Molding material mixture, molded part for foundry purposes and process of producing a molded part

A molding material mixture for use in a foundry, said molding material mixture consisting of a mold sand, a sodium hydroxide aqueous solution, a binding agent based on alkali silicate and additives, wherein the mold sand particles comprise a grain size of 0.1 to 1 mm, the molding material mixture contains 0.1 to 10 percent by weight of sodium hydroxide aqueous solution with reference to the weight of the sand, and wherein the sodium hydroxide aqueous solution comprises a concentration of 20 to 40 percent by weight, the molding material mixture contains 0.1 to 5% of binding agent based on alkali silicate with a solid matter percentage of 20 to 70%, the molding material mixture, as the additive, contains 0.1 to 3 percent by weight of a suspension with a solid matter percentage of 30 to 70% of amorphous, spherical SiO2 in two grain size classifications in the suspension with a first grain size classification A containing SiO2 particles with a grain size ranging between 1 and 5 micrometers and with a second grain size classification B containing SiO2 particles with a grain size ranging between 0.01 and 0.05 micrometers, and wherein, the volumetric amount of amorphous, spherical SiO2 particles having the first grain size is in the range of 0.8 to 1.2 times the volumetric amount of amorphous, spherical SiO2 particles having the second grain size.