# Influence of Partial and Total Replacement of Used Foundry Sand in Self-Compacting Concrete
## 4 Conclusions  
In this research, the fresh state properties, mechanical properties and microstructure of self-compacting concrete made with UFS as partial and total replacement for natural fine aggregates was studied. After analyzing the results, the following conclusions can be drawn:  
* The partial and total replacement of natural fine aggregates by UFS resulted in an increase in the diameter of the SF, by 46% and 50%, respectively, and in a decrease in t500 and V-funnel times by 60% and 80%, respectively, which is due to the lack of fines in UFS compared to natural sand, resulting in a decrease in the friction between the aggregates and the cement paste.
* There is a strong relationship between the t500 test and V-funnel test, which will help in predicting the results of the V-funnel test based on the t\({}_{500}\) test. L-box results improve with the partial and total replacement of UFS compared to the control concrete.
* It is observed that at early ages, SCC-UFS-100 and SCC-UFS-50 have higher compressive strength than SCC-C, with SCC-UFS-50 being the one that presents the highest strength at 7 and 28 days, which is due to the lower compaction capacity of SCC-C, resulting in a less resistant material.  
Figure 9: Micrographs obtained at 3000\(\times\) and 7000\(\times\).  
* Splitting tensile strength results showed that there is no great difference between SCC-C and SCC-UFS-50, but SCC-UFS-100 shows a decrease of 27%.
* SEM images proved that the incorporation of UFS enhances the concrete, creating a denser and less porous material compared to the control mix. This explains the higher compressive strength of concrete with UFS.
* In general, the partial and total incorporation of UFS in self-compacting concrete resulted in improved fresh state properties and mechanical properties. Self-compacting concrete with 50%vol. showed better mechanical properties and adequate fresh state properties. Likewise, the findings in this study show that total UFS replacement achieves an improvement in the fresh state properties and mechanical properties compared to control concrete, making this by-product a sustainable alternative to the use of natural fine aggregates.
* For the construction industry, the concretes developed in this research have been classified as SF2 and VS1, so they can be used in elements such as walls, columns, tunnel linings, foundation slabs and pavements. In addition, the use of a raw material, in this case natural sand, could be reduced to a lesser proportion.
* For the foundry industry, the use of its by-product creates a solution to avoid landfill contamination and monetary savings by avoiding paying for the deposit to landfill.
* Future studies could be carried out to analyze the durability of these concretes, with water penetration, oxygen permeability and carbonation being of particular interest.  
Conceptualization, G.G.D.A. and C.T.; methodology, G.G.D.A. and J.A.S.-A.; software, P.T.; validation, J.A.S.-A., C.T. and R.C.; formal analysis, C.T., G.G.D.A. and J.A.S.-A.; investigation, G.G.D.A. and A.A.; resources, C.T.; data curation, G.G.D.A. and P.T; writing--original draft preparation, G.G.D.A., J.A.S.-A. and P.T.; writing--review and editing, P.T., C.T. and A.A.; visualization, C.T. and R.C.; supervision, C.T. and R.C.; project administration, C.T. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The authors would like to thank Reinosa Forgings & Castings for the foundry sand used in this study and the Mexican National Council of Science and Technology (CONACYT) for the support granted during the realization of this work.  
The authors declare no conflict of interest.