Characterization and Modeling of Pores and Surfaces in Cement Paste: Correlations to Processing and Properties

Cement-based materials have complex multi-component, multiscale structures that first form through chemical reaction and then continue to change with time. As with most classes of materials, the porosity of cement paste strongly influences its properties, including strength, shrinkage, creep, permeability and diffusion. Pores in cement paste range in size from nanometers to millimeters, and numerous investigations and models have been reported in the literature. This paper reviews some key concepts and models related to our understanding of the pore system and surface area. A major reason for the complexity of cement-based materials is that the principal reaction product, calcium silicate hydrate (C-S-H), forms with a significant volume fraction of internal, nanometer-scale pores. This gel pore system contains water that is also adsorbed to the solid surfaces, blurring the distinction between the solid phase and pores. The gel pore system changes not only with the chemical composition and extent of reaction, but also with changes in relative humidity, temperature, and applied load. Pores can be characterized by their surface area, size, volume fraction, saturation, and connectivity, but precise quantitative models are still not available. A useful approach for characterizing the structure of cement paste is to document the influence of time and external factors on structural changes. Scientific progress will be facilitated by the development of models that accurately describe the structure and use that structure to predict properties. This is particularly important because the composition and chemistry of commercial concretes is changing more rapidly than laboratory experimentation can document long-term properties such as durability. Some of the possible models are discussed.