Strength and Ultrasonic Characteristics of Alkali-Activated Fly Ash-Slag Geopolymer Concrete after Exposure to Elevated Temperatures: Fid-Bau Portal

Strength and Ultrasonic Characteristics of Alkali-Activated Fly Ash-Slag Geopolymer Concrete after Exposure to Elevated Temperatures

Ren, Weibo / Xu, Jinyu / Bai, Erlei

Abstract

Geopolymer concrete is an environment-friendly building material which has attracted increasing interest in many fields as a replacement for conventional concrete. The main objective of this paper is to study the residual behaviors of the alkali-activated ﬂy ash–slag geopolymer concrete (FSGC) under different heating temperatures and cooling regimes. Changes in weight, compressive strength and ultrasonic pulse velocity were firstly investigated. Then the wavelet packet technique was adopted to further analyze the measured ultrasonic signals in frequency domain. Finally, the microstructures were investigated by scanning electron microscopy. The results indicate that an increase in temperature makes the weight, compressive strength, and ultrasonic pulse velocity value of FSGC decrease, especially under extreme heat conditions. The deterioration of specimens cooled by watering is more obvious than that of specimens cooled naturally. Based on the experimental results, it can be concluded that the critical temperature for FSGC is 600°C, but this temperature may change with the contents of fly ash and slag. Also, with the increase of temperature and specimen damage, the higher frequency components of the signal attenuate steadily and signal energy stored in lower frequency bands increases. A wavelet packet energy spectrum-based index is sensitive to thermal damage, which can be used as a supplementary indicator to assess the damage degree of the heated FSGC. After high temperature exposure, FSGC undergoes distinct morphological destruction as confirmed by the developments of microcracks and the damaged aluminosilicate gel matrix.