using industrial raw materials and analytical-grade chemical reagents with varying Mg/P doping levels. Specifically, MgO was incorporated at 0.6%, 1.0%, 1.5%, 3.0%, and 5.0%, while P₂O₅ was added at 0.1%, 0.4%, 0.7%, 1.0%, and 1.3%. The raw mixes were fired at 1320 ℃ for 60 min to obtain clinker samples.

The effects of Mg/P co-doping on the sintering behavior and properties of high-sulfur BCSA clinker were investigated through X-ray diffraction combined with Rietveld refinement, scanning electron microscopy, and mechanical strength testing.

The Mg/P co-doping exhibits a significant synergistic effect, with an appropriate doping ratio effectively enhancing the content of highly reactive mineral phases, including cubic-calcium sulfoaluminate (c-C₄A₃$) and α'-dicalcium silicate (α'-C₂S). P₂O₅ demonstrates a favorable stabilizing effect on high-activity crystalline phases such as α'-C₂S and c-C₄A₃$, whereas MgO shows an adverse influence, particularly when the P₂O₅ content is low (P₂O₅ ≤ 0.4%). The maximum α'-C₂S formation in clinker is achieved at a doping level of 1% MgO and 1.3% P₂O₅. Moreover, Mg/P co-doping reduces the grain size of C₂S and C₄A₃$ minerals. However, excessive P₂O₅ doping 1.3% leads to noticeable melting corrosion, resulting in decreased crystallinity.

At low doping levels [w(MgO) ≤ 1.5%, w(P₂O₅) ≤ 0.7%], the 3 d compressive strength improves while the 7 d strength remains stable, with all samples showing varying degrees of enhancement in 28 d compressive strength, though excessive MgO doping leads to periclase formation and significant strength reduction. The 3 d and 28 d compressive strengths exhibit linear correlations with the total contents of C₄A₃$ and C₂S, respectively, with appropriate Mg/P doping promoting C₂S formation, particularly at 1.5% MgO and 0.4% P₂O₅, which yields the highest C₂S content and achieves a 28 d strength of 74.5 MPa.

Optimal Mg/P co-doping effectively reduces the decomposition temperature of CaCO₃ while enhancing its decomposition rate, and lowers the formation temperature of intermediate minerals C₅S₂$ and C₂S within the 900-1320 ℃ range, thereby promoting the formation of clinker minerals like C₂S. Furthermore, Mg/P doping demonstrates significant synergistic effects: P⁵⁺ selectively substitutes calcium and aluminum/silicon sites, stabilizing reactive phases such as α'-C₂S and c-C₄A₃$. As P₂O₅ content increases from 0.4% to 1.3%, the concentrations of α'-C₂S and c-C₄A₃$ markedly rise. In contrast, Mg²⁺ competes with P⁵⁺ for calcium sites, leading to reduced α'-C₂S and c-C₄A₃$ content with increasing MgO doping. The highest relative α'-C₂S content (51.5%) is achieved at 1% MgO and 1.3% P₂O₅ doping levels.

The 28 d compressive strength of doped clinker was improvd to varying degrees, initially increasing with MgO content but declining when excessive MgO forms periclase. Similarly, strength first rises then falls with increasing P₂O₅ content. Both 3 d and 28 d compressive strengths exhibit linear positive correlations with the total contents of C₄A₃$ and C₂S, respectively. Maximum C₂S formation and peak 28 d strength (74.5 MPa) occur at 1.5% MgO and 0.4% P₂O₅ doping. Comprehensive analysis of mineral formation, crystal phases, and performance reveals an optimal doping range: w(MgO)<3% and w(P₂O₅) 0.3%-1.3%, which balances clinker sintering efficiency and mechanical properties.