A copolymer containing, as monomer components, a) at least one sulfonic acid-containing compound, b) at least one nitrogen-containing N-vinylamide, one acrylamide or methacrylamide and c) at least one bifunctional or higher-functional vinyl ether is proposed. These copolymers are suitable in particular as thixotropic agents, antisegregation agents and water retention agents and overall as an additive in cement-containing but also gypsum-based systems. Such copolymers having a molecular weight of >40 000 g/mol can be used in particular as admixtures in the exploration of mineral oil and natural gas deposits.