Macro-micro degradation process of fly ash concrete under alternation of freeze-thaw cycles subjected to sulfate and carbonation

Concrete is one of the most extensively used building materials. In the cold salty lake region, concrete structures generally undergo freeze-thaw cycles subjected to sulfate in the winter and early spring, and experience carbonation during the rest of the year. It is necessary to study the degradation process of concrete during the alternation of freeze-thaw cycles subjected to sulfate and carbonation. In this article, two types of experiments were implemented: freeze-thaw cycles subjected to sulfate (SF), and the alteration of freeze-thaw cycles subjected to sulfate and carbonation (SFC). Computed tomography (CT), as a non-destructive testing technology, was adopted to reveal the rules of pore changes and show generated cracks after the alternation of freeze-thaw cycles subjected to sulfate and carbonation. CT scanning results showed that pore numbers, pore areas, and porosity of each cross-section changed to some extent. Due to the different distances from the top, the changes of each cross-section were found to be dissimilar. From the 2D images, SFC specimens were identified to have corner erosion and internal cracks, and were damaged more severely than SF specimens. The compressive strength loss of SF specimens was very limited in the early state; after a certain period of erosion, the compressive strength decreased rapidly. The compressive strength loss of specimens under SFC was larger than the specimens under SF. Carbonation constantly consumed calcium ions, which negatively affected the stability of porosity system. Freeze-thaw cycles caused aperture degradation, coarsened the pore structure, and with the additional interaction of sulfate erosion, aggravated the deterioration of concrete.