A molding material mixture for foundry purposes, comprising a mold sand, a sodium hydroxide solution, an alkali-silicate binding agent, and an additive suspension. The molding material mixture contains 0.1 to 10% by weight of sodium hydroxide solution with reference to the weight of the sand, 0.1 to 5% of binding agent with a solid matter percentage of 20 to 70%, and 0.1 to 3% by weight of the additive suspension. The additive suspension comprises 30 to 70% by volume of an amorphous, spherical SiO2 particles, which have a first grain size in the range of 1 to 5 micrometers, and a second grain size in the range of 0.01 to 0.05 micrometers.