The amount and geometry of pores in hardened cement paste phase, that affect the mechanical behaviour of concrete, have been considered in the form of two different parameters: porosity and capillary sorptivity coefficient (or water/cement ratio) where the latter represents the geometry of pores in hardened concrete. A mathematical model based on the damage mechanics approach has been developed to relate the compressive strength of concrete to these two parameters. Equations developed in this model have been applied to the values obtained in an available experimental work, in which porosities, sorptivity coefficients and strengths were measured for 40 different concrete mixtures. For indirect determination of the water-cement ratio in both hardening and hardened concretes, a petrographic examination method using optical fluorescence is presented. Additionally, five concrete mixtures with different water-cement ratios were used to determine their fluorescence intensities, sorptivity coefficients and compressive strengths. It is shown that the flourescence intensity is directly affected by the water-cement ratio of concrete. Good agreement has been found between experimental and calculated results.