Use of vitrified bottom ashes of municipal solid waste incinerators in bituminous mixtures in substitution of natural sands

Bottom ashes derived from an Italian Municipal Solid Waste Incinerator (MSWI) were vitrified at 1450 °C and subsequently employed in the laboratory, after adequate size reduction, as sand fractions in partial or total substitution of natural aggregates in bituminous mixtures for road pavements. The experimental plan focused on the effects associated to variations both of the binder content (4.5-5.5%) and of the percentage of sand substitution with vitrified MSWI ashes (comprised between 0 and 100%). Investigations were carried out by considering performance-related compaction, volumetric and mechanical properties which were assessed in the laboratory by employing a number of different characterization techniques. It was observed that the key factor which allows a detailed interpretation of test results is the minimum absorption which takes place within the mixtures at the vitrified ash-bitumen interface. As a consequence, mixtures with this type of secondary (recycled) aggregate may lead to substantial savings associated to the reduction of binder dosage and to consequent economical and environmental advantages. Even though mixtures containing vitrified MSWI ashes still need to be thoroughly investigated with a special attention devoted to mix-design and moisture susceptibility issues, the results obtained in this study appear to be promising. The presence of vitrified ashes does not negatively affect the stiffness and fatigue failure properties of the mixtures, while benefits deriving from their high angle of internal friction may be beneficial with respect to resistance to accumulation of permanent deformation.