Masonry cement is a special blended cement that is mixed in specific proportions with sand and water to form a strong binding mortar. This mortar is notable for its strength, durability, aesthetic appeal and resistance to atmospheric and chemical deterioration. It is widely used for mortar and stucco work in stonework, block and brick masonry, but is not considered suitable strength-wise for concrete masonry.
Originally, masonry cements were created as time-saving, more reliable alternatives to on-site preparations of Portland cement and hydrated lime. They are a blend of Portland cement clinker, limestone or hydraulic lime filler and various additives. This blend is manufactured under controlled conditions in cement factories.
This cement is also checked and tested to ensure it conforms to American Society for Testing and Materials (ASTM) standard specifications. The additives added in the manufacturing process give it durability as well as water-repellent and coloring properties. Masonry cements of different properties are used for different kinds of masonry works.
The ASTM classifies masonry cements into three main types: N, S, and M. Type N is used to prepare general purpose mortars that can be used in the construction of non-load bearing and exterior veneer walls. Type S and Type M are used in mortars for load bearing structures as well as below grade level and paving masonry.
These cements are machine-mixed in ASTM specified proportions to ensure good workability. Workability is important as the mortar must be plastic enough to be readily scooped and applied with a trowel. The mortar's board life, which is the time it remains workable, can be affected by conditions on the work site and cause it to dry at a faster rate.
Using dry mortar gives inadequate bonding support to the masonry units, and remixing it with water saps the cement bond of strength. Furthermore, the mortar becomes entirely unusable after two hours. For this reason, it is advisable to mix mortars only in small, required amounts.
Apart from the masonry cement's mortar properties, the durability of the masonry work depends on the mason's level of workmanship. It will help to mix a mortar that color complements the masonry units, apply it to adhere evenly between them and properly detail the different masonry joints. The finished masonry work will then be waterproof, and perhaps stand for a good many years without requiring much maintenance or repair.