Concerns about the use of residues from municipal solid waste incinerators (MSWI) in construction materials usually focus on the potential for heavy metals and organic chemicals to leach into drainage waters under the influence of rain.We hypothesised that high level of salts in the MSWI leachates may cause more of a problem, particularly on soil physico-chemical properties. Both bottom ash (BA) and Solidified Air Pollution Control residue (SAPCr) leachates were added to experimental grassland plots. The amounts of Na+ increased by up to 13% in soils supplemented with each leachate. A decrease of the soil total porosity (−14%) was evidence of a subsequent adverse physical effect of this strong salinity. The potential for the grass cover type (species composition or density) to limit this adverse effect was discussed. Laboratory tests allowed us to determine that undiluted SAPCr induced slaking of aggregates accompanied by a strong decrease of aggregate stability, to 49% of control values. Undiluted BA induced dispersion of clays and others fine particles, which are then dislodged and transported into pores, causing blockage and decreasing total porosity. Clay dispersion followed by aggregate collapse occurred when soil solution contaminated by SAPCr was diluted by rainwater. This work stressed the importance of accounting for mineral contaminants, such as salts, when conducting an assessment of waste reuse scenarios.