A portable mixing apparatus that employs disposable 5-gallon plastic buckets as containers for mixing mortar, grout, plaster and other materials. First and second motor-driven rollers support the buckets at a downwardly sloped angle. A separate mixing basket is placed within the buckets, and includes a plurality of blades that pass through the material as the bucket is rotated by the rollers. The leading roller is positioned lower than the trailing roller so as to engage the flexible wall of the bucket in an area where it is subject to outward pressure from the material during rotation. The rollers are provided with ribbed, non-slip covers having channels that accommodate water and debris on the exteriors of the buckets. The mixing basket is formed of a plurality of coaxial, longitudinally-spaced rings joined at their edges by longitudinal rods. The blades are mounted internal to the rings and extend generally parallel to the axis of the buckets, with the ends of the rods being angled in the direction of rotation. The drive rollers and motor are mounted in a frame having folding legs so that the assembly can be collapsed to a compact form for transportation or storage.