Thermal performance of metakaolin-based geopolymers: Volume stability and residual mechanical properties

For any structure to perform well under a fire scenario, the structural material should exhibit thermal stability at micro, meso and macro levels. Excellent thermal stability for geopolymers has been reported in literatures at micro-structural levels. However, in order to apply this novel material for structural applications, a step forward is to better understand the performance at a meso-level by understanding the volume stability and residual mechanical properties of this material subject to elevated temperatures. Thus, a series of metakaolin (MK)-based geopolymer specimens measuring 50 mm cubes with different Si/Al ratios were prepared. They were exposed to different heating profiles in an electric furnace. The volumetric shrinkage and crack pattern (which is an indication of volume stability) and the residual compressive strengths and densities of the specimens were measured. The volume instability observed at a meso-level was linked to the transformations taking place at the micro-level by studying the SEM images. A general observation was that higher Si/Al ratio geopolymers retained higher compressive strengths after high temperature exposure but experience higher volumetric shrinkage too. The crumbling of cement paste into small pieces at 900°C while geopolymer specimens retained satisfactory compressive strengths points towards the potential of geopolymers to perform soundly as a fire resistant material and also the need to impart sufficient volume stability and strengths when subjected to high temperatures.