Mechanical performance and microstructural properties of cement mortars containing MSWI BA as a minor additional constituent

Along with an increase in the amount of municipal waste being incinerated, the amount of Municipal Solid Waste Incineration Bottom Ash (MSWI BA) is growing every year. Combined with limited landfill sites in the Netherlands, this necessitates new applications for the MSWI BA disposal. The larger MSWI BA fractions (> 3 cm) can be successfully used as a natural aggregate substitute in concrete. At the same time, the fine MSWI BA fraction (< 3 cm) has several drawbacks (high porosity, high contaminant content, metallic aluminum, etc.), which make it difficult to use this way. However, if this fine MSWI BA fraction is subjected to treatment that eliminates these shortcomings, it can become a product suitable as a Minor Additional Constituent (MAC) for cement (< 5% w/w). The addition of MAC allows to use less cement without having a negative influence on its properties, therefore reducing the carbon footprint, and utilizing suitable waste materials (instead of landfilling them). In this paper, mortars and pastes containing the decontaminated (by sieving out the most contaminated fraction < 125 µm) and mechanically treated MSWI BA < 3 mm was used as a cement replacement in the range of 0–20%. Three types of milled fines were studies, two of them had a particle-size distribution close to CEMI 42.5 and 52.5 to determine the optimum percentage of cement replacement, while the third was not optimized. Effects of MSWI BA fine replacements were studied in terms of mechanical properties, hydration (isothermal calorimetry), microstructure (optical microscopy), and the environmental impact (the standard leaching test by ICP–AES and ion chromatography). The results showed that the mildly milled fines, with a cement replacement of 1%, are most suitable as a MAC as they have the lowest environmental impact, and no negative effects on the mechanical properties (compressive strength).