It was explained earlier that hardened cement is characterised by a porous structure, with a minimum porosity of some 28%, which is reached when all the capillary pores become completely filled with the cement gel (see section 2.4). This may occur, theoretically at least, in a well-cured paste made with a water to cement (W/C) ratio of about 0·40 or less. Otherwise, the porosity of the paste is much higher due to incomplete hydration and the use of higher W/ C ratios. In practice, and under normal conditions, this is usually the case, and a porosity in the order of some 50%, and more, is to be expected.