Biofuel ash in road stabilization - Lessons learned from six years of field testing

In 2009, pulp mill fly ash was used for stabilizing the road base of a low-volume gravel road. Six years after stabilization, a two-year monitoring program and a complementary study were conducted to study the environmental and technical properties of the road and road material. Environmental properties were studied through chemical analysis of road samples by X-ray diffraction (XRF), inductively coupled plasma (ICP) technique and colorimetric techniques. The strength development was studied by falling weight deflectometer (FWD) and compressive strength tests. Potassium (K) and sulphate sulfur (SO4-S) concentrations in road material decreased by 40 and 55%, respectively during this time. Absolute concentrations of most trace elements increased. Leaching of chloride (Cl) salts from road samples decreased with time, while leaching of magnesium (Mg) and calcium (Ca) increased. Leaching of trace elements was below 0.5 mg/kg at all sampling occasions. The bearing capacity increased by 30-50%, and the infiltration capacity decreased compared to a reference section. The results showed that the ash-stabilized sections performed better than conventionally upgraded sections and achieved increased bearing capacity over time. Since the acid neutralizing capacity of the stabilized layer was high and lumps of unreacted ash were still left in the road, it was concluded that the ash material could be utilized once more in a potential end-of-life road recycling.