Mechanical activation of fly ash from MSWI for utilization in cementitious materials

In the present study, the physicochemical characteristics of municipal solid waste incineration fly ashes (FA) from circulating fluidized bed (CFB) or grate furnaces are studied in detail. It is identified that the CFB FA, containing high amount of Si and Al, has better potential and properties for utilization than the grate FA, which is much richer in chlorides. Mechanical activation (MA) allows amending the properties of CFB FA, thus preparing for its subsequent utilization in cementitious materials. Compared to simple water washing, MA treatment of CFB FA further reduces the residual amount of chlorine in fly ash from 0.72 to 0.33 wt%, giving the possibility of doubling the capacity of cement kiln for fly ash disposal. The improvement in chlorine removal relates to the conversion of FA compounds from a crystalline to an amorphous state, increasing the solubility of sparingly soluble chlorides. During the curing of mortars, traces of aluminum or other nonferrous metals in CFB FA are oxidized, liberating lots of hydrogen gas; this would cause expansion problems and significantly reduce the flexural and compressive strength. MA treatment of CFB FA solves such expansion problems by exhausting these reactions in advance, respectively enhancing flexural and compressive strength from 5.7 and 35.3 MPa to 9.1 and 56.9 MPa, which is comparable to the performance of Ordinary Portland Cement. Finally, an innovative pre-treatment technique for CFB FA, combining wet ball milling and counter-current two-stage water washing, is proposed for facilitating its recycling.