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Abstract

In this doctoral thesis influence of different temperatures (5 °C - 60 °C) on the hydration of belite-ye’elimite-ferrite cement was studied. Fluctuations in temperature can cause changes in the phase assemblage, kinetics of hydration and mechanical properties. Besides temperature, other factors such as the cement clinker phase composition, calcium sulfate amount and the type of secondary raw material used in the raw meal affect the cement hydration, which was also studied in this work.
Cement clinkers with different phase compositions were synthesised from various natural and secondary raw materials. The cement was prepared by adding gypsum to the cement clinker based on the M-ratios selected. The kinetics of hydration was studied by isothermal calorimetry. The microstructure and phase composition of the hydrated cements at different hydration times were investigated by XRD, TGA, SEM/EDXS, FE/SEM, NMR, and TEM. The development of compressive strength of the cements was determined and porosity was assessed by MIP, while the rheological properties of cement pastes were investigated by rotational and oscillatory measurements. A thermodynamic model was established using GEMS and compared to the experimental data.
Elevated temperatures significantly accelerate early hydration. Ye’elimite and gypsum react rapidly at higher temperatures, leading to increased early compressive strength. The hydration of belite intensifies with temperature. Higher temperatures result in less ettringite due to its increased solubility, but more monosulfate. Higher temperatures also favor siliceous hydrogarnet over strätlingite and produce more C―S―H from belite, enhancing the compressive strength. At 60 °C, C―A―S―H is detected, as higher temperatures promote greater aluminum uptake from the solution. At 5°C, a dense, uniform microstructure is observed, whereas at higher temperatures it's more varied and intermixed. Although the chemical composition of ettringite and strätlingite remains unaffected by temperature fluctuations, it's observed that iron becomes more prominent at lower temperatures in siliceous hydrogarnet and C―S―H. At a low w/c, hydration of belite is slower at late ages, as ye’elimite consumes more water. In this case, the cements cured at 60 °C showed more residual gypsum, less ettringite and C―S―H at later ages, and lower strength, indicating a lower degree of hydration. More gypsum in the cement clinker accelerates hydration. A higher M-ratio increases ettringite and decreases monosulfate and strätlingite content, leading to a higher compressive strength. More belite produces a greater amount of siliceous hydrogarnet and C―S―H and less strätlingite. Cement with red mud shows accelerated hydration, due to more mayenite and alkali sulfates, compared to cement with natural raw materials and waste concrete. The impact of secondary raw materials on the hydration processes of belite and ferrite is noticeable at 5 °C but far less so at increased temperatures, while the impact on the hydration of ye’elimite is even smaller compared to belite. A rise in temperature leads to an enhanced storage modulus because of accelerated hydration, while its development is shifted to earlier times, due to the fast structural build-up formation at higher temperatures. The amount of ye’elimite increases viscosity, while the storage modulus is lower which was possibly attributed to its weaker gel structure.