The invention relates to a mineral composition for the preparation of foundry molds, comprising: (a) from 20% to 90% by weight of plaster, (b) from 10% to 80% by weight of a mineral component based on silica and/or alumina, and (c) from 0.5% to 4.8%, preferably from 1.5% to 4.5% and in particular between 2% and 4.5% by weight, of a mineral powder having a thermal conductivity (λ), at 20° C., of greater than 15 W/(m·K) and a specific surface area of greater than 10 m2/g, these percentages being relative to the total weight of the sum of the components (a), (b) and (c).