Mark B. knows a bit about cement so we asked him for an answer. Here it is.
Well, to start lets understand the difference between cement and concrete. Cement does not contain rocks, when we mix rocks with cement we create concrete.
An aggregate, rocks, gravel or sand, (chemically inactive stuff) is mixed with cement to change its properties making it more suitable for any given application. Strength, weight, cost and setting time are typical properties which change with amounts of water, cement and aggregate.
Cement becomes chemically active when mixed with water. The four main ingredients found in cement are calcium compounds. The ingredients are ANHYDROUS, that is they contain no water. When water is added to the mixture, a chemical reaction takes place, and long, needle-like crystals are formed. The crystals lock together , binding the cement and aggregate into a mass of great strength and hardness. A small amount of gypsum in the cement prevents it from setting too quickly when water is added. This allows people to use and work the cement before it hardens.
Ancient Egyptians used hydrated lime and gypsum (calcium sulfate), (plaster of Paris) cements. The Romans discovered by adding pozzlanna-a natural volcanic ash found near Mount Vesuvius that this made the concrete much much stronger and one that hardened under water. Now they could use concrete for making there bridges and aquaducts. This basic mix was used through out the "Dark Ages"- 11th to 15th centuries.
In 1824, Joseph Aspdin took out a patent in England on a process for making Portland cement. It was so named for its resemblance to the limestone quarried on the Isle of Portland in Britain. Portland cement is still used today for mortar when mixed with sand and water.
Answer #% % filename A146.DOC page% % page 1 of% % numpages 1 % % date \@ "MMMM d, yyyy"April 19, 1995