Evaluation of municipal solid waste incineration filter cake as supplementary cementitious material

Worldwide, the incineration of household waste generates high amounts of Municipal Solid Waste Incineration (MWSI) bottom ashes (BA). In some European countries, the Filter Cake (FC) is also collected (below 0.25 mm) by the wet sieving treatment of coarse BA (4–32 mm). By using centrifugation, the presence of readily soluble contaminants in FC is decreased. In addition, for reducing the leaching of heavy metals the use of FC in combination with cement represents a valuable option, due to cement immobilization capacity and the FC appropriate particle size and chemical composition. However, to the best of the authors’ knowledge, FC has not often investigated as binder replacement. Therefore, this paper addresses the suitability of FC as substitution for CEM I 52.5 R, between 1 and 20% replacement, by mass. Due to its porous structure, the FC lowers the bulk and particle density of the mixtures and increases the water demand, and widens the particle size distribution. The XRD analysis evidences the formation of layered double hydroxides (LDH) in the early stage of the reaction (1, 2, 7 days) incorporating contaminants as Cl. Kinetically, the hydration of C₃S is delayed according to the FC replacement without affecting the long-term flexural strength. Despite this, the presence of FC minimally contributes to the increase of cement reaction degree. Tested accordingly to the EN 124-57, the 28 days mortars fulfill the Dutch legislation for contaminants leaching as unshaped material, independently on the substitution rate.