The article investigates the hydration processes of free calcium and magnesium oxides, as well as anhydrite, with the formation of ettringite, portlandite, and calcite in the high-calcium fly ash from Berezovskaya GRES. The composition and properties of non-hydrated ash and its hydration products during hardening in water at a temperature of (20 ± 2)°C were determined using physico-chemical analysis methods. Kinetic constants for the reduction of calcium oxide and anhydrite, as well as for the formation of portlandite and ettringite, were determined. It was found that after 7 hours of hydration, 11.5% of active calcium oxide remained, and about 3% may not hydrate further. Free magnesium oxide practically did not change its activity during the entire observation period, indicating its low hydraulic activity.