Some physical and chemical characteristics of hardened cement concrete, which influence its strength and durability, are given quantitatively in terms of the chemical composition of the pozzolanic cement. The chemical reactions of pozzolanic activity and hydration are presented and used to determine the final quantity of CSH, calcium hydroxide, etc., produced per unit weight of pozzolan cement, and the minimum water-cement ratio required for hydration. Expressions are given for the final values of porosity of pozzolanic cement paste, mortar, or concrete, as affected by hydration, pozzolanic action, and carbonation, in terms of the composition of cement and of concrete or mortar mix. The effect of the pozzolans on the pore-size distribution of hardened paste is studied experimentally, and that effect on the degree of pore saturation is studied both experimentally and analytically. The effective gas diffusivity of pozzolanic cement mortar or concrete is measured, and a semi-empirical expression, developed earlier for ordinary portland cement (OPC) concrete, is fitted to the measured values. The applicability of a fundamental, yet simple, predictive model for concrete carbonation is verified experimentally, and its parameters are specified for pozzolanic cement concrete. Conclusions are drawn on the favorable or adverse effects of OPC pozzolan or aggregate pozzolan replacement on CSH-content, porosity, and rate of carbonation.