Microstructural Investigation of Coupled Sulfate and Freeze–Thaw Damage of Concrete Using Micro-Computed Tomography

Concrete structures in sub-zero temperatures can be severely damaged due to freeze–thaw cycles. The damage can be aggravated if the concrete is exposed to different sulfate environments. The damage is observed at a macroscale level; however, little attention has been paid to investigating the damage at the microscale level representing the early stage of damage. Therefore, the focus of this investigation is to study the microscale damage mechanisms of concrete subjected to up to 80 freeze–thaw cycles in different environments: water, potassium sulfate, and magnesium sulfate with 5 and 10% concentrations. It was observed that exposure to potassium sulfate significantly accelerated the frost damage leading to a complete disintegration of the samples, whereas the control specimen lost about 20% of its mass after 80 cycles. However, the typical mechanisms of frost damage were not altered, where the scaling damage started at the external surface and propagated toward its core. On the other hand, subjecting concrete to magnesium sulfate mitigated the severity of frost damage and changed its mechanisms, resulting in more expansion within the internal pores than the surface ones. Hence, the average mass loss of concrete after 80 freeze–thaw cycles in the magnesium sulfate environment was only 1.38%. Also, increasing the concentration of magnesium sulfate produced more frost damage.