be cast in cement

To be firmly or permanently established; to be unalterable or not subject to change. The healthcare law looks promising, but we'll have to wait until it's cast in cement before we know exactly what it will do. Keep in mind that these blueprints are not cast in cement—they're just to give you a feel for the design. Nothing is cast in cement, of course, but I do think you have a good shot at getting the job.