Aplicação dos ensaios de migração e de difusão natural para cálculo do coeficiente de difusão de íons sulfatos no estado estacionário e não estacionário no concreto

Abstract

Concrete may be subject to several processes that cause its degradation, which may originate in physical, chemical and mechanical phenomena. Among the processes of deterioration with origin in chemical phenomena is the sulfates attack of external origin. In this attack, sulfate ions react with the hydration components of the cement and form compounds with a volume greater than the initial ones, being able to generate expansion and rupture of the concrete, besides loss of mass and progressive decrease of the resistance. In order to better understand the transport of these ions, which involves the transport of sulfates, chemical reactions between fluid and solid and the degradation of the cement matrix, tests of diffusion and migration of sulfate ions in concrete were performed, which are currently largely restricted to chloride ions. The sulfate ion migration and diffusion tests were performed in concrete at steady and non-steady-state for solutions with 5 and 10% of SO42- for both sodium sulfate (Na2SO4) and magnesium (MgSO4). With the obtained data, the diffusion coefficient of concrete was calculated for all analyzed situations. The microstructure of the concretes after diffusion/migration of sulfate ions was also analyzed by mercury intrusion porosimetry, X-ray diffraction (XDR) and scanning electron microscopy, besides of the variation of compressive strength. The migration test was effective in providing accelerated sulfate results, with a maximum test time of 14 days. For a magnesium sulfate solution with 5% SO42- it was possible to determine the stationary diffusion coefficient with the smallest variation, but with an order of magnitude difference from the natural diffusion. With the sodium sulfate solution with 10% SO42- it was possible to determine the non-stationary diffusion coefficient with the smallest variation and the same order of magnitude of natural diffusion. The migration tests samples showed a reduction in compressive strength, in non-stationary diffusion there was an increase in resistance and in the stationary diffusion test there were practically no variations. The same characteristic was identified in all samples in the microstructure images and identified compounds, except for brucite, not identified in the stationary diffusion test. There was no pattern in the porosity behavior of the samples for the same solution in the different tests, nor for the same cation or concentration within each test, evidencing that the sample size and presence or absence of electrical voltage affect the kinetics of external sulfate attack.