Influence of repeated heating–cooling cycles and exposure duration on mechanical, electrical, and durability properties of geopolymer concrete

The growing emissions associated with cement production have necessitated the development and utilization of sustainable alternatives such as green concretes, notably geopolymers. Although numerous studies have investigated the effects of elevated temperatures on concrete, the compressive behavior of these materials under repeated heating–cooling cycles, particularly in high-temperature industrial and infrastructural applications, has rarely been addressed. In the present study, the influence of successive thermal cycling on the performance of geopolymer concrete incorporating ground granulated blast furnace slag (GGBFS) and fly ash (FA) was examined and compared with that of conventional concrete. The evaluated variables included the type of concrete (RC as the reference mix, G 1 with 100% GGBFS, and G 2 with a blend of 60% GGBFS and 40% FA, all designed with the same binder content), the number of thermal cycles (1, 3, and 6 cycles), exposure soaking time (2, 4, and 6 hours), and target temperatures (20, 300, and 600˚C). The performance indicators comprised compressive strength, electrical resistivity, water absorption, and porosity of the concrete specimens. The results revealed that the reduction in compressive strength due to thermal exposure was not significantly influenced by different soaking times. This trend was consistently observed in both geopolymer and conventional concretes across all thermal cycling regimes. At elevated temperatures, conventional concrete generally exhibited a lower loss in compressive strength compared to geopolymer concretes, and this trend remained consistent across different soaking times and thermal cycles. Furthermore, among the geopolymer mixes, the specimen containing only GGBFS demonstrated superior compressive strength at ambient conditions relative to the blend of GGBFS and FA; however, this behavior was completely reversed at higher temperatures. The influence of exposure soaking time and the number of thermal cycles on the reduction of electrical resistivity in both geopolymer and conventional concretes was found to be minor, with an average impact of less than 10%. By changing the concrete type from normal concrete to geopolymer concrete, the average electrical resistivity (considering all thermal cycles and soaking times) decreased by 78, 70, and 50% at ambient temperature, 300, and 600˚C, respectively. Water absorption and porosity in ordinary concrete were lower than those of geopolymer concretes under ambient conditions, but became higher at elevated temperatures. In addition, the effects of soaking time and the number of thermal cycles on these two parameters were more pronounced in geopolymer concretes than in conventional concrete. Finally, by applying nonlinear regression to the experimental dataset, high-accuracy predictive equations were developed to estimate the compressive strength of both geopolymer and conventional concretes as a function of temperature, number of thermal cycles, and soaking time. • The effect of repeated thermal cycles on performance of geopolymer and conventional concretes was investigated. • Conventional concrete exhibited lower compressive strength loss at high temperatures than geopolymer concretes, regardless of soaking time or thermal cycles. • GGBFS-based concrete outperformed FA-GGBFS mix at ambient conditions, but showed inferior strength retention at elevated temperatures. • Water absorption and porosity were initially higher in geopolymer concretes, but conventional concrete surpassed them under elevated temperature exposure. • Accurate models were developed to predict compressive strength as a function of temperature, thermal cycles, and soaking time.