An environment-friendly pretreatment process of municipal solid waste incineration fly ash to enhance the immobilization efficiency by alkali-activated slag cement

Municipal solid waste incineration (MSWI) fly ash can cause serious early volume expansion when stabilized and solidified by cementitious materials. Many pretreatment processes have been reported, such as alkali solution, milling and chelating etc. However, these processes may lead to second pollution or higher costs. This study proposed an original pretreatment process of MSWI fly ash, and investigated its effect on the immobilization efficiency by alkali-activated slag (AAS) cement. The possible reasons for volume instability were assumed and analyzed by gas chromatography, scanning electron microscopy, X-ray diffraction, hydrostatic testing and leaching tests. It was found that the 0.377 wt% of metallic aluminum is the main expansive source of volume expansion in early stage (within 24 h). Due to the amphoteric characteristic of Al, it is possible to eliminate it using either acid or alkaline. Considering the fact that alkali-activated system and the fly ash were both alkaline, this study chose water immersion to pre-treat the fly ash, aiming to avoid the possible neutralization process of acid treatment and the entrained extra alkali component of alkali treatment, which may greatly affect many characteristics of alkali-activated slag. After pretreatment, the solidified specimens showed lower water demand (with a water to cement ratio of merely 0.27) and higher compressive strength due to the refined and smoothed particles. The volume expansive rate was greatly reduced, giving the better volumetric stability and encapsulation rate. Up to 90% MSWI fly ash-added solidified bodies feature a volume expansive rate of less than 0.09%. The alkali-activated slag pastes can effectively immobilize up to 70% treated fly ash with the leaching rate of solidified particles lower than the limits recommended by Chinese standard, Identification for extraction toxicity (CSEPA GB 5085.1, 2007). Therefore, the pretreatment process is easy to operate and is a promising way to increase the immobilization efficiency of alkali-activated slag cement.