Robust synthesis of coal bottom ash-based geopolymers using additional microwave heating and curing for high compressive strength properties

Abstract

While coal bottom ash (CBA) contains an amount of amorphous silica and alumina as high as coal fly ash (CFA), its irregular particle shapes and excess unburnt carbon content compared to CFA are known to decrease the compressive strength of CBA-based geopolymers. Hence, we propose an advanced synthetic method of high-strength CBA-based geopolymer by appending microwave heating to conventional oven curing process without pretreatments for carbon removal. First, we blended finely crushed CBA with a moderate amount of 14 M NaOH alkali activator to make a mixture in a slightly wet state. Then, we fabricated precast samples by casting the mixture against 5-cm cubic molds with the help of a hand press. Next, the samples were hardened through the two-stage process of pre-dry-oven curing (12, 24 and 36 hr at 75 °C) and post-microwave-oven heating (up to 7 min under 700 W power). In essence, the specimens cured for 36 hr in the dry oven showed a considerable improvement in compressive strength just after being heated for 3 min in the microwave oven (from 12.8 to 40.5 MPa). This newly proposed synthetic method is proven to be very cost-effective for producing CBA-based geopolymer with high compressive strength.