The interest on alternative hydraulic road binders to replace cement is increasing these days aiming at reducing the carbon footprint of road projects. The PaperChain project, an EU funded project under the H2020 programme, tackles the use of one of these fly ashes, Waste Paper Fly Ash (WPFA), a waste stream coming from the energy recovery of paper rejects and sludge, as alternative binder for subgrade stabilisation and cement-modified subbase layers. WPFA has been extensively tested at laboratory scale and demonstrated at real scale in three field tests covering the three types of stabilised soils recognised in the Spanish Road Regulations, complying with all technical requirements. Nonetheless, the complete replacement of cement by WPFA is a challenge from the design point of view. This paper focuses on the construction and monitoring of one of those cases, specifically the one allocated for the pavement subgrade with the highest bearing capacity. Pairs of cement and WPFA-stabilised laboratory specimens have been tested showing a different hardening pattern and different reaction modulus evolution. Field-testing (load plate tests) and compressive resistance results confirmed this trend. These differences can result in positive effects on cracking development thanks to the slower hardening speed during the first days but the current design values should be reviewed when applied to WPFA, given the notable differences expected over the long term.