Masonry Magazine September 2002 Page. 30
Mixing Mortar
If the selected mortar has a high cement content, excessive shrinkage of mortar from the face shell of the masonry unit is more likely to occur.
"In fact, that is why they have control and expansion joints in brick and block walls; the wall is going to expand and contract and that movement needs to be accommodated for in the design," notes Carney. "And, if the selected mortar has a high cement content, excessive shrinkage of mortar from the face shell of the masonry unit is more likely to occur."
While mortar may seem like a simple product-most cakes use more ingredients-there are a surprising number of combinations that are found in both onsite and "factory" mortar. A little of this, a little of that, and the overall value of the mortar can change significantly. There are, obviously, specifications spelled out to assure that the mixture meets design requirements. ASTM specifies this for the U.S. and that information is readily available.
And there are regional preferences to be considered, determined both by the experiences of the mason and by local requirements. For example, some mortar mixes are not approved for use in certain states due to seismic conditions. "Most of our mortars are straight Portland lime mortars," says Stan Harwell, eastern regional manager for Spec Mix, based in Virginia Beach, Va. "However, in the southeast we formulate a lot of mortars with masonry cements. Those can only be used in non-seismic zones, not in seismic zone II or greater, such as California, Colorado, or Utah. Masonry cement is Portland cement with many different additives in it such as bonding agents, set controllers, water repellants, air-entraining agents, and inert fillers. Because masonry cement already has these additives put into it at the mill, we don't have to add anything."
Convenience and good workability are the reasons masonry cements are widely used. ASTM C 91 "Specification for Masonry Cement" defines masonry cement as "a hydraulic cement, primarily used in masonry and plastering construction, consisting of a mixture of Portland or blended hydraulic cement and plasticizing materials such as limestone, hydrated or hydraulic lime, together with other materials introduced to enhance one or more properties such as setting time, workability, water retention, and durability."
Masonry cement typically has a higher air holding capability than Portland lime cement. "Portland limes can, by ASTM standards, go up to 12 percent," comments Harwell. "By ASTM C 270, that same mortar, if we use a masonry cement, can go up to 18 percent of air by volume."
Because high air entrainment can significantly reduce the bond between the mortar and masonry units or reinforcement, BIA says the use of air-entrained Portland or blended hydraulic cements is not recommended. Most building codes have lower allowable flexural tensile stress values for mortars made with air-entrained Portland cement.
While ASTM C 91 provides specific criteria for physical requirements and performance properties of masonry cements, the constituents may vary depending on the manufacturer, local construction practices and climatic conditions. Model building codes generally allow flexural tensile stress values for masonry built with masonry cement mortars that are lower than those for masonry built with non air-entrained Portland cement and lime mortars. Therefore, the use of masonry cement should be based on the requirements of the specific application.
At companies like Spec Mix, the future will be one where mortars are premixed to property specifications and flexibly use additives as needed to accomplish specific tasks. It's what Harwell calls "prescription mortars, which have the ability to match mortars with the physical properties of the masonry unit, the structural requirements for the project, the weather conditions, and the desired workability for the mason."
He explains, "Why do we want to use a mortar in August, when it's very hot and dry, with a very absorptive unit like a brick-one that's very soft and will suck all the water out of the mortar fast? That increases the chance of having a dry out on the wall-all the water evaporating before the cement can completely hydrate.
"Then," he continues, "why would we want to use that same mortar in January, when it's cold and rainy, with a brick that has no absorption? As an industry we need to get to the point where we can manufacture prescription mortars by blending Portland cement, hydrated lime, and certain prescribed additives along with a good mason sand that will create a mortar that works well with specified units, whether brick or block, under job site conditions."
Carney adds, "If we can, we first have to influence architects or contractors to use property mortars.