Microstructure investigation of the Na/Ca aluminosilicate hydrate gels and its thermal compatibility in fly ash–GGBS cementitious binder

Abstract

The synthesis of geopolymers (Na/Ca alumino silicate hydrate gels) was carried out in different proportions of fly ash and GGBS: 0/100 (A), 50/50 (B) and 100/0 (C). The gel composition at elevated temperature was characterised by studying the microstructure through X-ray diffraction (XRD). In addition, Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were also employed for structural characterisation. Thermal expansion and thermal conductivity experiments were conducted through thermal gravimetric analysis/differential thermal analysis (TGA/DTA). The compressive strength gain in the composites after exposure to a temperature of 800 °C was compared and found to be higher in CpT than ApT samples, which is attributed to thermal transformation enabled by dehydration and dehydroxylation of the N-A-S-H gel. The thermal stress–induced microcracks were observed to be higher in ApT and BpT samples but lower in CpT samples as evidenced by SEM. The lines representing binding energy (BE) in XPS showed marked changes in their position, intensity and % composition related to crystal field of the elements and it is reflected in the compressive strength. It was found that the addition of slag in Bp decreases the binding energies of Si, Ca, Al, Na and O by changing the local environment in the network by charge delocalisation and reducing the intensity of Ca peaks to a greater extent at 800 °C. The O1s peak of all heated samples correspond to the non-bridging oxygen atoms. These research findings open up the possibility for the fabrication of thermally insulated ceramic type geopolymers.