Effect of sulfates on hydration and properties of belite-rich cement paste

It is common practice to use gypsum in cement production to control setting time. Traditionally it is believed that sulfate from gypsum combines with aluminates to form ettringite and monosulfate to prevent flash set. However, previous studies have confirmed that sulfates also affect the hydration and mechanical properties of pure alite paste. In view of recent research and development of cements with a low carbon footprint that are rich in belite, the question about the effect of sulfate on hydration and mechanical properties of belite arises. Moreover, this question becomes even more relevant considering the use of raw materials and fuels containing high content of sulfate-bearing impurities. In this work, the influence of the sulfate content on the hydration of belite-rich cement was studied by means of isothermal calorimetry and X-ray diffraction. The effect of sulfate content on compressive strength was also investigated. In the course of this study, an increase in hydration degree of belite with an increase of sulfate content in cement was observed. The compressive strength also increased with the addition of sulfate. The increase in hydration degree of belite and compressive strength of belite-rich paste was associated with the incorporation of sulfate in the cementitious gel as opposed to the ettringite formation.