Europe is the second world producer of pulp and paper, representing 23% of world production and an annual turnover of €180 billion (1.26% of the European GDP). [1]. According to the Confederation of EU Paper Industries (CEPI), Pulp and Paper Industry (PPI) rejects show a great potential as feedstock for the production of high value products in other industries, paper (fibres) or energy conversion. Nowadays, 55% of the rejects are burnt for energy production, land application accounts to 15%, and exploitation of residues in other industries to 10%, [2]. 1.65 million of tonnes of rejects generated by the PPI (15% of the total waste), [3], are still disposed to landfills. From this situation comes the idea of Paperchain Project, financed by the European Union through H2020 Programme. The overall objective of PAPERCHAIN is to develop five novel circular economy models centred in the valorisation of the reject streams generated by the PPI as secondary raw material for construction, mining and chemical industries. Particularly, the Spanish case is focused on demonstrating the use of wastepaper ash (WPA) as alternative binder to replace cement or quicklime in soil stabilization works in road projects. Three pilot cases have been built, from the less complex and easier to reach the market (unpaved roads), to the most complex and regulated (highways). The objective of this paper is to show the studies accomplished for the first case, an in-situ stabilized soil S-EST3, aimed at defining the job mix formula, and the results obtained from the field trial executed with the stabilized layer.