Pore size distribution is an important factor that affects the moisture diffusion and permeability properties of cement-based materials. For cement paste, it is assumed that the mesopores (100 nm–0.01 mm) and macropores (0.01 mm–1 cm) can be neglected compared to the micropores ($<100\mathrm{nm}$ ). Based on the surface energy theory, moisture chemical potential of pore water is introduced to explain the liquid-gas equilibrium in pores with different radius. Using chemical potential as an intermedium, a quantitative relationship between micropore size distribution and water adsorption isotherms can be established. The micropore size distribution can be treated as an explanation of the moisture adsorption behavior, and by using adsorption isotherms, the micropore size distribution can be estimated conveniently. Mortar and concrete can be regarded as a combination of cement paste and aggregates; thus, besides the micropores, the mesopores and macropores are also taken into consideration. Finally, the general empirical estimation equations for pore-size distribution are developed for cement, mortar, and concrete, which can be used for refined modeling and simulating of durability-related issues, such as frost action, water permeability, and drying shrinkage.

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