The invention relates to an additive composition, which comprises a thickener and a thixotropic agent and is characterized in that the thickener is cellulose or a derivative thereof and that the thixotropic agent is a pyrogenically produced silicic acid, the surface of which is modified with groups of general formula (I) —SiRaR1bOR2c (I), in which a can equal 1, 2, or 3, b can equal 0, 1, or 2, and c can equal 0, 1, or 2, wherein a+b+c=3, and R can be a monovalent, optionally monounsaturated or polyunsaturated, optionally branched hydrocarbon group having 1 to 24 carbon atoms, R1 can be a likewise monovalent, optionally monounsaturated or polyunsaturated, optionally branched hydrocarbon group having 1 to 20 carbon atoms, and R2 can be a hydrogen atom, a monovalent, optionally monounsaturated or polyunsaturated, optionally branched hydrocarbon group having 1 to 20 carbon atoms, or a bond to another Si atom, with the stipulation that at least one of the groups R or R1 is a hydrocarbon group having more than 3 carbon atoms.