Taxa de exalação de radônio-222 de concreto e argamassa de cimento usados na construção civil

Abstract

Radon is a gas presents in the atmosphere and it is the second largest lung cancer caused due to the dose deposited in the lung tissue. This gas has radioactive decay products of short half-life that accumulate in the organism, contributing to the effective dose. The main sources of radon are soil, building materials and groundwater. Residential environments are built with materials that were manufactured from the raw materials found in the soil. Therefore, these materials can significantly contribute to the concentration of indoor radon, if the radon exhalation rate is high. This study consisted of the determination of the exhalation rate of 222Rn and the radionuclide activity concentration obtained by gamma spectrometric measurements. The material objects of study of this research, widely used in various masonry buildings, are proof specimens of concrete and cement mortar. The methodology used in this study allowed to compare the exhalation rate of radon between two models of exhalation, called 1D model and 3D model. The materials used were characterized in relation to compressive strength, in order to observe if the physical characteristics were in agreement with the materials used in construction. Measurements of porosity, permeability and elemental composition were also carried. The measures of the exhalation rate of radon (222Rn) were taken by AlphaGuard equipment in a closed system. The value of the exhalation rate was calculated through graphs of experimental data. A concrete test cell - a cube with solid walls and hollow interior, with the aim of simulating an environment in small dimensions, with material characterized wall - was built to simulate the situation in an environment where the concentration of radon indoor comes from only the concrete. Qualitative results of EDXRF show that the analyzed materials have the same composition. The radon exhalation rate, in Bq∙h-1∙m-2, found to the concrete was: 2.55 ± 0.03 for the 1D model and 0.461 ± 0.008 for the 3D model. The exhalation rate of radon, in Bq∙h-1∙m-2, found for the cement mortar was: 1.58 ± 0.03 for the 1D model and 0.439 ± 0.011 for the 3D model. The calculated activity index was 0.3395 ± 0.0017 and 0.3106 ± 0.0017 and the equivalent radio activity was 89.8 ± 0.4 Bq/kg and 82.8 ± 0.4 Bq/kg for concrete and mortar cement, respectively. The indoor concentration of 222Rn from the test cell, extrapolated to the dimensions of a human living environment, was 112 ± 9 Bq/m3, below 200 Bq/m3, recommended value by the International Commission on Radiological Protection (ICRP) and below 148 Bq/m3, recommended limit by the US Environmental Protection Agency (EPA). Even so, this value is significant, since it is only relative to the contribution of concrete walls. These results show that concrete and cement mortar can significantly contribute to the concentration of indoor radon of environments that are built with these materials.