Mortar

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Overview

Mortar is the bonding agent that integrates a masonry wall. It must be strong, durable, capable of keeping the wall intact, and it must create a water-resistant barrier. Mortar as recommended by the Brick Institute of America and other masonry experts is made from the following ingredients.

Mortar Ingredients

Mortar specification is covered by the ASTM C270 "Specification for Mortar for Unit Masonry" and CAN/CSA A179 "Mortar and Grout for Unit Masonry". Note that the Canadian equivalent references are included in brackets behind the US ASTM references.

Portland Cement

A hydraulic cement, contributes to durability, high early strength and high compressive strength. It is one of the principal cementitious ingredients for mortars. Only three of the eight types covered by ASTM C150, Standard Specifications for Portland Cement, are recommended by the BIA.

Three types recommended by the BIA:

• Type I (Type 10) - For general use when the special properties of Types II and III are not required.
• Type II (Type 20) - For use when moderate salt water resistance or moderate heat of hydration is desired; sometimes used in hot weather as an alternative to adding a retarder.
• Type III (Type 30) - For use when high early strength is desired; may be used in cold weather.

Hydrated Lime

Type S, covered by ASTM C207 sets only upon contact with air, contributes to workability, water retentivity (preventing water from evaporating too quickly from the mortar), elasticity, bond strength and resisting water penetration through the brickwork. It is also a cementitious ingredient, however it takes much longer than Portland cement to develop its strength.

High lime mortars create a phenomenon called autogenous healing. When rainwater, which always contains carbon dioxide, is absorbed into the mortar, it either absorbs directly into the mortar or dissolves minute amounts of hydrated lime or limestone. This solution penetrates into cracks or voids while the hydrated lime combines chemically with the carbon dioxide to form limestone which becomes slightly larger than the original hydrated lime. The limestone hardens as the air dries it and bonds to adjacent surfaces in the voids. Eventually the voids or cracks may become filled. This slow process continues for years. The higher the lime content the more effective the healing process.

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