Aspects of concrete porosity revisited

Aspects of the pore structure of conventional concrete are reviewed. The appearance and distribution of pores are illustrated in the context of the microstructure of normal concretes as viewed in backscatter mode scanning electron microscopy. A significant portion of at least the larger pores found in concrete is considered to be derived from the hollow shell (Hadley grain) hydration mechanism. These pores therefore do not represent remnants of the original space between cement grains, the accepted concept of the origin of "capillary pores." A brief review of the characteristics of surface fractals is provided and experiments are described, indicating that the surfaces constituting the boundaries of at least the larger pores in concrete are fractal in nature, at least over a limited range of self-similarity. It appears that the characteristic fractal dimension describing the pore surfaces is almost constant, regardless of age, water:cement ratio, or most other variables. (C) 1999 Elsevier Science Ltd. All rights reserved.