A comparative study of geopolymers synthesized from OXY-combustion and chemical looping combustion bottom ashes

The generation of geopolymers from OXY-FBC and CLC bottom ashes has not yet been reported. In this study, geopolymers from OXY-FBC and CLC bottom ash were synthesized and compared with geopolymers from FBC bottom ash. The bottom ashes used in this study were generated from the combustion of high ash South African coal. FBC and OXY-FBC bottom ashes were divided into two, viz: the first half was coarse, while the second half was grinded into fine particles. The bottom ashes were mixed with sodium silicate (Na₂SiO₂) and sodium hydroxide solutions (5M, 10M and 15M) and the pastes were cured at 60 °C for 10 days. The properties of the geopolymers were characterized using: TGA, FTIR and SEM-EDX techniques. TGA analysis showed that FBC geopolymer with 5M NaOH had the least amount of % weight loss, which indicates that it had a better thermal stability than synthesizing geopolymers with higher NaOH concentrations. Coarse FBC geopolymer (C-A1) had a higher thermal stability than fine FBC geopolymer (F-B1), therefore indicating that there is no need to grind the ash. OXY-FBC showed the opposite, therefore indicating the need to grind the ash in order to attain a better thermal stability of the geopolymer. The EDS analysis showed that the geopolymers produced an N-A-S-H gel and an incomplete N-A-S-H gel, instead of the C-(A)-S-H gel. Geopolymers synthesized from 5M NaOH (C-A1, F-B1 and C-C1), 10M NaOH (F-D2) and 15M NaOH (F-E3) had the most degree of geopolymerisation as seen on the FTIR spectrum. Geopolymers (F-D2) synthesized from 10M NaOH and fine OXY-FBC bottom ash had greater degree of geopolymerisation.