To qualify for reuse as construction material, waste materials must comply to both technical requirements of the application as environmental quality criteria. While the former depend on the final use and are set by technical product standards, the latter are imposed by legislation as maximum total concentrations and leaching limits.
In Flanders, bottom ashes from municipal solid waste incineration require treatment in order to meet the environmental criteria in terms of heavy metal leaching. Conventional treatments involve extraction of coarse (>2 mm) ferrous and non-ferrous metals by magnetic or eddy-current separation. The quality can be further improved by washing the ashes to remove heavy metal loaden fines or by density separation of fine metals using dry or wet jigging. As a final step, ageing of the treated mineral fraction is often applied to further reduce leaching of heavy metals. Indeed, the beneficial effect of ageing was shown to result from slow hydration and carbonation reactions that can immobilize heavy metals by adsorption, encapsulation and pH reduction induced precipitation.
Technically, most bottom ashes readily meet specifications for low grade use such as back-fill, embankment or road-base. For higher value end-uses additional issues remain to be resolved. In case of use as aggregate in concrete, the presence of residual aluminium metal has been shown to lead to volumetric instability of the concrete matrix, leading to durability problems such as crack and pop-out formation. Prior to use as cement or concrete constituent the physical removal or chemical neutralization of aluminium metal is therefore key.
The present study aims to treat MSWI ashes for use in carbonate bonded construction materials. A milling treatment was followed by sieving enabled to recover coarser (>0.125 µm) copper and aluminium particles from the mineral fraction. Copper, steel and residual organics remained as hard, comminution resistant materials, while coarser malleable aluminium metal particles were retained as flaky particles. An additional chemical treatment was required to remove fine aluminium metal in order to avoid expansion during carbonation treatment. Two different methods were tested to enhance Al oxidation rate: treatment with a 1 M NaOH solution, and temperature treatments at 40 and 60 °C. Both methods were shown to effectively remove the fine Al metal.
The treated MSWI bottom ashes were subsequently mixed with 20-40% stainless steel slags, compacted as blocks and carbonated at elevated temperatures and CO2 pressure in an autoclave. The resulting carbonated blocks were characterized in terms of mineralogy, carbonation degree, microstructure and compressive strength. The results show that compressive strengths up to 48 MPa are reached for temperature treated bottom ashes, while alkali-treatment of ashes resulted in strengths up to 65 MPa. In the former strength develops mainly through carbonation, in the latter alkali activation of the waste glass present in the ashes resulted in additional and synergetic strength development.
Interaction and chemical compatibility with other materials and structures , New products, applications and machinery , Processing possibilities and logistics
