Foundry sand is used in the manufacture of mold formwork for the production of ferrous and non-ferrous casting materials and it is mainly made up of silica sand and binders of a natural or chemical nature. The natural binder is mostly bentonite clay mixed with water with a carbonaceous additive to enhance the casting surface [1], while chemical binders include phenolic-urethanes, sodium silicates, epoxy resins and furfuryl alcohol [2]. Depending on the binder additive used, these will receive the name of green foundry sands (GFS) or chemical foundry sands (CFS), respectively [2,3]. During the casting process, the sand from the molds is reused until its properties are no longer suitable. It has been reported that this use is around 8 to 10 times at temperatures about 1500 °C [4], which is when a by-product known in the literature as waste foundry sand (WFS) or used foundry sand (UFS) is generated. In this research work the term UFS will be used. The UFS generated on an annual basis has been reported to be 100 MT [5], while Sandhu and Siddique [6] reported 62.64 MT. Dyer et al. [7] reported that 0.60 tons of UFS are generated per 1 ton of steel production.The physical-chemical properties of UFS will depend of the casting process, type of metal, fines content, number of times that the sand has been reused and the industry sector [2]. Overall, their properties are suitable for the replacement of the fine aggregate fraction [8]. It is because of the high quantities of UFS produced and its good physical and chemical properties that researchers have studied its applications in the civil engineering sector. Researchers have found that UFS is suitable for layers in pavement structures, reducing costs and reducing CO2 emissions compared with natural sand [9]. It can also be used in subgrade fill [1], pavement structures [8], where chemical composition and leachate analysis show that there is no hazard in their implementation [8], and cement-treated bases [10], where UFS’s feasibility as a fine aggregate has been proved.Regarding mortars, Cevik et al. [11] found that at early ages (3 days of curing), the partial incorporation of UFS (15% and 30%) increases the compressive strength by 13.51% and 12.35% (respectively) but at 28 days, the compressive strength of the mortars with 15% of UFS decreases 5.45% compared to the reference mortar. In another study, Monosi et al. [12] reported a decrease in the fresh state properties of 8%, 19% and 22% when the UFS replacement in mortars with a w/c ratio of 0.5 was 10%, 20% and 30%, respectively. For the compressive strength, a decrease of 2%, 20% and 30% was reported with WFS replacement of 10%, 20% and 30%, respectively [12]. The loss of properties in the fresh state properties and mechanical properties of mortars when replacing natural sand with UFS is attributed to the loose links between the cement paste and the aggregate due the fine powder of carbon and clay of the binders used [13]. Vazquez et al. [14], reported a decrease of 70% in compressive strength at 7, 14 and 28 days with mortars with total replacement of natural sand by UFS from an aluminum plant. This is due to the reaction between cement and the metalized aluminum, creating hydrogen gas and resulting in microfractures within the cement matrix [14]. UFS has also been used in the manufacture of masonry elements as clay replacement up to 50%wt. [15,16] and total replacement [17]. It also has been reported that the hazardous components are inertised during the firing process [18].As for conventional concrete, the increase or decrease in fresh state properties and mechanical properties such as compressive strength depends on the type of UFS, whether or not there was a previous treatment and the substitution rate [2]. In relation to fresh state properties, Ahmad et al. [19] stated that the decrease in fresh state properties is due to the fineness of UFS [19]. Bilal et al. [20] reported a decrease of 31.25% in the slump test when 40% of natural sand was replaced by UFS [20]. For mechanical properties, Ahmad 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, Siddique 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, Siddique and Sandhu [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, Parashar et al. [29] reported decreases in fresh state properties, SCC’s behavior and mechanical properties when increasing replacement percentages up to 40%. Ashis and Verma [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, UFS negatively 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. Sandhu and Siddique [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. Makul [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 Şahmaran 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. In the absence of further studies using full UFS replacement, the authors of this paper considered using chemical UFS to investigate whether its effect on SCC can be beneficial for the mechanical and fresh properties.

The novelty of this work is to contrast the results of other authors who have indicated that the use of large volumes of UFS negatively affects the fresh and hardened properties of SCC. The aim of this work will be to test whether chemical UFS in partial and total proportions are suitable for certain applications in construction based on the determination of their fresh state properties and their mechanical properties.

Therefore, the aim of this work is to investigate whether there can be a contrast in the total replacement of UFS in the manufacture of SCC when the UFS type is QFS. To achieve this, a novel dosing method has been proposed and experiments have been carried out with longer mixing times to ensure the correct homogeneity of the materials.