Described are polymers and cement admixtures which reduce drop in fluidity with time. Said admixture comprises at least one polymer of the present invention and at least one polymer A. The polymer of the present invention comprises side chains of which at least 10 weight-% can be cleaved in alkaline medium. The polymer of the present invention is an acrylic polymer which comprises side chains connected to the backbone by ester linkages. These side chains may be alkyl, hydroxy alkyl, cycloalkyl or polyoxyalkylene groups. Polymer A is a cement dispersing agent. The combination of polymer A with the polymer of the present invention in a weight ratio of 0.1:10-10:1, and preferably 1:10-10:1 reduces the drop in fluidity with time of cementitious compositions.