Abstract: As part of the increasing efforts to promote recycling or reutilization of residues generated from industrial activities, for environmental reasons, the effect of the partial replacement of sand was studied by (a) Marble reject samples, (b) Granite reject samples and (c) Mixtures of marbles and granite rejects with an unknown composition, resulting from processing of different marbles and granites, on the mechanical properties of concrete masses. With that aim, samples were collected, dried and grinded to obtain a grain similar to that of the sand used in preparation of concrete masses. Grinded samples were then used to prepare masses with a composition similar to the reference material, with content of residues varying from 10-40% sand replacement and compressive and flexural strength of prepared specimens measured. Compressive strength results of specimens with 28 days curing age were then statistically tested for eventual differences using ANOVA, with a confidence of 95%. Results for the compressive strength show a similar behaviour to that of the reference material, with compressive strength increasing with curing age. Granite residues show a much better performance than marble ones. This was attributed to granite composition which is closer to that of sand. Granite assayed between 50 and 60% of SiO2 plus Al2O3. Agreement between results is more pronounced with increasing curing age. Addition of residues up to 30% give, in general, values of the compressive strength over the minimum value prescribed on the norm. Flexural strength gave, in general, results higher than conventional masses but more disperse.