A laboratory investigation of steel to fly ash-based geopolymer paste bonding behavior after exposure to elevated temperatures

Geopolymers are generally agreed to provide good steel-to-concrete bonding performance and fire resistance. However, few previous studies on geopolymer investigated the steel-to-concrete bonding behavior and fire-resistance at very high temperatures (>800 °C), hence cannot reveal the damage of a severe fire accident. This paper presents the results of an extensive experimental study carried out to investigate the effect of elevated temperature on the thermal-physical behaviors and mechanical properties of fly ash-based geopolymer paste. After being exposed up to 1200 °C according to the RATB fire curve, the damage stages of geopolymer paste (GPC) and ordinary Portland cement (OPC) were investigated through the unconfined compressive strength test, push-out test, thermogravimetric analysis (TGA), differential thermal analysis (DTA) and scanning electron microscopy (SEM) analysis. Based on the results and existing literature, there was a less mass loss and a better compressive strength for GPC paste after the treatment, which led to a better bonding behavior of GPC compared with OPC paste. In addition, a positive linear relationship between the bond strength and the compressive strength was found for both GPC and OPC. Furthermore, the stable properties of phases change and porous micro-structure in GPC identified by SEM and XRD methods were important for a better bond behavior at elevated temperatures. Thus, the GPC offers a feasible alternative to OPC in practical fire-resistant building applications.