The sulfate source in calcium sulfoaluminate (CSA) cement is an important factor that determines its hydration characteristics, and the properties of CSA cement can be controlled via adjusting the molar ratio of calcium sulfate to ye'elimite (generally referred to as the "M-value"). Sulphosilicate cement is a novel cementitious material developed via incorporating ternesite (C₅S₂＄) and free-calcium sulfate (f-C＄) into the mineral composition of CSA cement. The calculation of the M-value must comprehensively account for their contents in the clinker due to the presence of SO₃ in C₅S₂＄ and f-C＄. This study optimized the conventional calculation formula for the M-value and systematically investigated its influence on the compressive strength and expansion behavior of sulphosilicate cement. In addition, the mechanism underlying the effect of anhydrite dosage on cement hydration was also elucidated through the evolution of hydration products.

In this experiment, sulphosilicate clinker was synthesized with limestone, fly ash, phosphogypsum, and bauxite as raw materials. The raw materials were calcined at 1250 ℃ for 1 h, followed by rapid air quenching to obtain the clinker. Five groups of sulphosilicate cement with M-values of 0, 0.9, 1.8, 2.7, and 3.6 were designed. Mortar specimens were prepared to evaluate compressive strength and expansion rate, while paste specimens were prepared for analyzing hydration product characteristics at specific ages. The hydration heat release was measured by an isothermal calorimeter. The phase compositions and microstructure of the hydration products were analyzed by X-ray diffractometer, thermal behavior analyzer and field-emission environmental scanning electron microscope. The pore structure was performed by a mercury intrusion porosimeter.

Among the various sulphoaluminate cement groups, the M_1.8 specimen demonstrates the maximum compressive strength of 43.9 MPa after 3-d hydration. As hydration progresses, the compressive strength of the M_2.7 and M_3.6 groups shows a significant growth, with the M_3.6 group reaching 56.7 MPa at 14 d. However, the M_3.6 specimen exhibits a notable strength retrogression, which, is decreased by 4.3 MPa, after 28-d hydration. In contrast, the compressive strength of the M_0.9 group increases markedly from 14 d to 28 d, with an improvement of 12.3 MPa.

For specimens with the M-value below 1, the early hydration products at 3 d are dominated by AFm phases with insufficient AFt content, resulting in an initial shrinkage. As hydration progresses, the dissolution of SO₃ from anhydrite and C₅S₂＄ promotes AFt formation, leading to a measurable expansion after 7 d. At low M-values, AFt contributes to pore refinement, while the reaction of C₅S₂＄ and C₂S in later stages generates C- (A)-S-H gel and stratlingite (C₂ASH₈), refining the microstructure of hydration products and enhancing long-term strength. In contrast, specimens with higher M-values (i.e., 1.8, 2.7, and 3.6) exhibit expansion as early as 3 d. Increased anhydrite content and prolonged hydration amplify the crystallographic stress exerted by AFt crystals on the surrounding matrix, causing coarsening of pores, and elevated total porosity, and resulting in strength retrogression during 14–28 d of curing.

This study was to propose a formula for calculating the gypsum coefficient M-value in sulphosilicate cement, i.e., C_G = 0.13 × M × (Y-A×2.24-T×1.27)/S_g, establishing the relationship between gypsum dosage and cement performance, while elucidating the influence of M-values on the mechanical properties and hydration mechanisms. The experimental results demonstrated that within the optimal M-range of 0.9–1.8, cement could achieve continuous strength development for all curing ages. Higher M-values (2.7–3.6) enhanced mid-term strength but induced strength regression at 28 d. Increasing M-values effectively suppressed early-age shrinkage and induced progressive expansion, and expansion rates were positively correlated to M-values at all hydration stages. Hydration products primarily consisted of AFt, AH₃, and C-(A)-S-H gel. At M₀ and M_0.9, AFm phases formed during early hydration, followed by C₂ASH₈ in later stages. Conversely, M-values of ≥1.8 accelerated C₄A₃＄ hydration but inhibited both C₅S₂＄ dissolution and C₂ASH₈ formation. Microstructurally, M_0.9 specimens exhibited pore refinement and reduced porosity through gel-phase densification, enhancing strength development, whereas M_3.6 samples suffered from coarse pore proliferation due to excessive anhydrite content promoting oversized AFt crystals, ultimately degrading 28-d strength. At the M-value of 0.9–1.8, the system achieved balanced high strength and minimal volumetric deformation, making it suitable for general engineering applications. Elevating the M-value to 1.8–2.7 induced a controlled expansion behavior, enabling targeted use in shrinkage compensation scenarios. However, at the M value of 3.6, delayed strength retrogression and microstructural deterioration could occur, thus establishing 3.6 as a critical upper limit for M-values.

The existing cement clinker systems have some challenges. The production process of calcium sulfoaluminate (CSA) clinker, which primarily contains mineral C₄A₃＄, requires a significant amount of high-quality bauxite resources, thus leading to a higher production cost. Compared to CSA, a high belite calcium sulfoaluminate (BCSA) cement reduces the demand for high-grade bauxite during production, but has a slower strength development over time. To ensure the synergistic development of both early age performance and strength development in clinker systems, calcium sulfoaluminate-modified Portland cement (designated as S.M.P.) is further developed. However, alite (C₃S) remains a dominant mineral, resulting in relatively high carbon emissions.

For the compositional and performance characteristics of S.M.P. and BCSA clinker, this study proposed a new system for preparing a low-calcium calcium sulfoaluminate-modified Portland cement clinker with belite (C₂S) as a dominant mineral (i.e., C₃S/C₂S–C₄A₃S–C₄AF–CaSO₄).

This study also utilized minerals such as C₃S and C₂S from Portland cement clinker as crystal seed additives. The effect of adding Portland cement clinker on the sintering process and the mineral composition structure of the low-calcium calcium sulfoaluminate-modified Portland cement clinker was investigated. In addition, the underlying mechanisms were explored to provide a theoretical and technical guidance for the design and development of low-calcium cement clinkers.

The designed mineral composition of the calcium sulfoaluminate-modified low-calcium Portland cement clinker was 45%–50% (in mass, the same below) of C₂S, 5%–10% of C₃S, 25% of C₄A₃＄, 10% of C₄AF, and 10% of f-CaSO₄. Calcium sulfoaluminate-modified low-calcium Portland raw meals were prepared with industrial raw materials. Portland cement clinker was used as a crystal seed at different incorporation contents of 0%, 1%, 3%, 5%, 10%, and 12%.

The mineral composition and microstructure of calcium sulfoaluminate-modified low-calcium Portland cement clinker were determined by in-situ high-temperature X-ray diffraction (XRD), linear shrinkage measurement, free lime titration, scanning electron microscopy (SEM) and thermal analysis.

The linear shrinkage of the samples generally increases gradually with the increase in clinker addition percentage from 0% to 10%. The linear shrinkage remains relatively unchanged at >12% of clinker additions. The XRD patterns of the synthesized clinker samples within the 20° to 51° range reveal that the diffraction peak intensity of β-C₂S gradually increases, while the intensity of α′-C₂S and C₄A₃＄ peaks decreases, and no significant diffraction peaks for C₃S appear as the clinker crystal seed content increases.

The analysis by rietveld whole-pattern fitting indicates that the addition of Portland cement clinker seeds does not lead to a significant increase in C₃S content. There is a notable increase in C₂S content and a gradual decrease in C₄A₃＄ content, accompanied by a relative increase in the iron phase content.

The high-temperature in-situ XRD patterns indicate that at 950 ℃, an original C₃S diffraction peak disappears, instead of relatively weak C₂S diffraction peaks at approximately 27.5° and 38.0°. This indicates that C₃S decomposes into C₂S and CaO at 950 ℃.

According to the analysis by backscattered electron–energy dispersive spectroscopy (BSE–EDS), the solid solubility of aluminum, sulfur and iron in C₂S increases with increasing the addition of clinker crystal seeds, thus favoring the stability of β-C₂S. Furthermore, the presence of sulfur impedes a reaction between C₂S and CaO, thus inhibiting the formation of C₃S. An increase in the iron phase content and the Al/Fe ratio within this phase, coupled with enhanced Al solubility in C₂S, reduces the amount of Al available for sulfate reaction.

The incorporation of Portand cement clinker seeds facilitated the formation of liquid phases, favoring the liquid-phase sintering of clinker and enhancing the densification of the microstructure of the synthesized clinker samples. This incorporation also increased the grain sizes of C₂S and C₄A₃＄. The introduction of clinker seeds lowered the decomposition temperature of CaCO₃, promoting the formation of C₂S and C₄AF. Consequently, the contents of C₂S and the iron phase increased sulfur-aluminous modified low-calcium Portand cement clinker. However, the decomposition of C₃S in clinker crystal seeds at lower temperatures prevented an effective promotion of C₃S formation alongside C₄A₃＄.

The content of the iron phase and the Al/Fe ratio increased with increasing the addition of clinker crystal seeds. Increased Al solubility in C₂S led to a reduction in the Al available for sulfate reactions, thereby decreasing the formation of C₄A₃＄. This effect was particularly pronounced when the clinker crystal seed content reached 10%, resulting in a significant reduction in C₄A₃＄ content.

The inclusion of Portland cement clinker seeds led to a decrease in the content of highly reactive α′-C₂S and c-C₄A₃＄ in the clinker. As the clinker crystal seed content increased, this reduction becomes dominant. This was primarily due to an increase in the calcium-to-silicon ratio in C₂S and an increased dissolution of Al, Fe, and S in C₂S, stabilizing β-C₂S but destabilizing α′-C₂S, and was not conducive to C₃S formation. Concurrently, a decreased Fe dissolution in C₄A₃＄ favored the stabilization of orthorhombic C₄A₃＄ rather than cubic C₄A₃＄.