mortar, in building, mixture of lime or cement with sand and water, used as a bedding and adhesive between adjacent pieces of stone, brick, or other material in masonry construction. Lime mortar, a common variety, consists usually of one volume of well-slaked lime to three or four volumes of sand, thoroughly mixed with sufficient water to make a uniform paste easily handled on a trowel. Lime mortar hardens by absorption of carbon dioxide from the air. Once universally used, lime mortar is now less important because it does not have the property of setting underwater and because of its comparatively low strength. It has largely been supplanted by cement mortar, commonly made of one volume of Portland cement to two or three volumes of sand, usually with a quantity of lime paste added to give a more workable mix. Cement mortar, besides having a high strength, generally equal to that of brick itself, has the very great advantage of setting or hardening underwater. Other varieties include gauge mortar, for rapid setting, composed of plaster of Paris used either pure or combined with lime or with lime and sand, and grout, a thin liquid mixture of lime or cement, poured into masonry to fill up small interstices. Primitive mortars took various forms: in early Egypt, Nile mud was used as an adhesive; the Mesopotamians used bitumen (the slime mentioned in Genesis) or sometimes a mixture of clay, water, and chopped straw, to cement together their unbaked bricks; Greeks of the Mycenaean era probably employed a soft bituminous clay. The advanced Greek buildings are notable for their construction without mortar, the huge blocks of stone being consummately fitted with dry beds. The Romans likewise used little mortar in cut stonework or vaulting but in later periods bedded the rough stone of their mass masonry in strong cement mortar. In medieval times and in all periods since, mortar of some sort has been almost universally used in masonry construction.
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