Degradation mechanism of cement-based materials under the effects of stray current, chloride and sulfate

Abstract

When infrastructure is exposed to a complicated environment containing stray current, chloride and sulfate, it may be subjected to severe degradation. In order to analyze the interaction of different factors and the degradation mechanism of cement-based materials in a complicated environment, the blended solution with four molar concentration ratios of Cl-/SO42- (0, 0.5, 1, and 2) were prepared to simulate a range of aggressive medium. Meanwhile, 40 V DC (direct current) electrical field was applied to cement-based materials to simulate stray current. After exposure, the transport properties, macro-properties, and microstructures of specimens were observed and measured. Additionally, the types and amounts of degradation products were tested to further analyze the degradation mechanism of cement-based materials. Experimental results show that chloride in the blended solution could inhibit SO42- transport into cement-based materials, causing the SO42- concentration in a specimen to decrease with the increase of Cl-/SO42- ratio. Therefore, the specimens appeared less degraded in terms of macro-properties. But the results of microstructural analysis showed that degradation products and cracks were generated in the interior of specimens. Friedel’s salt was detected in the specimens exposed to the solutions containing chloride. The formation of Friedel’s salt consumed calcium aluminate hydrate, which caused the amount of Al-phase involved in the formation of ettringite to reduce. Consequently, the amount of ettringite was reduced, and the degradation of compressive strength and appearance was also reduced. Additionally, an electrical field could induce the ion concentration in pore solution to change and even break the ion chemical equilibrium, which resulted in the decomposition of ettringite and Friedel’s salt. However, Friedel’s salt can also transform into ettringite at a specific condition. Therefore, the amounts of ettringite and Friedel’s salt in specimens were changed over the exposure time.