The results of own studies concerning the application of a new additive to the CO2-hardened sodium water glass foundry sands are presented. The new additive, which is a composition of aqueous solutions of modified polyalcohols, has been designated by the symbol “B” and is used as an agent improving the sand knocking out properties. The scope of studies included various mechanical and technological properties of foundry sand mixtures, such as permeability, friability, life cycle of cores and knocking out properties. Two types of water glass with different values of the silica modulus and density, designated as R145 and R150, were tested. Moulding sands used in the tests were made with the additive “B”. For comparison, a reference sand mixture with water glass but without the additive “B” was also prepared.