New insights of MgCl₂ attack to cement mortar in the condition of low air pressure

Almost all the studies about the deterioration of cement-based materials by chloride salts are conducted in the condition of curing and exposing the samples in the normal air pressure. In this study, Portland cement paste and mortar were firstly cured in the environment with low air pressure (60 kPa) and then transferred to the immersion in the10 wt.% MgCl₂ solutions under the same air pressure up to 6 months. Experimental results revealed a new corrosion mechanism during the MgCl₂-induced deterioration of semi-immersed mortar, i.e., the formation Mg₂(OH)₃Cl·4H₂O with fireworks-like shaped (surface) and dandelion-like shaped (inside), leading to a nearly intact surface and cracked interior with peeling onion shape. Besides, the generation of Mg₁₀(OH)₁₈Cl₂·5H₂O with higher alkalinity in the full-immersed cement paste resulted from the presence of Mg(OH)₂ layer on the surface. Two types of white sediments found on the surface of paste and mortar are respectively composed of Mg(OH)₂ and M − S[sbnd]H detected by XRD, SEM/EDS, and TG-HDSC. Finally, the accumulation of MgCl₂-induced corrosion products resulted in significantly deteriorations in pore structure as well as compromised compressive strength.