Performance of sustainable ternary blended cement containing municipal solid waste incineration fly ash coupled with slag, coal fly ash or metakaolin

Abstract

Developing low-clinker cements can effectively reduce carbon footprint of the cement industry. Additionally, more sustainable methods over landfilling are needed to tackle the increasing annual generation of municipal solid waste incineration fly ash (IFA). In this study, carbonated-washed IFA coupled with slag (SG), coal fly ash (CFA) or metakaolin (MK) were used to substitute 60 % of Portland cement to form ternary blended cement. Performance of the blended cements was investigated and compared with pure Portland cement in terms of hydration kinetics, binding capacity, hydration mechanisms, pore characteristics, microstructure, and leaching toxicity. Results indicated that in the blended cement systems, ettringite was stabilized by the formation of hemicarboaluminate as a result of the reaction between calcium carbonate in the IFA and aluminate in SG, CFA and MK. The SG displayed the best synergy with IFA, refining pore structure through pozzolanic reaction and increased formation of ettringite and hemicarboaluminate. For an optimal 40 % SG+20 % IFA blended cement, the mortar compressive strength reached 90 % of that for pure Portland cement at 90 days. Although CFA had abundant aluminate, its low reactivity restrained the performance of the blended cement, resulting in poorer pore structure and lower compressive strength. The MK had abundant reactive silicate and aluminate, but the rapid consumption of calcium hydroxide by the pozzolanic reaction and hemicarboaluminate formation restrained further reaction, limiting the performance of the blended cement at the later stage. Besides, the heavy metal leachability and dioxin level in the produced mortar satisfy the prerequisites to be categorized as nonhazardous according to HJ 1134. This study demonstrated the promising potential of the blended cements to simultaneously divert IFA from landfills and reduce the clinker content of cement.