Effects of Carbonation and Freeze-thaw Cycles on Microstructure of Concrete

The hydraulic concrete durability under the alternation of freeze-thaw and carbonation has been systematically investigated in this work, where both the micro part and the microscopic characteristics of concrete interface were analyzed based on computed tomography(CT) test and scanning electron microscopy(SEM). Average CT numbers of each section, declined at water-cement ratio of 0.35, increased at 0.45, and changed a little at 0.55. The specimen in the absence of fly ash exhibited less types of hydration products and the surface was observed to be a needle-like ettringite, with a relatively dense overall structure. However, with the increase of fly ash content, pores and micro-cracks of specimen structure increased. Hexagonal flake calcium hydroxide, present in the specimen after the first carbonation, was negligible in the test pieces of the first freezethaw where the main hydration products were ettringite and calcium silicate gel. Regular hexagonal plates of calcium hydroxide exhibited in the interior of the specimen in which charring first occurred but calcium hydroxide rarely existed in the interior of the specimen in which freeze-thaw first occurred.