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A basic knowledge of block production can help architects, masonry contractors, and engineers. RAW MATERIALS Aggregates used to produce normal-weight block include sand, gravel, and crushed stone. Aggregates used in lightweight block include expanded shale, clay, slate, and blast-furnace slag; sintered fly ash; coal cinders; and natural materials such as pumice and scoria. BATCHING AND MIXING Two types of mixers commonly are used: planetary (sometimes called a pan) and horizontal drum. A planetary mixer resembles a large, shallow bowl and is lined with wear plates. A horizontal drum mixer resembles an oversized coffee can turned on its side. To assure batch consistency, water usually is added to the dry mix using a computer-controlled electronic water meter. Admixtures, such as water repellents and coloring agents, also are added at this time. MOLDING After a batch is mixed it is transferred to a hopper at the top of a block machine, where it is fed into molds. Molds consist of a mold box with replaceable liners. Block are molded in one- piece molds and are pushed out of their molds at the end of the molding process. Compacted and consolidated block are pushed out of the mold onto a pallet and transported to a curing environment. At this point, the block are referred to as "green." CURING Once a curing rack is filled with pallets of green block, the entire rack is transported, usually on a rail-mounted automated rack transporter system, to either a low-pressure or high- pressure (autoclave) steam-curing kiln.