Red mud, from an alumina plant at Point Comfort, Texas, molded into balls and heated in a muffle furnace to 2300 to 2400°F (1260 to 1316°C), partially crystallizes and forms a strong, heavy, synthetic aggregate. Attempts to produce a lightweight aggregate by adding small amounts of gas-forming materials to the red mud before heating were unsuccessful. The additives used and the products formed are described. Apparently the red mud does not soften until heated to nearly 2400°F. Gases given off at lower temperatures either blow the aggregates apart or escape through cracks without expanding the mass.