Thermal stability and microstructural changes in 5 years aged cement paste subjected to high temperature plateaus up to 1000 °C as studied by thermal analysis and X-ray diffraction

Abstract

This paper studies the thermal effects on the mineralogical properties as well as the microstructure of 5 year-aged cement pastes, unprotected from contact with the air, heated up to various temperature up to 1000 °C in steps of 100 °C for a constant period of 6 h by studying the TGA/DTG and XRD curves. In order to simulate a real situation of a fire, in this research, the state of the specimens is in a block and the cooling regime is an air-cooling regime. The results show that ageing of the cement paste induces higher carbonation of the cement paste and higher uptake of humidity. The new portlandite formed during cooling and the decarbonation reaction continue to exist up to 1000 °C and 800 °C temperature plateaus, respectively. XRD tests show the presence of the C-S-H gel for all temperature plateaus and the coexistence with its products β-C2S and C3S beyond the 500 °C temperature plateau. In order to proof the effect of ageing, the state of samples and the impact of the cooling regime in the thermal stability and in the microstructure of the cement paste after being subjected to high temperatures, a comparison has been made between the results of powdered cement paste aged for 2 years and cooled in air and the results of blocks of cement paste, also aged for 2 years, but cooled at the desiccator.