Effects of high temperatures on mortar specimens containing Portland cement and GGBFS

During a fire, concrete is submitted to very high temperatures which can lead to structural collapse. The aim of this research is to study the behaviour of mortar specimens made of Portland cement and ground granulated blast furnace slag (GGBFS) which are heated to 900 °C. The samples are submitted to compressive and flexural strength tests and the microstructure is analysed by thermogravimetric analysis (TG and DTA), analysis and mercury intrusion porosimetry. The results indicate that a substitution rate of Portland cement by GGBFS of over 50 % gives residual compressive strengths that, after heating to 600 °C, are close to initial strengths before heating. The greater quantities of bound water, an acceleration of the hydration process due to heating and a lower porosity for specimens with GGBFS explain this increase. At 900 °C, the strong degradation of the microstructure results in strengths of less than 20 % of the initial strengths for specimens with or without GGBFS. Also, after cooling, the GGBFS reduces lime rehydration because of the decomposition of the portlandite between 450 and 550 °C, thus avoiding the risk of concrete cracking.