The present invention provides a composition and method for modifying a hydratable cementitious composition such as concrete or mortar using at least three different, distinct carboxylate polymers, even when clay is present in the concrete or mortar that would otherwise diminish dosage efficiency of polycarboxylate polymer used as dispersant. The three polycarboxylate polymers, designated as Polymer I, Polymer II, and Polymer II, are derived from monomer Components A, B, and C wherein Component A is an unsaturated carboxylic acid, Component B is a polyoxyalkylene, and Component C is an unsaturated carboxylate ester. The component molar ratio ranges for A:B:C are different as between Polymers I and II; while a distinct component molar ratio A:B+C is identified for Polymer III. When treated with these three different polycarboxylate polymers, the hydratable cementitious composition is surprisingly enhanced in terms of initial workability and slump retention, especially if clay is present.