Calcium-activated geopolymer ceramics: Study of powder calcination and amorphous gel phases

Burnt coal cinder (BCC), the main solid waste in coal and electric power industries was used to prepare the geopolymer ceramics (GC) in this study. The optimum preparation technology of BCC-based GC and the effects of calcination and calcification on the compressive strength of BCC-based GC were studied. Through the calcification of BCC with CaO, NaAlO2 and sodium silicate solution (SS) were separately used as the co-activator and silica additive to prepare BCC-based GC. The optimal single-factor conditions determined by the experiment were BCC:CaO:NaAlO2:SS = 18:3:2:4 (g:g:g:mL) and the liquid-solid ratio was 0.46. The compressive strength of BCC-based GC was 32.78 MPa (28 days). It was obtained that the calcination of calcified BCC below the temperature of 1000 degrees C can effectively remove the fixed carbon in BCC but will destroy amorphous active aluminosilicate. The microstructure study showed that BCC reacted in the ternary system of Ca-Al-Si to form a variety of amorphous products (C-(A)-S-H and (N, C) -A-S-H). Along with the three-dimensional amorphous gel structure as the dominant phase, the coexistence and interlacing of various amorphous gels made the microstructure more compact and increased the compressive strength of the BCC-based GC. GCs with various amorphous phases have been successfully prepared at low temperature, and it also provided a new idea for the utilization of low-activity silicate solid wastes.