PORE STRUCTURES AND SURFACE AREAS OF HARDENED PORTLAND CEMENT PASTES BY NITROGEN ADSORPTION

Abstract

The investigations were performed on five hardened pastes of a type I (normal) portland cement, having initial weight ratios of water to cement of 0.35, 0.40, 0.50, 0.57, and 0.70, and hydrated for 12, 12, 12, [Formula: see text], and [Formula: see text] years, respectively. The percentage hydration ranged from 90 to 98%.Nitrogen adsorption and desorption isotherms were determined for all pastes at the boiling point of nitrogen, from low pressures to almost the saturation pressure. The BET surface areas were calculated, and the pore space accessible to nitrogen was obtained by a slight extrapolation of the isotherms. In addition, pore structure analysis was made for three of the five pastes by the method of Cranston and Inkley.The specific surface areas of the five pastes were also determined by water vapor adsorption, and the total porosity to water was measured. For every paste, the water areas and porosities were greater than the nitrogen areas and porosities.The main conclusions from the results were as follows.(1) There is a wide range of pore sizes in the pastes.(2) Nitrogen is excluded from a part of the pore system by two mechanisms; some of the pores are too narrow and others have too narrow entrances to admit nitrogen.(3) The hydraulic radius (the volume of the pore system divided by the surface area of the pore walls) for the system of small pores increases with increasing water to cement ratio.(4) The hydraulic radius of the system of large pores also increases with increasing water to cement ratio, but the increase is smaller than for the small pores.(5) The paste with water to cement ratio of 0.35 has enough pore space for complete hydration.Some practical implications of the above conclusions are discussed.