Abstract: A mixing method uses, and a colloidal grout mixing apparatus comprises, an upright cylindrical vessel in which a mixer is located. The mixer comprises a shaft extending substantially co-axially with the longitudinal axis of the vessel, and a paddle assembly mounted on the shaft. The paddle assembly produces a vortex in liquid in the vessel, and also provides a high shear region near the base of the vessel. A non-shear pump is connected to the base to discharge grout from the vessel. The grout may be discharged to a drum containing radioactive waste and mounted on a vibratory platform.