Effect of substitution of fly ash for quartz in triaxial kaolin–quartz–feldspar system

Quartz was progressively replaced by 5, 10 and 15 wt.% fly ash (a byproduct of Indian thermal power plant) in a traditional triaxial porcelain composition consisting of kaolinitic clay, quartz and feldspar. The effects of this substitution upon firing at different temperatures (1150–1300 °C) were investigated by measuring the linear shrinkage, bulk density, porosity and flexural strength. Quantitative estimation of mullite and residual quartz content was done by XRD and microstructures of some selected samples were studied using SEM. The results show that fly ash containing samples achieved higher densities (max. 2.46 g cm−3) in the entire temperature range (1150–1300 °C) of firing and matured early than a traditional porcelain composition. The maximum flexural strength (70.5 MPa) was obtained at 1300 °C in the 15 wt.% fly ash containing sample compared with (61.1 MPa) obtained in the traditional porcelain sample. XRD studies reveal higher mullitization in fly ash containing samples. SEM photomicrographs of the polished and etched specimens of 1300 °C heated samples show the presence of quartz grains and secondary mullite needles embedded in feldspathic glassy matrix. Quartz grains (25–50 μm) are associated with circumferential cracks around them.