Widely used in every industrialized nation but the United States and Canada, autoclaved cellular concrete (ACC) was invented in Sweden in 1914. This lightweight building material was first commercialized in Sweden and Germany in the 1920s; after World War II, it became popular in the rest of Europe, then spread throughout the world. Also known as autoclaved aerated concrete (AAC), the material is common in residential, commercial, and industrial applications--in low-rise loadbearing walls and high-rise separation and curtain walls. Its popularity is understandable. Although ACC has low compressive strengths and must be covered with siding, stucco, or another weather-resistant coating, the product has excellent insulating, fire-resisting, and sound-dampening properties. The product can be sawed and nailed like wood. What's more, with densities typically ranging from 25 to 40 pounds per cubic foot, ACC block is only 1/4 to 1/3 the weight of conventional concrete block, even though the units are solid. Despite the product's merits, European manufacturers tried unsuccessfully to introduce ACC to the United States in the 1950s. The early 1970s saw another aborted attempt to launch production in North America. Using Swedish technology, a company in Montreal began to manufacture ACC panels, which were used to construct several warehouses and factory buildings in the United States. However, the Canadian plant shut down due to a labor dispute, and builders comfortable with the product no longer had a source of supply.