Radiometric Investigation of Black Sands as Naturally Occurring Radioactive Materials for Building Materials in Aguas Dulces, Uruguay

Aguas Dulces sand is frequently used as a component for preparing building materials, leading us to study it and characterize it as a Naturally Occurring Radioactive Material (NORM). Particle size distribution was determined by dry sieving and mineralogical composition was evaluated by X-ray powder diffraction and scanning electron microscopy. Natural radionuclides composition was evaluated by gamma-spectrometry. The higher the grain size of the sand, the lower the heavy mineral concentration. Ilmenite, rutile, zircon, and quartz, were identified in the 50–100 µm grain size fractions, ilmenite, quartz, albite and orthoclase were identified in the 100–200 µm grain size fractions, and quartz, albite, sanidine and anorthite were identified in the 200–315 µm sized grain fractions. Thorium and Uranium series activity concentrations reached a maximum for the 50–100 µm sized fractions, which exceed by one order of magnitude the activity concentration in the whole sample and in any other fraction. This result is in agreement with the heavy mineral composition. Finally an activity concentration index (I) of 1.27 ± 0.10, corresponding to the integral sand, was determined. This value exceeds the one recommended when using the sand as building material, and makes it necessary to study I when it is used as a component of different building materials (bricks, cinder blocks, cement, etc.).