Deterioration of ambient-cured and heat-cured fly ash geopolymer concrete by high temperature exposure and prediction of its residual compressive strength

This paper presents an investigation on the performance of ambient-cured and heat-cured low-calcium fly ash geopolymer concrete after exposure to elevated temperatures. The concrete specimens were heated to 100, 200, 400, 600, 800 and 1000 degrees C at a heating rate of 5 degrees C/min. The effect of high temperature exposure on geopolymer concrete was studied by visual inspection, mass loss, extent of cracking, residual strength and microstructure investigation. The total length of cross-section cracks and surface cracks reached their peak values at 800 degrees C and then declined at 1000 degrees C. The results show that all the concrete specimens could be heated at 600 degrees C for 2 h without strength loss. Heat-cured geopolymer concrete specimens showed higher residual compressive strengths than the ambient-cured specimens for all the exposure temperatures. A crushing index of 7.7% could be regarded as the threshold value for coarse aggregate to maintain the initial compressive strength of concrete up to 600 degrees C. Thus, the microstructural damage as observed by SEM images, dehydration of geopolymer shown by thermogravimetric analysis and the reduced strength of coarse aggregate are considered as the contributing factors for strength losses at temperatures higher than 600 degrees C. Finally, two prediction equations are proposed which correlated well with the experimental results of this study and those reported in published literature. (C) 2020 Published by Elsevier Ltd.