Abstract: New additive composition, in liquid or powder form, for thin slabs, plasters, concretes, prefabricated elements and refrectory elements, with insulating and impermeabilizing properties and with antishrink characteristics, comprising a bonding agent and an additive comprising a mixture of carbonates containing 59-75% CaCO.sub.3, 25-37% Si(CO.sub.3).sub.2, 0-2% Al.sub.2 (CO.sub.3).sub.3, 0-1% MgCO.sub.3 and 0-1% Fe.sub.2 (CO.sub.3).sub.3.