Study on Concrete Deterioration in Different NaCl-Na2SO4 Solutions and the Mechanism of Cl- Diffusion.

Fei Zhang,Feng Wei,Zhiping Hu,Long Liu,Li Dai,Xin Wen,Xiaoguang Li

doi:10.3390/ma14175054

Fei Zhang, Feng Wei + Show 5 more

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https://doi.org/10.3390/ma14175054

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Abstract

The diffusion of sulfate (SO42−) and chloride (Cl−) ions from rivers, salt lakes and saline soil into reinforced concrete is one of the main factors that contributes to the corrosion of steel reinforcing bars, thus reducing their mechanical properties. This work experimentally investigated the corrosion process involving various concentrations of NaCl-Na2SO4 leading to the coupled erosion of concrete. The appearance, weight, and mechanical properties of the concrete were measured throughout the erosion process, and the Cl− and SO42− contents in concrete were determined using Cl− rapid testing and spectrophotometry, respectively. Scanning electron microscopy, energy spectrometry, X-ray diffractometry, and mercury porosimetry were also employed to analyze microstructural changes and complex mineral combinations in these samples. The results showed that with higher Na2SO4 concentration and longer exposure time, the mass, compressive strength, and relative dynamic elastic modulus gradually increased and large pores gradually transitioned to medium and small pores. When the Na2SO4 mass fraction in the salt solution was ≥10 wt%, there was a downward trend in the mechanical properties after exposure for a certain period of time. The Cl− diffusion rate was thus related to Na2SO4 concentration. When the Na2SO4 mass fraction in solution was ≤5 wt% and exposure time short, SO42− and cement hydration/corrosion products hindered Cl− migration. In a concentrated Na2SO4 environment (≥10 wt%), the Cl− diffusion rate was accelerated in the later stages of exposure. These experiments further revealed that the Cl− migration rate was higher than that of SO42−.

Highlights

Chloride ion (Cl− ) invasion and corrosion is one of the main causes for the deterioration of reinforced concrete structures in the marine environment [1,2,3]
The weight change rate was calculated according to the relationship: (Wt − W0 ) × 100%/W0
The observed changes in the concrete samples’ appearance after different corrosion times in solutions of 5 wt% NaCl and 0, 2.5, 5, 10, or 15 wt% Na2 SO4 indicated that the surface pores in the CS0.5, CS1, CS2, and CS3 samples gradually increased with corrosion time

Summary

Introduction

Chloride ion (Cl− ) invasion and corrosion is one of the main causes for the deterioration of reinforced concrete structures in the marine environment [1,2,3]. Salt lakes and saline (alkaline) areas contain higher concentrations of Na2 SO4 and NaCl [10]. Cl− content in rivers is about 19.0 g/L and the SO4 2 − content is about 3 g/L; the Cl−. Content in salt lakes is about 19–23 g/L, and the SO4 2 − content is about 22.2–36.4 g/L [11]. The combined invasion and corrosion of Na2 SO4 and NaCl exacerbate the deterioration of reinforced concrete more significantly than the effects of only NaCl corrosion [15,16,17,18], highlighting the complexity of Cl− diffusion in concrete

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