The Influence of Gypsum Content on the Hydration and Properties of Belite-Ye’elimite-Ferrite (BYF) Clinker

Belite-ye’elimite-ferrite (BYF) cements are considered an environmentally friendly alternative to ordinary Portland cement due to their lower carbon dioxide (CO₂) emissions and reduced energy requirements. However, their hydration mechanism and its effect on strength development have still to be clarified. Therefore, this study aimed to investigate the influence of gypsum contents on the hydration behavior and strength development of laboratory-prepared BYF cement. The studied cements were made from a clinker, produced at 1250 ℃, containing approximately 50% of belite (C₂S), 30% of ye’elimite (C₄A₃$), and 20% of ferrite (C₄AF) as the main phases. Phosphorus pentoxide and lithium oxide were added to the clinker to stabilize the α’_H-C₂S. The phase assemblage of the cement pastes was characterized by quantitative X-ray diffraction using Rietveld refinement at different ages. TGA and DSC methods were used to confirm the XRD data and to characterize the main amorphous hydrate phases. The hydration kinetics was studied using isothermal calorimetry for seven days. The strength development of BYF mortars with a water-to-cement ratio of 0.50 was studied for up to 56 days. The results showed that the optimal content of gypsum was 18 wt % due to the higher strengths at all ages and higher heat generation during hydration. Increasing the amount of gypsum increased the content of ettringite. In contrast, the content of strätlingite, the main hydration product of belite, decreased, and the content of unreacted belite in the cement increased. This indicates that the amount of gypsum strongly affects the hydration rate of belite.