<p>Portland cement concrete building structures are the most common and are used in various conditions. However, their durability varies significantly depending on the operating conditions. The paper is devoted to studying the reasons for the twofold decrease in compressive strength of concrete constantly in contact with hot water. Complex studies (scanning electronic microscopy, x-ray diffraction, and differential thermal analysis) have established the features of the phase composition of Portland cement hydration products depending on heat and humidity conditions. Well-crystallized crystalline hydrates of various compositions and morphologies have been revealed, and druses of large prismatic crystals of α-hydrate of dicalcium silicate have been investigated, the formation of which creates significant stresses that cause a decrease in the strength of the cement paste during long-term hardening in hot water. The main cause clarifying this is the thermal instability of highly dispersed particles of calcium silicate hydrates that provide the Portland cement binding properties. That brings intra- and interphase recrystallization of primary hydrate new growth products, accompanied by impairment of the binding properties. The current study is important for the operation of concrete reservoirs of industrial and thermal power plants, which need special protection methods to increase their durability.</p>