Effect of GGBFS and Fly Ash on Elevated Temperature Resistance of Pumice-Based Geopolymers

The current study investigated the effects of geopolymer composites formulated from pumice dust partially replaced by ground granulated blast furnace slag (GGBFS) and fly ash (FA) at levels of 10%, 20%, 30%, and 40% by weight. The mixtures were evaluated for flowability, compressive strength (7, 28, and 56 days), density, and water absorption (28 and 56 days) at ambient temperatures. Moreover, compressive strength, mass loss, density, and water absorption were evaluated after exposure of the mixtures to elevated temperatures (250 °C, 500 °C, and 750 °C) at 28 days. All specimens were initially cured at 60 °C for 24 h, followed by storage under ambient laboratory conditions until testing. The inclusion of GGBFS into the mixtures decreased flowability, and the inclusion of FA resulted in its improvement. At ambient temperature, GGBFS-based mixtures, which were high in calcium content, exhibited substantially superior compressive strength and reduced absorption relative to FA-based mixtures due to the development of dense C-A-S-H gel networks. However, the compressive strength of FA-based mixtures considerably increased when exposed to a temperature of 250 °C. Moreover, at 750 °C, the FA-based mixtures showed superior residual strength (up to 18.1 MPa), lower mass loss, and reduced absorption, indicating enhanced thermal stability due to the dominance of thermally resistant N-A-S-H gels. X-ray diffraction results further supported these trends by showing the rapid deterioration of calcium-rich phases under heat and the comparative stability of aluminosilicate structures in FA-based systems. Overall, the inclusion of up to 40% GGBFS is beneficial for early strength and densification, whereas the incorporation of up to 40% FA improves durability and mechanical retention under high-temperature conditions.