Study on water sorptivity of the surface layer of concrete

This paper presents the results from a study of water sorptivity of concrete surface layer. The sorptivity is characterized by a surface sorptivity index as measured by Autoclam. In this study, different types of concrete were immersed in ultrapure water and NaCl solution prior to the sorptivity test. The influences of several factors on the value and evolution of concrete surface sorptivity index are discussed. It is found that: concrete surface sorptivity is a function of the pore structure, higher porosity and lower tortuosity lead to higher surface sorptivity; as cured in moist condition for 1 month, the surface sorptivity is an increasing function of w/c in plain cement concretes, and an increasing function of fly ash replacement if w/b is kept constant; surface sorptivity increases as immersed in ultrapure water in the first month of immersion due to leaching, and decreases thereafter as the continuous hydration of cementitious materials makes the pore structure finer and finer; the immersion in NaCl solution limits the effect of leaching because of the formation of calcium oxychloride compounds, and results in lower long-term surface sorptivity index as compared with the ultrapure water immersion, due to the formation of Friedel's salt which reduces the pore volume and blocks the pore network.