High temperature well cement compositions and methods

In a method of cementing a subterranean zone penetrated by a well bore with a retarded cement composition which is substantially non-thinning at high temperatures comprised of hydraulic cement, sufficient water to form a pumpable slurry, a set retarder and a delayed viscosifying agent which yields viscosity at high temperatures comprised of a treated galactomannan gum, and said cement composition is formed on the surface, pumped into the subterranean zone by way of the well bore and allowed to set therein, the improvement which comprises utilizing as the delayed viscosifying agent a galactomannan gum which has been depolymerized and treated with a complexing agent whereby the hydration rate of said depolymerized gum is retarded at temperatures below about 120.degree. F. to 140.degree. F. and at pH levels above about 10, but the hydration rate increases at temperatures above about 120.degree. F. to 140.degree. F., said treated depolymerized gum being present in said composition in an amount sufficient to offset the thinning of said composition at temperatures above about 120.degree. F. to 140.degree. F. by the hydration of said gum.