Recycling of ashes from solid biomass combustion as filler in bituminous mixtures

Fly and bottom ash are the main by-products arising from the combustion of solid biomass. Since the production of energy from this source is increasing worldwide, the processing and disposal of the resulting ash have become environmental and economic issues. Fly and bottom ash are of interest as a construction material because they are composed of very fine particles similar to the fillers normally employed in bituminous and cementitious mixtures. This research investigates the potential use of ash from biomass combustion as filler in bituminous mixtures. The morphological, physical and chemical characteristics of twenty-one different types of ash and two traditional fillers (calcium carbonate and a recovered bituminous mixtures plant filler) were evaluated and discussed. Based on this characterization, several types of ash were selected in order to cover the whole range of morphological, physical and chemical properties. These ashes were subsequently used to create bituminous mastics (filler-bitumen blends), which were studied with the aim of obtaining indicators of bituminous mixture performance. A variety of rheological tests were performed to evaluate workability, rutting resistance, fatigue resistance, low temperature cracking resistance and moisture damage resistance of bituminous mastics. Experimental results showed a wide range of values for almost all the characteristics of the ash and a low correlation with biomass type in terms of origin and chemical composition. A sieving process was found to be necessary in order to improve the properties of the raw materials in terms of grading. Furthermore, mastics containing ashes with high values of porosity of a compacted sample of dry filler (Rigden voids) were found to have low workability, fatigue resistance and low temperature cracking resistance. It was found that in order to limit the Rigden voids values, a milling process should be adopted before biomass ashes reuse. The low content of organic matter and harmful fines and the effectiveness of sieving and milling treatments suggest that biomass ash may be considered as replacements for natural fillers in bituminous mixtures.