Sintering lignite fly and bottom ashes via two-step versus conventional process

Lignite combustion class-C (highly-calcareous) fly ash and bottom ash were sintered employing two-step sintering (TSS) and compared to conventionally sintered ones. TSS sintering is a new promising approach mainly used to obtain effectively densified ceramics. This alternative process is generally characterized by the absence of the final stage of grain growth occurring upon conventional sintering and by the development of nanograin microstructure. The ceramic microstructures successfully obtained in the present research were characterized by means of X-ray diffraction and scanning electron microscopy coupled with energy dispersive X-ray analysis as well as by density measurements. The effectiveness of the solidification process was thoroughly studied, and the specific microstructural features attained are compared between each other and evaluated in relation to the sintering method applied. The results show that the valorization of lignite calcareous ashes into ceramic materials is feasible through different sintering techniques.