Hydration Mechanism And Microstructural Evolution Of Ye’elimite-gypsum-calcium Oxide System

Based on the national strategic objective of "double carbon".The design and promotion of low carbon cements presents an unprecedented opportunity and challenge,as the high carbon emissions of the cement industry need to be addressed.High belite sulphoaluminate cements is a new type of low carbon cement that contain high levels of the low-carbon mineral belite phase.However,the reduced of the highly reactive ye’elimite mineral in high belite sulphoaluminate cements has resulted in slow overall hydration of the cement,resulting in low early and late strengths that are difficult to meet service requirements.By introducing free calcium oxide into high belite sulphoaluminate cements,which interacts with ye’elimite and gypsum in the clinker,the cement hydration rate can be increased and strength development promoted,which can effectively solve the problems of slow hydration rate and low strength of ordinary high belite sulphoaluminate cements.However,if the free calcium oxide content is not properly controlled,the cement is prone to problems such as flash setting or excessive expansion,which adversely affects the mechanical properties and volumetric stability of the cement stone.By studying the hydration mechanism of ye’elimite,gypsum and calcium oxide,and establishing the link between hydration products,microstructure and macroscopic properties,the efficient use of mechanisms of ye’elimite and the scientific use of calcium oxide can be explored to promote the sustainable development of the sulphate aluminate cement industry.In this study,the hydration mechanism and microstructure of the ternary system were investigated by adjusting the initial molar values of each raw material in the ye’elimite-gypsum-calcium oxide ternary system,using the molar ratio of calcium oxide to gypsum,the molar ratio of gypsum to ye’elimite and the molar ratio of calcium oxide to ye’elimite as variables.The theoretical model of the microstructure evolution of the ternary system was established through theoretical calculations and validated using test results,and the matching relationship between the hydration products,microstructure and macroscopic properties in the ternary system was investigated.The study of the ternary system was used as a theoretical guide for the optimal design of high belite sulphoaluminate cements,and the key mechanism of the role of calcium oxide in the strength development of cement stone was explored.The main conclusions drawn from this study are as follows.(1)In the ternary system,the presence of gypsum and calcium oxide allows for the efficient use of ye’elimite.With a small increase in the content of gypsum and calcium oxide,the degree of hydration of ye’elimite increases,the amount of ettringite production increases,the porosity decreases and the compressive strength of the test piece increases.(2)As the volume ratio of crystals to gel in the hydration product and the expansion rate of the hydration product β value increase,the compressive strength of the specimens both increase and then decrease.(3)In the high belite sulphoaluminate cement system,the lower calcium oxide content is favorable to the early strength development of the cement stone and the higher calcium oxide content is favorable to the later strength development of the cement stone.