The present work aims to investigate the influence of the replacement of sand fractions by iron ore tailings (IOT) on concrete. The reactivity of tailings in the presence of cement, the variation of porosity and workability, and the filler effect of the material were studied. For this purpose, a series of chemical, physical, mechanical and thermal tests were conducted. The IOT was characterized by its chemical composition, particle size and specific weight. It is found that the material is formed by silica and hematite and has a specific weight similar to that of sand, although its grains are ten times smaller. To evaluate the reactivity of the IOT, the results of several experiments were compared. Axial compression tests were performed on cement pastes with replacement of 10 percent, 20 percent and 30 percent cement by IOT at different ages. The IOT was chemically tested by the modified Chapelle test and the pozzolanic activity index was determined. In addition, IOT concrete fragments at 28 and 90 days were tested by thermogravimetry and the hydration products were compared to the reference sample. The degree of crystallinity of the IOT was determined by X-ray diffraction. All evidence indicates that the material is inert. The effect of IOT on the workability of concrete and cement paste was tested by slump test and flow table, respectively, and an increase in workability was detected by increasing the IOT fraction. Porosity was evaluated by the water immersion test on concretes with different IOT fractions, and by computed tomography, which determined the pore distribution. In general, a tendency to reduce porosity was observed as IOT fraction increased. The mechanical strength of the concrete was evaluated for 10 percent, 20 percent and 30 percent sand replacements by IOT and the results were compared to the replacement of the same sand fractions by inert material, in the IOT granulometry. The filler effect caused an increase in resistance, with the best result being the 20 percent IOT fraction. It was also evaluated the flexural behavior in prismatic concrete specimens with addition of steel fibers in the fraction of 30 kg/m cubic and 20 percent of sand substitution by IOT. The results of maximum loads, residual stresses and toughness were on average very similar in the IOT and reference samples.