A mixture is used in conjunction with water is used for preparing a slurry that hydrates to form an exterior gypsum cement. The mixture includes 30-70% by weight hydraulic cement, 30-70% by weight calcined gypsum and 0.05-2.5% polycarboxylate. When the mixture is added to water less than 40 weight % based on the weight of the mixture, a slurry is formed that expands very little while it cures.