Although roads are not a particularly suitable use for residues for logistical reasons, they are the type of civil works with the best developed set of requirements, against which constructions with combustion residues may be assessed. Fly ash, preferably from solid biofuels or sludges from the pulp and paper industry, has been used in non-surfaced gravel roads and bottom ash is now also beginning to be used.

Non-surfaced roads have been built using fly ash in Sweden for some time. However, through the introduction of the Finnish experience with this technology (Lahtinen 2001) as the starting point within the Ash Programme, a convenient impulse was given to renewed development. In successive projects, fly ash from biomass has been characterised, recipes have been developed and a few test roads have been built (Macsik and Svedberg 2006; Macsik et al. 2009). A short summary of the results is as follows: bearing capacity and freeze-thaw resistance have increased, fly ash replaces natural materials of about twice its volume, which leads to a significant reduction in weight and height. For the good properties of biomass fly ash to be optimally exploited, and for conservation of fly ash resources, ash should be used to stabilise bad or worn-out road materials. Adverse impacts on the environment could not be observed during the monitoring of construction and use of the roads.

Bottom ash from solid biofuels is also used for roadwork. An example is a private road north of Norrtalje, where approximately 5,000 t of mixed bottom and fly ash from a grate furnace have been used since 2006; see Fig. 11.1. It was possible to run 40-t lorries on the road even when it was flooded by melting snow, which is remarkably good. Infiltration in the body of the work is very slow, which indicates that the environmental impact should be minor. The impact of an ash pile stored for 7 years on the place has been investigated: the uptake of heavy metals in ash by plants and berries did not lead to any increased levels in the plants.

The ash of solid biofuels also has binding properties and it has been utilised in concrete applications. One project involved replacing Portland cement in panel stope mine filling, where large blocks or “stopes” of ore are removed, creating a large cavity. These stopes are backfilled using concrete (cement and mine tail­ings) to stabilise the mine. In full-scale trials, biomass fly ash from grate furnaces could replace 50% of the Portland cement. The other use of solid biofuel ashes demonstrated in the Ash Programme is as filler in low-quality concrete. However, the chloride content of the ash may pose corrosion problems for steel reinforce­ment bars.

Fig. 11.1