Methods and compositions for sealing subterranean zones penetrated by well bores are provided. The methods basically comprise the steps of introducing a sealing composition into the subterranean zone comprised of a hydraulic cement, sufficient water to form a slurry and a rapidly acting thixotropy providing gelling agent, and allowing the sealing composition to remain at rest in the zone whereby the sealing composition rapidly gels and then sets into a hard impermeable mass therein.