In the years 1908-1909, a drinking water treatment station was built for the city of Poznan, which was then 196. 000, taking it from the Warta River. The plan dimensions are 65.5 × 21.5 m, and the height is 9.0 m above and 5.0 underground. It is a pearl of industrial architecture of the end of the century. In the underground part, a large-size reinforced concrete tank with a ribbed ceiling supported in the middle with reinforced concrete columns was constructed throughout the projection. In order to estimate the durability of the concrete, after several decades of influence of very high relative air humidity, it is very significant to check the changes in its microstructure, which influence its strength parameters. The SEM, EDS and XRD analyses were done. In the lower layers of the concrete a calcium hydroxide concentration is observed, which outside the barrier changes into the calcium carbonate. The migration of the Portlandite in the direction of the inside of the tank with very high relative air humidity (>95%) results from the mass diffusion caused by different concentration of the porous liquid in the outer and inner layers of the barriers. Those are very slow processes, that last tens of years during the exploitation of the tank. They are beneficial in terms of the gradual alkalinity increase, which protects the steel against corrosion, but they can be harmful due to a local increase of the porosity and the decrease of the concrete strength.