Phosphate-containing polycarboxylate polymer dispersant compositions of the invention comprise a polymer made from polyoxyalkylene groups, acrylic acid groups, mono-ester groups, and optionally di-ester and tri-ester groups, all in specific molar ratios. The polymer dispersants of the invention achieve quick mix-in dispersibility, particularly within hydratable cementitious compositions such as concrete, in comparison with polycarboxylate polymer dispersants that do not contain phosphate groups, while having improved initial slump as well as slump retention compared to phosphate-containing polymers of the prior art.