This paper compared the thermal stability between fly ash (FA) and fly ash-ladle furnace slag (FA-LS) geopolymers. FA-LS geopolymer was prepared by mixing FA and LS (FA:LS weight ratio of 80:20) with an alkali activator. Geopolymers were aged at room temperature for 28 days before being exposed to high temperatures (200 °C – 1000 °C). Unexposed FA and FA-LS geopolymers had a compressive strength of 38.9 MPa and 40.5 MPa, respectively. The FA and FA-LS geopolymers retained 61.6% and 91.3% compressive strength, respectively, when exposed to temperatures up to 1000 °C. FA-LS geopolymers experienced smaller variation in the density (2.6 – 5.5%) and pores (17.4 – 23.0%) compared to FA geopolymers (density and porosity of 2.9 – 25.2% and 19.0 – 30.0%, respectively). The formation of crystalline peaks, densification of matrix, pores and their connectivity, cracks and dimensional changes influenced the compressive strength of exposed geopolymers. FA-LS geopolymers could be potentially applied as heat-resistance material.
