The durability of construction works build by using concrete/mortar with the use of Portland cement is dependent on the structural and compositional properties of these materials, as well as on the environment where they are located. Carbonation, acid attack of some ions (sulphates) and alkali-silica reactions are some of the most frequent degradation processes. Two study cases, i.e. the dam on Râşca Mică Valley (Cluj County) and the concrete pedestals in the internal yard of the central building of the “Babeş – Bolyai” University (UBB) in Cluj-Napoca are presented, in the view of illustrating some mineralogical aspects of the above-mentioned degradation processes. Carbonation represents the main degradation process taking place in both cases, and it is due to the reaction between the calcium hydroxide (portlandite) with the atmospheric CO₂ dissolved in the pore and capillary water within the concrete/mortar. Calcite results from this process, as pore and fissure filling, as matrix for the concrete/mortar, or as efflorescence at the surface. The deposition of calcite is obviously zoned, as compared to the mortar/air interface while the levigation of solubilized calcium from the concrete/mortar is probably a stimulant for the alkali-silica reactions. The sulphide attack was noticed only in the case of the UBB building, as a consequence of the act that the sulphate anionic group was released in the specific microenvironment by degradation of sulphide ore samples located in open air in the close vicinity. This process resulted in the formation of secondary sulphates: ettringite and gypsum. Ettringite was mainly deposited in the pores of concrete as acicular-fibrous crystals, this being an argument for its subsequent formation after the hardening of the concrete. The secondary silica gels have been microscopically identified in the mortar from the Râşca Mică valley dam as infillings in the mortar’s pores and fissures, subsequent to the deposition of calcite. The gels show processes of partial crystallization, evidenced by the optical diffuse anisotropy.