use of mortar of cement and sand to form the water contacted innenbereiches of biologically aufbereitbaren waters, in particular of schwimmteichen, naturpools or fish tank.when cement is a pure kalziumaluminat cement and sand dolomitsand and / or quartz sand, using the kalziumaluminat cement a maximum of 2 wt.% to sulphates, phosphates and silicates containing and his share in dry 5 wt.% to 30 wt.%.