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Advertising supplement provided by Portland Cement Association

by Jennifer Grover Prokopy

Properties of Masonry Mortars

Masonry mortars perform more than just the obvious function of joining masonry units to form a lasting structure. They create tight seals between the units to protect against air and moisture entry; bond with any steel reinforcement, ties or anchor bolts; provide a desirable aesthetic quality through color contrasts or shadow lines; and compensate for size variations in the masonry units.

 

La Cascada II. Phoenix, AZ. Architects: Todd & Associates. Photo by Richard Abrams Photography.

Masonry mortars are comprised of three key ingredients: one or more cementitious materials; clean, well-graded masonry sand; and sufficient water to produce a plastic, workable mixture. Until the late 19th century, lime was the primary cementitious material used. Sand-lime mortars took months or years to harden, but were acceptable for the massive projects and slow-paced construction schedules of those times. Mortars began to evolve with the addition of a small amount of portland cement to “sweeten” the lime; by the later 19th century, lime was being used to “sweeten” the portland cement, creating a mortar that hardened faster and stronger.

Today, the most common cementitious materials used to make masonry mortar are:

• Masonry cement
• Mortar cement
• Portland cement and lime

Masonry cement is a factory-prepared hydraulic cement. It enhances properties like board life, workability, water retention, and durability. White and colored masonry cements are widely available, and provide a full palette for architects. Compared to portland cement and lime, masonry cements simplify jobsite production of mortar because the cementitious materials are preblended into a single product.

For more demanding structural applications and in high seismic areas, project planners sometimes specify mortar cement. Like masonry cement, mortar cement is a factory-prepared hydraulic cement, but it is the only masonry material with ASTM-specified bond performance criteria.

No matter what cementitious material is chosen—masonry cement, mortar cement, portland cement and lime, or combinations of portland cement with masonry cement or mortar cement—acceptable results are easily achievable, allowing applicable specifications to be met when appropriate design procedures are followed. Under the proportion specification of ASTM C270, the cementitious material is blended with damp, loose mortar sand at a ratio of 1 to between 2 1/4 and 3 parts (see Table 1).