# Influence of Partial and Total Replacement of Used Foundry Sand in Self-Compacting Concrete
## 1 Introduction  
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 \({}^{\circ}\)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 [PERSON] and [PERSON] [6] reported 62.64 MT. [PERSON] 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 engineeringsector. Researchers have found that UFS is suitable for layers in pavement structures, reducing costs and reducing CO\({}_{2}\) 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, [PERSON] 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, [PERSON] 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]. [PERSON] 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, [PERSON] et al. [19] stated that the decrease in fresh state properties is due to the finiteness of UFS [19]. [PERSON] et al.