The invention relates to a polymer which is obtainable by copolymerizing monomers comprising at least one ethylenically unsaturated monomer which comprises at least one acid group and at least one ethylenically unsaturated monomer (II) having polyether groups with 5 to 35 repeating units and at least one ethylenically unsaturated monomer (III) having polyether groups with 45 to 150 repeating units, the molar ratio of monomer (II) to monomer (III) being between 75:25 and 99.5:0.5. Further disclosed are the use of the polymer of the invention as an admixture for inorganic binder compositions, and a composition in powder form comprising an inorganic binder and the polymer of the invention.