Quantifying Effect of Later Curing on Pores of Paste Subject to Early-Age Freeze-Thaw Cycles by Different Techniques

Freeze-thaw cycles can damage the microstructure of concrete and reduce the service life of structures, which is particularly detrimental for concrete at very early ages. What size range of pores can be affected by the early-age freeze-thaw cycles and whether the later-age curing can recover the damage to pores are critical questions for understanding the mechanism and avoiding frost damage. Typical low and high water-to-cement (w/c) ratio pastes were investigated experimentally in this study in terms of the pore-size distribution. The pastes were subject to freeze-thaw cycles at the ages of 1 and 7 days and then sealed cured to the age of 40 days for pore-size distribution measurement by the techniques of mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM) with backscattered electron (BSE), and X-ray computer tomography (X-ray CT). The detectable pore-size range by MIP, BSE, and X-ray CT in hardened paste was identified. The pore characteristics within each size range were quantified. It was found that the pores in high w/c $\mathrm{ratio}=0.5$ paste have been coarsened significantly compared to the low w/c $\mathrm{ratio}=0.3$ paste, and the later sealed curing cannot recover the damage. Combination of different techniques can achieve a full-scale quantification of pore size. The total porosity of the samples cannot be directly determined by adding different porosity obtained by different techniques. The results of this study will offer suggestions for material design and curing strategies for concrete structures that are prone to experiencing freeze-thaw cycles at very early ages.