Optimizing Mortar Formulations for Maximum Performance

Words: Nick BlohowiakConcrete masonry units (CMUs) and mortar go together like bread and butter. For thousands of years, engineers, architects and tradesmen have married these two components to form a system used to build everything from the ancient Egyptian pyramids to local schools. Although masonry is one of the oldest and most proven methods of construction, the past few decades have brought major technological innovations that have improved not only labor practices, but also the integrity and aesthetic value of the building materials themselves.

Prior to the advent of factory pre-blended mortars in the late 20th century, mason contractors required a specified formula for measuring the proper ratios of cements and aggregates to be mixed on a jobsite, based on ASTM C270 Table 1 Proportion Specification Requirement, written by architects and engineers. The Proportion Specification, commonly referred to as the “recipe method,” allowed an on-site laborer to meter each mortar ingredient based on a cubic foot measurement system. This approach does not allow for regional variance in raw materials and severely challenges the ability of specifiers and mason contractors to achieve prescription mortars that align with the performance and characteristics of the CMUs.

That all changed with the introduction of pre-blended masonry mortar, which replaces the guesswork of field mixing with pre-determined formulations that are based on clearly defined raw material weights, measured according to strict tolerances for performance and quality control. Producing mortars in a controlled environment, utilizing computerized batching equipment, allows for customized formulations that complement the unique properties of the masonry units specified and the controlled addition of performance-enhancing admixtures. Moreover, from the perspective of a project designer, specifier or engineer responsible for selecting the best mortar for a project, pre-blended mortar offers endless customizable mix possibilities that match CMU properties, while supporting the mason’s need to increase productivity.

Matching Pre-Blended Mortars and Masonry Units

All masonry units are not created equal, and they come in almost limitless colors, textures and finishes, each with different physical properties. However, available mortars have generally been limited to type S or N, and then further limited to a single recipe when field mixing. Factory-controlled, pre-blended mortars allow for the full use of ASTM C 270 Table 2 Property Specification while specifically matching the performance of the mortar to the CMU. When the mortars are not properly matched to the CMUs’ properties, the mason will struggle with productivity when constructing a masonry wall. Why is this so?

Brick are manufactured with different clays and baked at different temperatures to create various colors, textures, densities and hardness. Each absorbs water at a different rate, and that rate is tested and reported as the initial rate of absorption (IRA) of the brick. A low-IRA brick (5 grams per minute) will tend to swim or float if placed with a high-water-retention mortar; this is common with standard field-mixed type S or N mortars. In addition, the mason will have to wait extended periods of time for the mortar joints to harden before finishing, which slows productivity. Conversely, a high-IRA brick (greater than 30 gpm) may require the mason to immediately tool every few brick, before the mortar joints lose moisture to the unit, harden and become unworkable. Water retention of factory pre-blended mortars can be pre-engineered to match the absorption rates of brick on a project, thereby eliminating tooling issues and shrinkage cracking while increasing productivity and color consistency on the wall through consistent tooling times.

Surface mottling of a mortar joint.
Surface mottling of a mortar joint.
CMUs and mortars that share similar properties should be matched together on projects. For example, architectural CMUs are densified in the manufacturing process to help create a unique finished appearance. This process makes the CMU less permeable and prone to swim/float, thus slowing down the installation. This also leads to on-site tooling and curing difficulties, including unsightly surface mottling of the masonry joints (Figure 2), a condition where the cement paste cures at uneven rates at the surface of the joint only.

Pre-blended mortar formulations for natural stone can be customized for hard and soft stone.
Pre-blended mortar formulations for natural stone can be customized for hard and soft stone.
Stone masonry construction can benefit tremendously by transitioning beyond simple type S and N mortars (designed for basic brick and block construction) to performance-designed mortars that match the properties of stone masonry units (Figure 3). In masonry construction, stone generally falls into two categories: hard, dense stone and soft, porous stone. By factory controlling known mixes and testing for water retention and compressive strength properties, mortars can be designed to match the properties of the stone masonry units very closely. When pre-blended mortar is properly used to support the units so the wall can “set up” quickly, shrinkage cracking is eliminated and more stone can be laid in a single day.

High-Performance Ingredients in Pre-Blended Mortar

Performance-enhancing admixtures introduced into masonry mortars can be used to increase durability and flexibility or to help extend the range of conditions in which masonry walls can be constructed. It’s important to note that the inclusion of admixtures in masonry mortars changes the chemistry, which directly changes the performance of the mortar and often its final appearance. As mortars continue to evolve through the use of improved and higher-performing admixtures, controlling the addition rates in a factory setting ensures a consistent outcome on site.

Figure 4 – Mortar joint with cementitious paste intact (left), and mortar joint chemically etched in washing process (right).<br />
Figure 4 – Mortar joint with cementitious paste intact (left), and mortar joint chemically etched in washing process (right).
The most common admixtures used in masonry are water repellents, which are generally used in the production of architectural CMUs. Due to the water-repelling properties in architectural CMUs, they tend to act more like a plate of glass than a standard CMU, making them particularly tricky to clean. If the mortar joint is not properly enhanced with a water repellent and does not act in a manner similar to the CMU, it will absorb the cleaning solution at a much higher rate than the CMU. If untreated or improperly dosed, the cleaning process can cause the cementitious paste to be eaten away, exposing the aggregate in the mortar (Figure 4). To help avoid this undesirable discoloration, factory pre-blended mortar can be enhanced with a controlled dosage of water repellent to ensure performance from batch to batch (Figure 5).

Pre-blended mortar produced with dry admixtures, pigment and integral water repellent ensures quality control and performance.
Pre-blended mortar produced with dry admixtures, pigment and integral water repellent ensures quality control and performance.
Admixtures are also used in mortars designed to install thin masonry veneer, one of the most popular masonry claddings. Incorporating admixtures that increase bond strength can double the flexural bond properties of standard mortars, thereby increasing their flexibility to accommodate the movement and deflection common in this type of application.

Another invaluable admixture in pre-blended mortar is set accelerators. There is no such thing as “anti-freeze” for mortar, but set accelerators speed up the set time of mortar, so mason contractors can work in colder conditions. Specifically, the set accelerators allow the mortar to use the mixing water at an accelerated rate, thus hydrating the cement before temperatures drop enough to freeze the water in the mortar. Similarly, the use of set-delaying admixtures can slow the hydration of the cement in the mortar on hot days, keeping the cement from drying out before full hydration of the cement occurs. If the addition rates are not properly controlled, the outcome can be catastrophic. Only pre-blending facilities can guarantee that these admixtures have been properly weighed and controlled to achieve the desired result.

SPEC MIX Polymer Modified Adhered Veneer Mortar for superior shear bond strength.
SPEC MIX Polymer Modified Adhered Veneer Mortar for superior shear bond strength.

Conclusion

The introduction of factory pre-blended mortar is one of the most significant developments impacting the way building owners, designers and mason contractors select and construct masonry projects. Whether the architectural drawings call for brick, CMU, building stone or thin masonry veneer, pre-blended mortar manufacturers are able to engineer mix designs for structural wall or veneer systems with optimal mechanical bond, compressive strength and mortar joint color, allowing the designer’s vision to be translated onto the building.
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