Mortar is a a mixture of lime, cement, and sand, worked with sufficient water to form a uniform paste that can be handled with a trowel and applied between bricks, stones, concrete blocks, or other materials to bind them together in masonry walls and buildings. Good mortar sticks to the surface of the bricks or stones on which it is buttered and, when properly mixed and used, hardens to lock them together, becoming part of a solid masonry structure that is weather resistant and durable. It is laid thick between the courses and the ends of the bricks or stones.
There are several formulas for making mortar, all using cement, lime, or a mixture of the two, with sand. That most used now in brick construction consists of one part Portland cement, one part lime, and three parts sand. About 10 to 15 per cent of portland cement itself is hydrated lime, and a very strong mortar is made by using one part cement to three parts sand, but it is more easily worked if additional lime is mixed in. For some types of work, masons still prefer lime mortar, once the most commonly used. It is made by adding one part of slaked lime to three or four of sand, and mixing with water. This is the easiest to work, but it is comparatively low in strength. Plaster of paris is used in another variety, either with grout, a thin liquid mixture of lime or cement, or combined with lime and grout or with lime, sand, and grout. It sets rapidly and is useful for pointing, or patching, breaks in old mortar. Preblended commercial mortars are available, to which the user need only add water. They consist of lime, cement, sand, a waterproofer, bonding chemicals, pjasticity agents, and a variety of other ingredients to make them versatile. Forms of mortar are employed in plastering interiors and for stucco.
Primitive mortars were chiefly mud or a mixture of clay, water, and chopped straw, as in Egypt, Mesopotamia, and early Greece. Later Greek and early Roman stone structures were built without mortar, but the Romans eventually developed a strong cement mortar that gained favor and was used, with modifications, throughout medieval times and ever since.