Reivindicaciones
1. Additive for gypsum and special mortar that comprises a solid mineral component selected from among clay, characterized in that it comprises at least a modified natural gum, and in that the mineral component is a rheological grade clay selected from among attapulgite, sepiolite and mixtures thereof. 2. Additive according to claim 1, characterized in that the additive comprises at least 20-75% by weight of the solid mineral component; 25-80% by weight of modified natural gum. 3. Additive according to claim 1, characterized in that it comprises 35-60% by weight of the solid mineral component; 40-65% by weight of modified natural gum. 4. Additive according to claim 1, 2 or 3, characterized in that the solid mineral component is rheological grade sepiolite. 5. Additive according to claim 1, 2 or 3, characterized in that the modified natural gum is at least one combination of at least two monosaccharides, glucose, mannose, galactose, and glucuronic acid, modified by etherification. 6. Additive according to claim 6, characterized in that the combination of monosaccharides is sterified with an alkylene oxide selected from among ethylene oxide, propylene oxide and butylene oxide. 7. Additive according to claim 6, characterized in that the monosaccharide combination is a galactomanane. 8. Additive according to claim 1, 2 or 3, characterized in that the modified natural gum is modified guar gum. 9. A filled composition of conglomerate building material selected from among gypsum and special mortar, that comprises a fraction selected from among a cement fraction, a gypsum fraction and an aggregate fraction, characterized in that it also comprises 0.05-1.2% by weight of the additive defined in any of the claims 1 to 8. 10. A filled composition according to claim 9, characterized in that it comprises 0.2-0.9% by weight of the additive. 11. Use of an additive according to any of the claims 1-8, as a component in a special mortar. 12. Use of an additive according to any of claims 1-8 as a component in gypsum.