Fly and bottom ashes are the main by-products arising from the combustion of solid biomass. Since the production of energy from this source is increasing, the processing and disposal of the resulting ashes has become an environmental and economic issue. Such ashes are of interest as a construction material because they are composed of very fine particles similar to fillers normally employed in bituminous and cementitious mixtures. This research investigates the potential use of ash from biomass as filler in bituminous mixtures. The morphological, physical and chemical characteristics of 21 different ashes and two traditional fillers (calcium carbonate and "recovered" plant filler) were evaluated and discussed. Leaching tests, performed in order to quantify the release of pollutants, revealed that five ashes do not comply with the Italian environmental re-use limits. Experimental results show a wide range of values for almost all the investigated properties and a low correlation with biomass type in terms of origin and chemical composition. Furthermore, sieving and milling processes were found to improve the properties of the raw material in terms of grading and sample porosity. The effectiveness of these treatments and the low content of organic matter and harmful fines suggest that most of the biomass ashes investigated may be regarded as potential replacements for natural filler in bituminous mixtures.

The paper presents an extended research work on the re-use of bottom and fly ashes of different origins in substitution of fine aggregates in bituminous mixtures. Bottom ash from a north Italian municipal solid waste incinerator (MSWI), and biomass bottom and fly ashes from twelve different power plants were considered. The MSWI ashes were vitrified at 1450°C. Vitrified MSWI ash and biomass ashes were ground and sieved to different dimensions and used as filler or sand in bituminous mixtures. Physical-mechanical tests included in road materials specifications were carried out on aggregates, mastics and on the whole mixtures. In this last case, ashes were used in partial or total substitution of natural aggregates. The test results demonstrate that vitrified MSWI ash and biomass bottom and fly ashes can be effectively re-used as pavement materials in order to avoid their landfill disposal. Moreover, the vitrification process reduces the ashes absorption potential of binder and the biomass ashes can be re-used in their original state if they are first subjected to sieving selection