Developed in the early 1980s in the Netherlands, large calcium silicate blocks speed wall construction and reduce costs. Two workers can build 1,000 square feet of wall in an 8-hour workday. The blocks are made large for fast construction. Face dimensions range from 24x12 inches up to 36x24 inches. Thickness ranges from 4 to 12 inches. To lay such large blocks (which weigh from 230 to 600 pounds each), masons use a small transportable electric crane and mechanical grippers. Calcium silicate masonry is made from 94% sand or other siliceous material and 6% quicklime. Because the units are pressed in machined molds and hardened in an autoclave at a constant temperature and humidity, they have clean edges and consistent dimensions. Usually only the bed joints are mortared (using a conventional masonry mortar with fine sand). Mortar is not needed on head joints because they interlock by tongue and groove. Because of the large block size, thin bed joints, and unmortared head joints, this wall system uses only 25% of the mortar used in a conventional concrete masonry wall.