Production of autoclaved aerated concrete by using municipal solid waste incinerator fly ash as an alternative raw material

Autoclaved aerated concrete (AAC) is a modern building material that currently attracts much attention because of its low unit weight, thermal and sound insulation, fire resistance, and fast construction. The raw materials for AAC production conventionally include lime, silica sand, and cement, and an expansion agent is used to create porous structures. Autoclave curing is a critical process for enhancing the mechanical strength of the AAC, and tobermorite is the major hydration product. The fly ash (FA) generated from a municipal solid waste incinerator (MSWI) normally has large amounts of Ca and Si and should thus have the potential to be an alternative raw material for AAC production. However, MSWI FA also contains some impurities including chlorides, sulfates, and heavy metals, which may affect the formation of tobermorite and the properties of the AAC. In this study, a washing process was used to reduce impurities, and most chlorides (i.e., NaCl, KCl, and Ca(OH)Cl) were removed from the FA. The washed fly ash (WFA) had higher contents of Ca and Si than the FA did, and the particle size distribution of the WFA was toward finer particles. These results imply that the WFA should be more adequate for replacing AAC raw materials. When FA was used for AAC production, the bulk density and compressive strength greatly decreased with increasing amounts of FA. In contrast, the bulk density of the AAC specimens prepared with WFA slightly increased, whereas the compressive strength initially increased (at 10–40 wt.% WFA) but then sharply decreased (>40 wt.% WFA). Significant amounts of CaCO 3 , γ-CaSO 4 , and SiO 2 remained in the AAC specimens when high levels of WFA were used as alternative raw materials, and the formation of tobermorite was inhibited. The AAC specimens prepared with WFA had lower total porosities, but the pore size distribution shifted to a larger size region, which should be responsible for the reduction in compressive strength.