The high strength, watemproofness, and durability of medieval mason's mortars were at-tributed to the high dispersity of gypsum in the gazha stone, intensification of carbonation processes in the presence of calcite, and stractural similarity of the stone phases. Ettringite, hydrocalumite, thaumasite, creedite, calcium monocarbohydroaluminate, hydrated calcium aluminates, and hydrated silicates were identified in the gazha stone. -- AATA