## 2 Materials and Methods  
Most road pavements are of the flexible type and present a top layer with a bituminous mixture. This product is formed by different size aggregates (95 wt%) and bitumen (5 wt%) [10]. Industrial-scale production of the mixture begins with the transport of raw materials to the bituminous mixture preparation plant. Afterwards, aggregates and bitumen are heated and mixed in the plant, and then the mixture is loaded into a truck and transported to the application site. Finally, at the application site, the mixture is spread and compacted by a paver and a compaction cylinder, respectively [37]. Bituminous mixtures are currently standardized products with CE marking, based on defined requirements, characteristics, and test methods for this type of product (EN 13108-1) [38].  
Experimentally, this work involved the design, implementation, and monitoring of three bituminous mixtures of the AC14 surface type for application in the pavement wear top layer, one being the reference, with a standard bituminous mixture formulation, and the other two were produced with bituminous mixtures where the fine aggregate (limestone powder) in the standard formulation was partially replaced by dregs or grits, two wastes from the pulp and paper industry. This approach promotes the industrial symbiosis between two industrial companies located less than 25 km from each other, by using the PPI wastes as an alternative raw material in the construction sector. Before incorporation into the bituminous mixtures, a waste manager facility was engaged to dry and sieve the wastes, to eliminate the excess moisture which was initially 50% in dregs and 20% in grits, too much for the intended application. This real-scale test intends to change the linear economy paradigm in a sector that extensively uses natural aggregates, together with another one (PPI) that needs to avoid sending to landfill most of its wastes.  
Thus, dregs and grits were tested as alternative raw materials of natural aggregates in the production of bituminous mixes, while changes to the production conditions were checked along with the final characteristics and performance of the bituminous mixtures. Three wear top layers were produced on a full-scale pavement (250 m road), involving a traditional linear economy solution (natural aggregates) and a circular one (mixtures with wastes). Wastes content used to replace the fine aggregate was intentionally kept small (<4%) yet enough to assess their impact, with the purpose of valuing them locally, but without degrading the layer required features and avoiding long distance transportation (>50 km), so as not to aggravate CO2 emissions.  
Before the real-scale test, wastes and the traditional aggregate raw material (limestone) were characterized (particle size analysis, density, fines quality) in order to assess any differences. Before the pretreatment stage, samples were taken to assess moisture and particle distribution to guarantee an equivalent material to the traditional raw material. During the real-scale test, samples from the bituminous mixtures with and without wastes were taken to assess technical characteristics such as given by the Marshall test, porosity, and conserved strength. The mixtures environmental impact was assessed in situ for 1 year through leachate collection after road opening to traffic and at the laboratory by standard leaching tests.  
After technical validation of the asphalt top layer produced in both circular and linear context, sustainability evaluation of the developed solutions was made though LCA assessments of the environmental, economic, and social impacts caused by this simple industrial symbiosis.