Masons have successfully used mortar made from only sand, portland cement, and lime for more than a century. But as good as that simple mortar is, the properties can be improved by using various admixtures, including retarders and accelerators, to counteract the effects of cold and hot weather, and water repellents.
Mortar can be specified either based on prescribed proportions or by required performance properties. Masonry cements and mortar cements can only be specified based on the required performance since the precise materials are proprietary. In this sense, the masonry industry is leading the concrete business where performance-based mixes are less common than prescriptive ones.
Giving masons the freedom to select their mortar based on performance allows the use of a variety of sometimes unconventional ingredients. Contractors can select properties that speed up the work and increase the quality of masonry walls.
Mortar is not concrete—it needs a few additional properties to allow efficient construction of a quality masonry wall. The most important fresh mortar properties are workability and water retention; finished masonry needs better bond between the mortar and the units than could be achieved with a straight cement mortar.
That's why lime is added to the mortar. Mortar made with some proportion of lime (actually Type S calcined lime) has greater plasticity, bonds better with the units, and shrinks less that a straight cement mortar. The typical proportion of lime in a Type N mortar mix is ½ to 1¼ parts lime to one part cement, based on the proportion specification table of ASTM C 270, Mortar for Unit Masonry.
But if a mason chooses mortar based on performance requirements, there is flexibility in what can go into the mix. One change that some masons have found beneficial is to substitute a product called Easy Spred for some or all of the lime. This material is a finely ground clay that has a lubricating effect on the mortar.
In a mix that would require 50 pounds of lime, only seven pounds of Easy Spred is needed to achieve similar fresh and hardened properties. “Normally, Easy Spred is referenced under the ASTM C 270 property requirements,” says Charles Meadowcroft, president of Peninsula Products, which markets Easy Spred. “Or, if you blend cement and Easy Spred, you can qualify under ASTM C 1329 as a mortar cement and end up in the proportion table of C270.”
This product has been used across North America for more than 25 years. Masons report that it results in an easy-to-work-with mortar. Independent testing shows that the mortar has high strength and low shrinkage. Some masons have developed their own formulas using a combination of Easy Spred, lime, and portland cement to enhance workability and to reduce drying and shrinkage.
A masonry wall's greatest weakness is its inability to prevent water penetration. Water-repellent admixtures added to the mortar, and also often to concrete blocks during manufacturing, can lead to more watertight masonry walls. These admixtures, often provided in liquid form but sometimes as a dry powder or blended with dry mortar mix, are polymer emulsions that make the mortar much less permeable. Because water is unable to move easily through the mortar, this admixture also reduces or eliminates efflorescence. Water-repellent admixtures also increase the bond between the mortar and CMUs or brick, leading to even more water resistance.
Integral water repellents typically are added to the mortar during mixing. But Spec Mix, with its Spec Mix IWR, blends the water-repellent admixture into a dry portland cement-lime mortar mix that is available in 80-pound bags or 3000-pound bulk bags. In testing in conditions that simulated 62-mph winds and more than 5 inches of rain per hour, no water penetration was detected after four hours when using Spec Mix IWR and CMUs manufactured with water repellant. On projects where minimal water penetration and efflorescence are desired, integral water-repellent admixtures might be a good choice.
This article first appeared in CONCRETE & MASONRY CONSTRUCTION PRODUCTS. magazine. William D. Palmer is a former editor in chief of MASONRY CONSTRUCTION.