Foundries and metallurgic industries in general have ever been a very good allied to cement and concrete manufacturers: blast furnace slags and other by-products have been using as secondary raw materials in cement production or as artificial aggregates in concrete, field where foundry sand finds application. Used Foundry Sand is a by-product of ferrous and non-ferrous metal casting industries, where sand has been used as a molding material because of its thermal conductivity. Foundries successfully recycle and reuse the sand many times, after a cleaning process to separate reusable sand from other wastes and to discard particles of varying sizes. Although being partially a recycled material itself, after many times it loses its characteristics, especially the cleanliness and the uniformity: becoming unsuitable in the manufacturing process, it’s discarded as a non-hazardous waste.
Chemically, a typical foundry sand is composed by high quality silica (85-95%), small percentages of a binder (4-10%), a carbonaceous additive (graphite powder 2-10%) to improve the casting surface finish, and water (2-5%) to adjust plasticity. A binder is required to give the sand the ability to be compacted and shaped according to the mould pattern that is going to be produced. Classification of foundry sands depends upon the type of binder systems used in metal casting. Generally, two types of binder systems are used, and based on that foundry sands are classified as:
· GREEN SAND, if the binder is a clay, like bentonite (4-10%);
· CHEMICALLY BONDED SAND, where the binder is a chemical activated by a catalyst (1-3%). Most consists of organic, like phenolic, urethane or epoxy-resins, although some systems use inorganic binders (i.e. sodium silicate).
Foundry sand has a very uniform grain size distribution, finer than a typical natural fine sand (the majority ranging between 150 and 600 µm). In this review, I consider the most prominent papers published in the scientific literature on the investigations about the applicability of used foundry sand as partial replacement for natural sand in mortars and concretes, with the aim to establish the amount that could be added in mixtures without too heavy penalizations in terms of mechanical and durability performances. Green sand is the most commonly used recycled foundry sand, but also the other category has been studying and gives very promising results.
MIXTURES FOR “CONVENTIONAL” CONCRETES. Rafat Siddique is undoubtedly the scientist who carried out a lot of research works in the field of the use of foundry sand as a partial replacement of fine aggregates for its possible large-scale utilization in making concrete. He produced a lot of publications promoting the applicability of foundry sand in concrete formulations, obtaining marginal increases in the strength properties of plain concrete by the inclusion of used foundry sand as partial replacement of fine aggregate (sand): as example, an amount of 30% of foundry sand
allows a compressive strength gain of 10% at 28 days and of 20% at 365 days , and an improvement in abrasion resistance . This increase in strength could probably be due both to the more SiO2 amount and to the more fineness of foundry sand than regular sand, which results in a denser, less porous concrete matrix with a more prominent C-S-H framework.
Although other authors have obtained results in slight opposition, it can be concluded that a suitable recycling of discarded foundry sand as building construction material could be suggested the same, manufacturing structural mortars and concretes with it as a partial replacement of natural sand. S. Monosi et al. , studying the use of a foundry sand of grain size < 500 µm, discover that increasing its percentage, the concrete mixes need a higher dosage of superplasticizer in order to maintain a good workability. This is a common behaviour of foundry sand concretes: being green sand a heterogeneous mix of rounded and angular particles, being finer than natural sand and containing a clay binder and silica (both highly hydrophilic) that adsorb water on their surface, it is needed a higher dosage of superplasticizer for the maintenance of workability. A mechanical performance decrease, quite comparable to the replacement amount, can be noted at any foundry sand dosage above 10%: the strength reduction is due to the presence of the binder, composed by a very fine powder of carbon and clay, which promotes both a loss of contacts and links between the cement paste and the aggregate and a delay in cement hydration. The drop in compressive strength is about 20-30% (compared to that of the reference mix with only natural aggregates) and the modulus of elasticity knows a penalization of about 6%. Drying shrinkage increases with the decrease of mechanical performances. It is interesting to note that the negative influence ascribed to the presence of used foundry sand in reducing the compressive strength seems greater at lower w/c: as a consequence, negligible advantages could be achieved when w/c ratio is lower than 0.50, being this solution unsuitable due to the increase in superplasticizers addition. Also costs/benefits ratio of an eventual washing treatment of foundry sand should be well evaluated because, in author’s opinion, not carrying to a complete elimination of clay particles, it does not allow a total performance recovery.
FEASIBILITY STUDIES FOR SPECIAL CONCRETES (HSC). Y. Guney et al.  investigate the potential reuse of waste foundry sand in high-strength concrete production. The natural fine sand is replaced with foundry sand (5, 10 and 15% by mass) to achieve a high-strength concrete of about 65 MPa, gaining that value at 56 days of curing with 10% waste foundry sand replacement: this is the optimum re-allocation amount of the foundry sand used in this case study, explained with a decrease of voids in the concrete due to the foundry sand’s fineness. Tested for freeze-thawing resistance, the 10% foundry sand mix is slightly influenced by freezing and thawing cycles with respect to the other waste foundry sand replacement ratios: the reduction in compressive strength is only about 10% than the value of reference control mixtures.
EFFECT OF DIFFERENT SOURCES ON WASTE FOUNDRY SAND. S. Monosi et al.  investigate the possibility of reusing two types of Used Foundry Sands (UFSs), coming from two different processing stages of the same foundry, in the production of mortars and concretes for structural applications, added to mortars and concretes as fine aggregate replacement (ranging to 0% to 30% by weight). The first sand, referred as UFSa, was recovered directly from the moulds disposal, while the second one, referred as UFSb, from the aspiration process during the moulds crush. Regardless the UFS type and dosage, UFS addition decreases the compressive strength, due to the presence of binder in
foundry sand composed of a very fine powder of carbon and clay that causes a loss of contact and links between the cement paste and the aggregate. The reduction in compressive strengths is more evident in the presence of UFSb, due to its higher binder content: at the rate of 20%, the performance decrease at 28 days is about 40%. The washing of UFS significantly reduces the negative effects of UFS addition on mortar properties. If previously washed used foundry sand is employed, alkaline ions are solubilized in water and removed from sand, so their accelerating effect in the cement hydration at shorter curing times is eliminated and the reduction in mechanical performance is partially recovered at longer curing times. Probably when the washed UFS is used for mortar and concrete production, higher percentage than 30% substitution is possible to use.
SINERGY WITH OTHER BY-PRODUCTS. M. Sahmaran et al.  develop very eco-friendly SCC mixtures combining the use of foundry sand (SFS) as sand replacement material and fly ash (FA) in partial substitution of Portland cement. The beneficial effects of fly ash (in terms of filler effect and pozzolanic activity at longer ages resulting in better workability, higher ultimate strengths, increased durability and lower shrinkage) compensates for some possible detrimental effects of foundry sand. SCC mixtures composed by at least 30% of fly ash reach compressive strengths around 40 MPa at 28 days and 50 MPa at 90 days by using both SFS and FA. The advantage of this strategy is you can maximize the sustainability of concrete mixes, as the presence of FA enables the production of cost-effective, green SCC mixtures with proper fresh, mechanical and durability properties: strengths over 40 MPa can be reached even at 100% SFS replacement.
On the agenda of sustainability of concrete mixes there we can also include the work of Y. Aggarwal and R. Siddique , where they investigate the effect of waste foundry sand and bottom ash in equal quantities as partial replacement of fine aggregates in various percentages (0-60%), on concrete mechanical properties and durability characteristics. It is observed that the best compromise in compressive, splitting tensile strength, and flexural strength compared to that of the conventional concrete is achieved by substituting 30% of the natural fine aggregates (15% of foundry sand and 15% of bottom ash): in this case study is the foundry sand to have a compensatory effect on the side effects provided by bottom ash inclusion, giving an opportunity to use two by-products together and to achieve strength comparable to that of the reference mix and greater resistance to aggressive agents. The best mixture in any case is inarguably the 30% replacement mix also because of its morphology: it shows large formation of C-S-H gel around sand and ash particles, resulting in a dense microstructure. The fibrous C-S-H formation acts as a thick impermeable membrane for the ingress of chloride ions into concrete: this makes the concrete more resistant to aggressive environment.
CHEMICALLY BONDED SAND
T. Manoharan et al.  partially replace river sand with a chemically bonded foundry sand having sodium silicate as inorganic binder. They find that compressive, flexural strengths and modulus of elasticity (at 28 days) and durability properties are almost similar to the values of control mix with full amount of natural aggregates.
K. Rashid and S. Nazir  prepare concretes by replacing two types of used foundry sand: the one with sodium silicate, the other with phenolic resin as different binders. Being finer than natural sand, the chemically bonded foundry sands improve the workability of the concrete casted with it, in opposition to green sands containing a clay binder. Compressive strength is reduced with the increase in the amount of foundry sand with reference to conventional concretes, even if the drop in concretes with the inorganic binder is greater as compared to the foundry sand with phenolic resin. Experimental work shows that the best solution should be the mix with 20% of the phenolic resin foundry sand, which corresponds a marginal difference in compressive strengths by only 3% less at all ages.
Also M. Mavroulidou and D. Lawrence  asses salient properties of structural concrete with chemically bound foundry sand (phenolic resin), studying a wide range of its properties and durability characteristics. The results are encouraging regarding the feasibility of using such a chemically bound foundry sand in concrete: highly workable mixes with good mechanical properties, similar to those of mixes with regular natural sand, are obtained, without any adverse effect on durability. This is due both to the chemically bound foundry sand’s homogeneity in shape, more rounded than green sand, and to its better compaction grade. The possibility of using high contents of this type of sand in concrete (as opposed to green sand) gives promise for an additional outlet route for large quantities of this waste material with clear economic and environmental benefits.
Utilization of industrial by-products as alternative aggregates improves the sustainability of concrete mainly because it reduces not only the use of natural raw materials (considering that aggregate occupies about 60-75% of concrete’s total volume), but also CO2 emissions and energy consumption. Several economic, environmental and technical advantages can be achieved by the introduction of used foundry sand as fine aggregate in concrete.
The key conclusion from this review is that although caution must be exerted for application of waste materials in construction, the usually recommended threshold of 30% of waste foundry sand in concrete may have to be reviewed on a case to case basis, to potentially maximize environmental and economic benefits. Used foundry sand can be successfully introduced in high-strength concrete applications if the particle-size distribution is arranged carefully, thus providing similar properties to that of the high-strength concrete containing standard fine sand. The possibility of a combined use of waste foundry sand as artificial aggregate and other by-products with pozzolanic activity as cement replacement is one of the most promising pathways in the present context of sustainability in the construction sector.
 R. Siddique, G. de Schutter, A. Noumowe, “Effect of used-foundry sand on the mechanical properties of concrete”, Construction and Building Materials 23 (2008), 976-980.
 R. Siddique, G. Singh, “Abrasion Resistance and Strength Properties of Concrete containing Waste Foundry Sand”, Construction and Building Materials 28 (2012), 421-426.
 S. Monosi, D. Sani, F. Tittarelli, “Used foundry sand in cement mortars and concrete production”, The Open Waste Management Journal 3 (2010), 18-25.
 Y. Guney, Y.D. Sari, M. Yalcin, A. Tuncan, S. Donmez, “Reusage of waste foundry sand in high-strenght concrete”, Waste Management 30 (2010), 1705-1713.
 S. Monosi, F. Tittarelli, C. Giosué, M.L. Ruello, “Effect of two different sources and washing treatment on the properties of UFS by-products for mortar and concrete production”, Construction and Building Materials 44 (2013), 260-266.
 M. Sahmaran, M. Lachemi, T.K. Erdem, H.E. Yücel, “Use of spent foundry sand and fly ash for the development of green self-consolidating concrete”, Materials and Structures 44 (2011), 1193-1204.
 Y. Aggarwal, R. Siddique, “Microstructure and properties of concrete using bottom ash and waste foundry sand as partial replacement of fine aggregates”, Construction and Building Materials 54 (2014), 210-223.
 T. Manoharan, D. Laksmanan, K. Mysalmy, P. Sivakumar, A. Sircar, “Engineering properties of concrete with partial utilization of used foundry sand”, Waste Management 71 (2018), 454-460.
 K. Rashid, S. Nazir, “A sustainable approach to optimum utilization of used foundry sand in concrete”, Science and Engineering of Composite Materials 25 (5), (2018), 927-937.
 M. Mavroulidou, D. Lawrence, “Can waste foundry sand fully replace structural concrete sand?”, Journal of Material Cycles and Waste Management 21 (2019), 594-605.