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This paper presents a basic physical characterization of coarse and fine aggregates that take part in the preparation of concrete. Specifically, the investigation deals with arids coming from a quarry next to the city of San Luis, in Argentine. The material is characterised taking into account its origin, i.e., if its grading curve is the result of natural degradation (natural aggregate) or it comes from a grinding process (crushed aggregates). Apparent and real densities, shape and compacity are some of the parameters measured as a function of size and origin. The differences found when the origin of the material is taken into account, are discussed. Once the material is characterized, several mixtures using different ratios of natural and crushed aggregates to add to concrete are prepared. Six cores for each mixture ratio were prepared and uniaxial compression tests were performed over the cores. The non monotonic behaviour of the material resistance is obtained as a function of the percentage of crushed aggregates present in concrete. This behaviour is discussed and justified on the basis of the previous physical characterization of the aggregates.