Alkali-activated geopolymer materials prepared from coal gangue and municipal solid waste incineration byproducts

Abstract

Coal gangue and municipal solid waste incineration byproducts (fly ash and bottom ash) are considered as solid and hazardous waste, respectively, and the accumulation of these wastes leads to significant environmental issues. Coal gangue is rich in Si and Al but lacks of Ca, making it challenging to create alkali-activated cementitious materials with good mechanical performances. Fly ash and bottom ash are abundant in Ca. The feasibility of using fly ash and bottom ash as calcium sources for preparing alkali-activated coal gangue-based geopolymers is investigated in the present work. The unconfined compressive strength (UCS) test and microscopic tests (XRD, FTIR, TGA, SEM and ICP-OES) are conducted to study the strength and reaction mechanism of coal gangue and fly ash/bottom ash (CGFA/BA) geopolymers. The results show that the strength of CGFA/BA geopolymers initially increases and then decreases with an increase in fly ash and bottom ash content, and the optimal dosage of fly ash and bottom ash is 15% and 30%, respectively. The liquid-solid ratio and alkali solution (NaOH/Na2SiO3) ratio have a significant influence on the strength of the CGFA/BA geopolymers and the optimal alkali solution ratio is 25:75, while the liquid-solid ratio of 0.45 and 0.60 is recommended for CGFA and CGBA geopolymers, respectively. The CGFA geopolymer with 15% fly ash, 25:75 alkali solution ratio, 0.45 liquid–solid ratio and 8 mol/L sodium hydroxide concentration obtains the highest UCS of 30.28 Mpa. The heavy metal immobilization efficiency (IE) of the CGFA geopolymer is better than that of the CGBA geopolymer. The IE of heavy metals of the CGFA geopolymer is larger than 84.66% and that of the CGFA geopolymer is larger than 55.95%. In the CGFA geopolymer, the IE is Hg>Pb>Zn>Cd>Cr>Ba, while in the CGBA geopolymer, the IE is Hg>Cd>Ba>Cr>Zn>Pb.