Study on the influence of early frost attack on the performance of cement mortar

Abstract

The loss of strength and durability caused by early frost attack is the macro performance of internal microstructure deterioration. Understanding the influence mechanism of early frost attack at the micro level is necessary to avoid or minimize the early frost attack to macro performance. In this study, the influence of early frost attack on the strength, water sorptivity, chloride permeability, and microstructure of cement mortar was investigated. The fresh mortars were pre-cured for 10, 12, 14, and 16 h and then frozen at −10 °C for 7 days to simulate an early frost attack. After freezing, the standard curing (20 °C) was carried out until the testing. It is found that after standard curing of 28 days, the strength of early-frozen mortar can reach more than 95 % of the 28-day strength of unfrozen mortar. Interestingly, the sorptivity coefficient and total charge passed of early-frozen mortar are much higher than the level of unfrozen mortar, and the loss of resistance to water penetration and chloride ion penetration exceeds 20 %, indicating that early frost attack causes more severe damage to the durability than to the strength of mortar. Mercury intrusion porosimetry (MIP) results show that early frost attack can increase the pore volume of >50 nm and coarsen the pores within 10–50 nm. Correspondingly, key pore parameters (critical pore diameter, threshold pore diameter, and tortuosity) that affect the durability of mortar are also negatively affected. Energy dispersive spectroscopy (EDS) analysis also shows that after suffering early frost attack, the interfacial transition zone (ITZ) thickness of mortar is increased from 4 to 10 μm to 8–13 μm, and the probability of high Ca/Si ratios (4–10) in the ITZ is also increased. Finally, it is concluded that the coupling effect of pore structure coarsening and ITZ degradation leads to more severe durability loss.