Mercury porosimetry - An inappropriate method for the measurement of pore size distributions in cement-based materials

The conditions that must be met for mercury intrusion porosimetry (MIP) measurements to provide valid estimates of the pore size distribution of porous solids are reviewed. Evidence is presented indicating that these conditions are not satisfied in cement-based systems. In cement-based systems, nearly all of the mercury intrusion is held up until pressure corresponding to the threshold diameter is reached; subsequently, large and small pores are filled indiscriminately. Air voids, in sizes up to several hundred micrometers, are present in most pastes in substantial volume, unless the pastes were mixed under vacuum; these air voids are also not intruded until the threshold pressure is reached, and are recorded as fine (ca. 0.1 mum) pores. It is concluded that MIP measurements are useful only to provide threshold diameters and intrudable pore space measurements, which can serve as comparative indices for the connectivity and capacity of the pore systems in hydrated cements. MIP measurements should be abandoned as measures of the actual pore sizes present. (C) 2000 Elsevier Science Ltd. All rights reserved.