In the years 1908-1909 a large reinforced concrete tank for drinking water was built in Pozna(n).The plan dimensions are 65.5×21.5 m, and the height is 9.0 m above and 5.0 underground.Those were the times when the first bold constructions from the cement concrete were built in the world, very few of which have been preserved till the present day.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.This year the authors of the project conducted a material and structural analysis of the technical condition of the over hundred year old tank structure, which proved that the compressive strength of the concrete is lowered, and there are numerous scratches of the walls and the ceiling,and also some accumulations of white lines and icicles outside.The SEM, EDS and XRD analyses proved that the phase composition of the concrete in the barrier thickness is variable.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.