Influence of Magnesium Chloride on Corrosion of Concrete and Steel Reinforcement in Concrete

摘要

This paper investigates the effect of magnesium chloride on corrosion of concrete and steel reinforcement in concrete. For implementation of the planned research, prepared in parallel were two types of cylindrical samples of dimensions 80 mm × 40 mm, samples of cement paste for corrosion testing of concrete, and cement mortar samples with steel reinforcement for testing the corrosion intensity of the steel reinforcement in the concrete. The samples had varied water-cement ratio, 0.5 and 0.7 (ratio of water mass to cement mass). Twenty-four hours after preparation, the samples were extracted from the mould and then immersed in the following solutions: 5 % MgCl₂, 5 % NaCl, 1 % MgCl₂, distilled water. The samples were treated in these solutions for the next 9 months. After 9 months, the cement paste samples were extracted from the solution, dried in an oven at 105 °C to constant weight, cooled in a desiccator, and ground. In order to study the corrosion of concrete, the cement paste samples, after grinding, were analysed using X-ray diffraction analysis (XRD), and thermal analysis (DTA-TG/DTG). The XRD and DTA-TG/DTG analysis suggested the formation of monochloridealuminate hydrate, 3 CaO???Al₂O₂???CaCl₂???10?H₂O, in the cement paste samples treated in solutions of MgCl₂ with mass fractions 1 % and 5 %, in water-cement ratios (v/c = 0.5 and v/c = 0.7). Monochloridealuminate hydrate formed in the reaction chlorides with hydrates of alumina, thus causing the corrosion of concrete. The XRD and DTA-TG/DTG analysis proved that increased concentrations of MgCl₂, and increased water-cement ratio had led to an increase in the corrosion intensity of concrete on samples of the cement paste. That can be concluded from the diffraction lines of portlandite and monochloridealuminate hydrate. The cement paste samples treated in solutions of higher MgCl₂ concentrations and higher water-cement ratio showed lower intensity of the diffraction lines of portlandite and high intensity diffraction lines of monochloridealuminate hydrate. The samples of the cement paste treated in solutions with higher MgCl₂ concentrations and prepared with higher water-cement ratio showed higher portlandite consumption and formation of monochloridealuminate hydrate in large quantities, as compared to samples of cement paste treated in solutions with lower MgCl₂ concentration and lower water-cement ratio. The DTA/DTG analyses proved very aggressive action of Mg2+ ions on concrete (cement paste) from the solutions of higher MgCl₂ mass fractions, 5 % MgCl₂. The solution with a 5 % MgCl₂ led to the destruction of not only Ca(OH)₂ and hydrates of alumina, but other hydrates of cement minerals of the cement paste. Nine months after immersion in the above solution, the cement mortar samples were extracted from the solution and immersed 24 hours in saturated Ca(OH)₂. After 24 hours, the rate of corrosion on the steel reinforcement in samples of cement mortar was examined using electrochemical method of potentiodynamic polarization. Potentiodynamic polarization proved that increased MgCl₂ concentrations and increaseed water-cement ratio had led to an increase in corrosion intensity on the steel reinforcement in cement mortar. This is the result of intensive corrosion of concrete on samples treated in solutions with higher MgCl₂ concentrations and higher water-cement ratio.