The rheological and physical properties of cast magnesial concretes and the relationship between these properties have been studied. It is established that with an increase in the compressive load, the shear deformation of the cast concretes decreases sharply and the concretes become more rigid. In the concretes studied, when rapid elastic deformation predominates, slow elastic deformation is observed to a lesser extent; and the plastic deformation of the cast concretes is quite insignificant. The viscosity of the experimental concretes is virtually the same and is independent of the composition of the concrete. The cast concretes are characterized by an adequate ultimate compressive strength, adequate thermal shock resistance after drying, a low deformation temperature under a load of 2 kgf/cm2, and by very low gas permeability.