Effects of waste ground glass and lime on the crystallinity and strength of geopolymers

This study investigates the effects of adding waste ground glass and lime on the microstructural properties of amorphous gels transitioning into crystallised zeolites in fly-ash-based geopolymers. Three different mix compositions were synthesised using fly ash, waste ground glass powder and calcium hydroxide. These mixtures were subjected to accelerated aging by thermal curing (at a temperature of 80°C), the treatment lasting for 56 d. Crystalline zeolitic phases were detected by Fourier transform infrared spectroscopy and X-ray diffraction. The results show that the transition from amorphous gels into more ordered structures readily occurred in the compositions employing only waste ground glass or calcium hydroxide, whereas a combination of waste ground glass and calcium hydroxide hindered the transition. Moreover, the topographical images of the compositions indicate a greater degree of roughness in the mixture with a lower degree of transition from the amorphous gels into more ordered structures. It was revealed that crystallisation had no harmful effect on the strength stability of the compositions cured for 28 d at a temperature of 80°C. In fact, the maximum increase in the compressive strength occurred due to the formation of crystalline zeolites in fly-ash-based geopolymers.