Ancient structures such as the pyramids were built by stacking stones one upon the other with painstaking precision. For example, at the ruins of Machu Picchu in the Peruvian Andes, the stones are so closely matched that you cannot slide a feeler gauge between two blocks. People assume that mortar is only used to glue bricks together, however, Master Builders will tell you that apart from adding to structural strength, mortar is a labour-saving substance designed to keep bricks apart. There are several formulas for mortar. These vary, depending on geographical location, climate, and structural design.
Since Type M mortar has a high cement to lime ratio, it is the strongest mortar mix approved by the Building Code Requirements for General Use in Masonry Structures. It is suitable for load-bearing walls and below-grade structures such as basement walls, foundations, walkways and retaining walls, however, its lack of plasticity makes it difficult to work with. Type M mortar is composed of 1 part cement, 1/4 part lime, and 3 3/4 parts sand.
Type N is a medium strength mortar used for non-loadbearing above-grade walls and do-it-yourself projects, such as chimneys, garden walls, barbeques and pergolas. This mix has good bonding capabilities and is easy to work with. Type N mortar is composed of 1 part cement, 1 1/4 parts lime, and 6 3/4 parts sand.
Type O is the weakest grade of mortar in general use, therefore most building codes do not approve this mix for use in exterior construction or for load-bearing walls. In addition, it should not be used in applications subject to freezing or in the presence of excessive moisture. On the other hand, its high lime content makes it highly plastic and easy to work with, making it ideal for interior non-loadbearing walls and decorative restoration projects. Type O mortar is composed of 1 part cement, 2 parts lime, and 9 parts sand.
Type S mortar is a good general purpose product with high flexibility and bond strength. It is fully interchangeable with type N mortar. In addition, its low lime content imparts excellent resistance to severe weathering and freezing conditions. It is suitable for interior construction including load-bearing walls, unless type M mortar is specified. Type S mortar is composed of 1 part cement, 1/2 part lime, and 4 1/2 parts sand.