# Influence of Partial and Total Replacement of Used Foundry Sand in Self-Compacting Concrete
## 1 Introduction
[20] reported a decrease of 31.25% in the slump test when 40% of natural sand was replaced by UFS [20]. For mechanical properties, [PERSON] et al. [19] also studied up to 50% replacement of natural sand by UFS, reporting a compressive strength decrease at all ages because of the impurities within the sand [19]. By contrast, [PERSON] et al. [21] studied up to 20% replacement of natural sand by UFS in concrete, reporting that the mechanical properties increase at all ages as the replacement percentage increased, which is due to the more fine particles in the UFS, creating a denser matrix [21]. Self-compacting concrete (SCC) is a special type of concrete that does not require vibration to be compacted; that is, it can flow under its own weight and, even in the presence of congested reinforcements, it completely fills and compacts the formwork [22]. It has been studied with other by-products such as electric arc furnace slags [23; 24], where its feasibility for use as coarse aggregate has been demonstrated, showing adequate fresh state properties and mechanical behavior. Likewise, SCC with recycled aggregates of precast concrete as coarse aggregates has displayed mechanical properties [25] and durability [26] similar to SCC with natural aggregates. Moreover, SCC with recycled aggregates from railway wastes has shown adequate fresh state properties, mechanical properties and durability [27]. Regarding UFS in SCC, [PERSON] and [PERSON] [28] studied SCC with 5, 10 and 15% replacement of natural sand by UFS, reporting an increase in compressive strength and splitting tensile strength at all ages (7, 28 and 56 days). The slump flow increases 3% with 10% and 15% UFS replacement. By contrast, [PERSON] et al. [29] reported decreases in fresh state properties, SCC's behavior and mechanical properties when increasing replacement percentages up to 40%. [PERSON] and [PERSON] [30] studied SCC with up to 50% replacement of UFS at intervals of 10% and the results indicated that the incorporation of UFS does not affect the passing ability and viscosity, but negatively affects the flowability as the UFS replacement proportion increases. A decrease in the segregation of the SCC was also reported with an increase in UFS incorporation, due the low specific density of the UFS, avoiding settling [30]. In the same study [30], it was reported that at early ages, UFSnegatively affects compressive strength, but for longer ageing it does the contrary. This is due to the presence of a high silica content which slows down the pozzolanic reactions. However, with longer ageing, an improvement in compressive strength was observed. [PERSON] and [PERSON] [6] analyzed the replacement rate from 5% to 30% of UFS in SCC. It was reported that the fresh state properties and mechanical properties decreased as the replacement rate increased due to the increase in the specific surface area of UFS, leading to improper hydration and formation of voids. A 100% replacement of natural fine aggregate by UFS can be proposed as a solution to landfill disposal and, at the same time, result in eco-efficient concrete. [PERSON] [31] concluded that UFS can be used in ready mixed concrete plants and precast concrete yards. Moreover, it has been reported that the use of UFS led to 50-60% cost saving compared to natural sand [3]. Because of the high production of UFS, researchers have studied its application in cement-based materials. This has led to the development of more eco-efficient concretes which improve environmental conditions by avoiding the deposit of this by-product in landfills. The replacement of natural sand by UFS in conventional concrete and SCC is usually around 30-50%. Not many studies have experimented with large volumes because it is reported that the properties of fresh and hardened concrete are negatively affected. In this regard, one study reported in 2011 by [PERSON] et al. [32] shows a total replacement of UFS. In that study, the fresh state and mechanical properties of the SCC decreased compared to the control concrete.