Both lime and gypsum are among the oldest of construction materials. Various forms of lime were used in mortar and plaster and to make concrete by the early Egyptians and Greeks. It was also used extensively by the Romans, in China and in other societies (Lea 1970). The problem of adequately discussing lime arises from the large number of ways this material has been used for construction in the past, many of which have now lapsed in the West although some of these older techniques are still used in other parts of the world (Hill et al. 1992). The use of lime in repair to historic buildings, however, is increasing (Leslie and Gibbons 2000) and petrographic analysis of historic mortars has become a signi‡cant tool in conservation work. Lime is also discussed in Chapter 4, Section 4.2.1.