Microscopic characterization of ice morphology in entrained air voids

In this paper, the morphology of ice as it appears in the entrained air voids of hydrated portland cement paste is examined in detail. The main experimental technique employed in this work is the low-temperature scanning electron microscope, which allows for the imaging of frozen, hydrated, cement-paste specimens. Four main classes of cement-paste specimens were examined. First, normal cement-paste specimens are examined when frozen after 4 days of moist curing. Herein, discrete ice crystals were seen growing from the air void wall, making an average contact angle of 91 degrees with the air void wall. When bulk ice was observed in the air voids of these specimens, it appeared to be smooth in texture and without striations. Second, cement-paste specimens made with CaCl₂ dissolved in the mixing water were examined to determine the effect of deicing salts used on concrete surfaces. For these specimens, striations were seen in the bulk ice found in the air voids, assumed to be eutectic lines that form when the discrete crystals grow together and coalesce. The third type of specimen is early hydration specimens, where normal cement-paste specimens were frozen after 4 h of hydration. Herein, ice crystals were seen to grow outside of the air void shell in the air void transition zone. These crystals pushed the air void shell out of their way, indicating the relative weakness and impermeability of the air void shell at 4 h of age, compared with the mature air voids in the first two sections. The last type of specimen is normal cement paste subjected to freezing-and-thawing cycles. After three freezing-and-thawing cycles, large hydration products were observed in the air voids; these hydrates could be significant enough in volume to affect the frost-protecting capacity of the air voids.