## 1 Introduction  
Foundry sand (FS) is the material used in the manufacturing of mould boxes in the sand-casting process of ferrous and nonferrous materials. The main constituents of FS are silica sand and binders, which can be of natural or chemical origin [1]. When organic binders such as bentonite clays are used along with carbon powder to enhance the cast surface, the FS is called green foundry sand (GFS). When chemical binders are used instead, such as phenolic-urethanes, epoxy resins, or sodium silicates, the FS is called chemical foundry sand (CFS) [2]. The GFS or CFS is compacted against a mould that leaves a cavity in the shape of the part required, and then molten metal is poured into it. Once the metal has cooled, the sand box is broken up by the \"shakeout\" method. By means of vacuum belts, a magnet collects the metal debris, and the sand is used several times again in the manufacture of moulds until its properties are no longer suitable for the casting process, creating a by-product known as used foundry sand (UFS) [3]. The construction industry is responsible for 39% of CO\({}_{2}\) emissions and for more than 50% of the extracted natural resources [4]. In this regard, the use of alternative cementitious materials [5] and the reuse of solid waste from the construction, steel and foundry industry benefits the environment by reducing the exploitation of natural aggregates, reducing CO\({}_{2}\) emissions emitted by the machinery and transport of aggregates from the quarry to the construction site. Moreover, the use of waste tire [6; 7], marble [8; 9], waste lathe fibres [10], waste lathe scraps [11], waste glass [12; 13], coal bottom ash [14] and PET [15] has been studied as a substitute of coarse and fine aggregates in the concrete manufacturing towards a more eco-friendly material. According to [PERSON] [16], 100 Mt of UFS are generated annually and mostly landfilled, thus creating an environmental cost [17]. The physical and chemical properties of UFS vary depending on the foundry process or the metal and the binder used [1]. However, it has been reported that most of the UFS is categorised as nonhazardous [18], and its physical properties are suitable for use in cement-based materials such as low-strength materials [19], cement-treated bases [20], conventional concrete [21; 22] and self-compacting concrete [23; 24]. Regarding mortars, diverse authors have experimented with UFS and additions such as ceramic mould shells and paraffin waxes wastes [25], polyurethane residues [26], high phenolic contents [27] and geopolymers [28]. [PERSON], [PERSON] and [PERSON] [3] used a replacement of up to 30% of natural sand (NS) by UFS with a fixed W/C ratio of 0.5, reporting a decrease in the workability of 8, 19 and 22% as the UFS content increased by 10, 20 and 30%, respectively. At 28 days, the compressive strength of the mortars decreased when the replacement was higher than 20%, which was attributed to the presence of fine powder carbon and clay within the binder, causing a loosening of contacts and links between the aggregates and cement matrix [3]. However, with up to 10% replacement of UFS the compressive strength of the mortar was not affected significantly. [PERSON] et al. [29] used UFS from an aluminium foundry as a total replacement for NS in the manufacture of mortars. The compressive strength results at 7, 14 and 28 days reported that the total inclusion of UFS decreased the compressive strength by 71, 77 and 76% compared to the control mortar. This was attributed to the reaction of cement with aluminium, which produces hydrogen gas that creates microcracks in the cementitious matrix [29]. [PERSON] et al. [30] manufactured mortars with up to 60% replacement (in steps of 15%) of NS by UFS and a fixed W/C ratio of 0.5. Compressive strength results at 3 days showed an increase of 13 and 12% when 15 and 30% of UFS were used, respectively. However, at 28 days, the compressive strength of the mortars decreased compared to the control mortar. [PERSON] et al.