## 2 Experimental Approach  
In the case of precast concrete, the concrete is produced in a producer's manufacturing facilities, moulded into elements of different shapes and sizes, which is then be assembled in the work field intended for the planned use. The various types of precast concrete elements today are standardized products and a target of CE marking, due to the existence of standards that define its requirements, characteristics, and test methods for this type of products. For example, columns are linear elements, regulated by the product standard EN 13225 which defines, among other parameters, a minimum class (C20/25) for concrete to be used in reinforced elements, such as columns [33].  
From an experimental point of view, this work involved the design, implementation and monitoring of four frames in an industrial scale with elements in precast concrete. One frame, used as reference, was manufactured with the standard formulation of precast concrete, and three other frames were manufactured using the same precast concrete developed with a waste (lime ash) fully replacing all the natural limestone filler used in the standard precast concrete formulation (4.4 wt.% of filler). The aim was thus to promote, in a circular economy perspective, the industrial symbiosis between two industrial companies located less than 25 km from each other, making the waste of one of them (from the pulp and paper sector) into a raw material alternative in the construction sector company (concrete products manufacturer).  
This represents a change in the paradigm of the current linear economy model of concrete production, which uses natural aggregates in its production combined with the fact that the pulp and paper sector still sends lime ash and other wastes to landfill disposal [13]. Hence, this was tested as a replacing raw material for natural filler in the production of precast concrete, to create an alternative to the traditional raw material, but without changing the processing conditions and the final characteristics of the precast concrete. Concrete elements were produced on an industrial scale following a traditional linear economy model (without wastes) or following a circular model (with wastes). Note that the amount of waste (lime ash) used in this work to replace the natural filler was small (around 4 wt.%) but intentionally so, it was enough to produce an impact while intending to locally exhaust the waste produced in the cellulose plant, without resorting to long-distance transports (>25 km).  
Prior to implementation, the waste and the traditional raw material (natural limestone filler) were also characterised in terms of particle size analysis, density and mineralogical composition, to assess any eventual differences. During full-scale implementation, concrete specimens with and without waste were also collected to assess fundamental characteristics, such as mechanical compressive strength, hardened concrete density, water absorption and the capillarity coefficient of the concrete. All the four frames together with the characteristics of the precast concrete were monitored for 1 year. After this technical validation of the concrete produced in both contexts (circular and linear model), the sustainability of the developed circular solution was evaluated by comparing it with the linear solution. This evaluation was based on a life cycle (LC) assessment of environmental, economic and social impacts that this simple industrial symbiosis process causes.