In 2021, Spain mixed ash from waste paper and sludge into a one-kilometre peri-urban road; its roadbed met the country's highest required E3 class

A Spanish road project used Waste Paper Fly Ash (WPFA), a residue derived from paper rejects and sludge, as a substitute for cement in a stabilized road subgrade. Built in Villamayor de Gállego in Zaragoza, the pilot covered a 1,000-metre-long and 5.7-metre-wide section of a peri-urban road. As testing showed, the WPFA-stabilized layer achieved an average static modulus above 334 MPa, the highest class specified for the section, and exceeded the 300 MPa threshold for the E3 subgrade category.The work was part of the EU-funded Paper Chain project, which examined whether WPFA could work as a hydraulic binder, replacing conventional cement work for stabilizing soils used beneath road pavements. Having previously investigated WPFA at laboratory scale and in other field trials, the researchers, however, focused on the construction and monitoring of one Spanish pilot. Although the road section didn’t carry high intensity of traffic, it was used by agricultural vehicles and trucks travelling to a local service area. The WPFA solution replaced a cement-stabilized soil classified as S-EST3 under Spanish road regulations, with a double bituminous treatment over the improved subgrade.For the laboratory mix, the researchers determined that the stabilized soil should contain 5% WPFA and 8.2% water, while the reference mixture contained 3% CEM IV/B (Q) 32.5N cement and 7% water. Unlike the cement mixture, the WPFA mixture required a 30-minute delay before compaction, followed by compaction to the maximum Modified Proctor dry density.The WPFA layer reached the required strength and subgrade classLaboratory specimens made from the WPFA mixture reached an average 1.8 MPa against the minimum requirement of 1.5 MPa and met the Spanish requirement for seven-day unconfined compressive strength. The stiffness of the completed layer was assessed through load-plate tests and in 14 days the WPFA section recorded an average Ev2 above 334 MPa. According to the study, the Spanish regulations required thresholds of more than 60 MPa for E1, more than 120 MPa for E2 and more than 300 MPa for E3. The measured value therefore placed the WPFA-stabilized roadbed in the E3 category.Construction involved the removal of 10 centimetres of the original surface layer which was replaced with borrowed soil. Using a dosing machine the WPFA was spread and mixed with the underlying soil to a depth of 25 centimetres. The mixture was compacted using four passes of a vibratory roller, followed by grading and a final static roller pass, before the double bituminous treatment was applied.After construction, technical inspections continued. Six months after its completion, no damage associated with the WPFA was identified. The second inspection came 15 months after completion and described the pavement in good condition overall, with only minor defects, such as potholes and loose aggregates, unrelated to the stabilization. The cement reference section, meanwhile, showed transverse cracks at four-metre intervals, while no cracking or swelling was observed in the WPFA section.Environmental monitoring tracked leaching and material stabilityResearchers also sampled the construction materials, surrounding soils, vegetation, groundwater and nearby surface waters before and after construction. The monitoring plan examined potential pathways for WPFA particles and focused on airborne dispersion during spreading and water transport via runoff. The concentrations of trace elements found in soils and dust collected from vegetation were similar before and after construction and no conclusive evidence was gathered by the researchers pertaining to the dispersal of WPFA through the identified construction pathways. Furthermore, all measured concentration remained below the threshold limits established for urban use soils in Aragon.Except for antimony, leaching tests indicated that the WPFA-stabilized material met the criteria for inert waste. However, leachate collected from piezometers after heavy rainfall showed very low metal mobility, while in the percolation test all metals remained below the inert-waste thresholds. Two years post-construction, follow-up plate-load tests found that the average static modulus of the WPFA section had reached 336 MPa, reportedly a 17% rise. The researchers concluded that WPFA could serve as an alternative hydraulic binder, but cautioned against using it in soils with high sulfate to avoid ettringite formation.You use AI every day. Now get your AI Quotient. Take the AIQ test.