Response of alkali activated low calcium fly-ash based geopolymer concrete under compressive load at elevated temperatures

Abstract

For geopolymer cement (GPC) concrete to become a viable building material in the main stream construction industry, reliable stress–strain curves need to be established. This paper presents the stress–strain curves for GPC concrete at ambient and elevated temperatures of up to 800 °C. Prediction models for capturing the stress–strain response of GPC concrete at ambient temperatures, based on widely accepted OPC models, are also presented here. At high temperatures testing only covered one type of temperature-load history; that is: samples were heated up to test temperatures then loaded to failure under displacement control. Between 20 and 200 °C all the tested samples underwent a decrease in strength. However, samples tested between 200 and 400 °C manifested a moderate to significant gain in strength. At 800 °C all samples underwent a decrease in strength. The initial loss of strength may be attributed to the loss of water from the GPC concrete samples, which is supported by thermogravimetric analysis of geopolymer samples. Between 200 and 400 °C, the increase in the compressive strength of all tested concrete mixtures is attributed to further geopolymerization, which has been proven by differential scanning calorimetry results. The loss of strength at 800 °C is attributed to possible disintegration of the geopolymer gel and formation of new phases within the geopolymer system.