Effect of two different sources and washing treatment on the properties of UFS by-products for mortar and concrete production

• UFS properties are very dependent on its source, even in the same foundry.
• The reduction in compressive strength is related to the replacement amount and is greater when lower w/c are adopted.
• The compressive strengths is acceptable for ordinary conglomerates and the heavy metals release meets the compliance.
• If washed UFS is used, the negative effects of UFS addition is partially recovered.
• The UFS washing water can be reused in the same concrete mixing plant for concrete production.

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 was investigated. The UFSs were physically and chemically characterized and then added to mortars and concretes as fine aggregate replacement at increasing dosages (ranging from 0% to 30% by weight). Different water–cement ratios w/c as well as the addition of previously washed UFSs were also considered. The resulting washing waters were used to manufacture cement pastes in order to investigate the effect of soluble UFSs ions on the hydration kinetics of cement. Leaching tests in de-ionized water were realized in order to assess the environmental impact of the final materials. The main results show that UFS properties are very dependent on their source, even in the same foundry. The reduction in compressive strength is greater when lower w/c are adopted. High content of soluble alkaline ions in UFS can induce an accelerating effect in cement hydration. However, if previously washed UFS is employed, the accelerating effect at shorter curing time is eliminated and the reduction in mechanical performance at longer curing time partially recovered. The release in de-ionized water satisfies the enforced standard for all the specimens. At these percentages of use, mortars and concrete for structural applications can still be manufactured and the use in construction is beneficial over disposal.