A well cementing composition and method are shown in which a hydraulic cement, water, and a heat actuated matrix control admix are mixed together to provide a cement slurry composition having improved cementing characteristics over a broad temperature range. The admix contains a polyvinyl acetate-polyvinyl alcohol polymer which is insoluble in the slurry composition at ambient temperatures and which has greater than about 95% acetate groups converted to hydroxyl groups. As the circulating temperature rises in the range from about 90.degree. to 140.degree. F., the polymer begins to solubilize and gradually thickens the slurry to combat thermal thinning, fluid loss and settling. A companion cellulosic fluid loss material can be utilized, which is soluble in the slurry at ambient temperatures, to provide initial fluid loss control.