Abstract: The specification discloses a self-hardening mold sand having surface stability and high compressive strength yet containing a comparatively small amount of water. The mold sand is prepared by mixing a self-hardening binder of clinker powder, containing 11CaO.7Al.sub.2 O.sub.3. CaX.sub.2 (where X is a halogen atom) and a sulphate, with a binder additive consisting of a surface active agent of which the main component is .beta.-naphthalene-sulphonic acid formaldehyde condensate salt, and then mixing this binder and binder additive with sand, water and preferably a retarder consisting of an organic carboxylic acid or boric acid.