In cement raw mix system crystalline SiO2 (quartz) is the
most thermally inert component. It has been reported that SiO2
was less microwave-energy receptive at lower temperature [5]
and the tan d of SiO2 increased greatly as the temperature
rises above 11008C [7]. It has also been reported that SiO2
gels heat and melt quickly during microwave sintering [8].