Behaviour of strain hardening geopolymer composites at elevated temperatures

Abstract

Strain hardening geopolymer composite (SHGC) is a sustainable fibre-reinforced material that exhibits superior strain-hardening and multiple cracking behaviour under tension. The geopolymer binder synthesised through alkali-activation of aluminosilicate precursors sourced from industrial by-products is chemically stable at elevated temperatures. This paper presents a systematic experimental study on behaviour of fly ash-slag based SHGC reinforced with polyvinyl alcohol (PVA) fibres exposed to elevated temperatures up to 800 °C through weight loss, uniaxial compressive, ultrasonic pulse velocity (UPV) and uniaxial tensile tests as well as thermal analysis using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and derivative thermogravimetry (DTG), X-ray diffraction (XRD), scanning electron microscope (SEM) and mercury intrusion porosimetry (MIP). Results indicate that the compressive strength of SHGC is increased until exposure to 250 °C, followed by a decline up to 600 °C and a regain at 800 °C. The strength gain mechanisms include further geopolymerisation that refines the microstructure, fibre bridging action and sintering that densifies the binder gels, while the strength reduction can be attributed to the damage induced by internal moisture removals and evaporation of fibres, decomposition of calcium compounds, and empty channels introduced by vanished fibres. Besides, the SHGC specimens exposed up to 250 °C exhibit high ductility performance with saturated microcracks when subjected to uniaxial tension, whilst those at 105 °C demonstrate the best strain-hardening degree because of the desired fibre-matrix interaction and enhanced matrix strength. Moreover, no spalling can be observed in SHGC due to its relatively porous structure.