By Rich Solomon
Choosing a mortar color to compliment the wide range of colors found in clay brick, colored block and stone will enhance the appeal of a masonry structure. Brick, colored block and stone contain thousands of hues that change throughout the day as the reflection of light is altered by brightness and angle. The same is true with colored mortar. Mortar color should be utilized as a design tool to create a different look, depending upon the color shade selected. Remember, mortar joints make up 18 percent to 20 percent of the wall surface. Using colored mortar will dramatically change the overall appearance of the structure.
Most people are not aware of the normal variations in regular uncolored mortar. When color is added to the equation, expectations become greater. The first step toward achieving a consistent mortar color is using the same ingredients and procedures throughout the job. The appearance of the mortar color joint is impacted by many factors other than the addition of iron oxide pigments to the mix. Variations in cement, sand, water-to-cement ratio, rate of hydration, environmental conditions, tooling of the mortar joints, and cleaning procedures can change the appearance of the colored mortar.
Masonry and Portland cement are produced with different manufacturing processes offering multiple shades from light gray to dark gray color tones. The cement color shade will have a pronounced direct effect toward the mortar color. Adding color to light gray cements will produce brighter and cleaner color shades. Conversely, utilizing dark gray cement will limit your ability to produce light color shades, such as buff or pink. If a light pastel color is specified, more than likely white cement must be used to obtain the desired mortar color. In the recent past, there has been tight cement production levels with increased demand for cement, which has created a shift toward darker shades of cement. To maintain uniformity throughout the job, visual inspections of each cement delivery should be compared against future shipments. In addition, stay with the same cement manufacturer throughout the job. The use of the lightest shade of gray Portland or masonry cement will enhance the vibrancy of the finished mortar color.
Mortar mix design
The ASTM C270 Specification for Mortar Unit Masonry includes Prepared Masonry (ASTM C91) Type N,S,& M Masonry Cement and Type O,N,S, & M Portland Cement (ASTM C150) — Lime (ASTM C207) Mortars plus Sand (ASTM C144) used in each mix design. As with the shade of cement affecting the mortar color, the type O, N, S, & M strength mortars and sand shade will also influence the colored mortar shade. The cement to lime ratio will determine the lightness of the mortar. The more cement in the mix the darker the mortar. Thus, type M strength mortar will be the darkest, type S second darkest, type N third darkest, and type O lightest with the highest concentration of lime. With more Type S mortars being specified, the ability to produce lighter colored mortar joints such as buffs, tans and pink shades are becoming more difficult without utilizing white cement or a light gray portland cement. As with cement, the shade of sand will change the shade of colored mortar. Light-colored sand will allow a brighter more intense color. Conversely, darker sand will mute the color shade. Sand with more fines will also mute the color shade due to the increased surface area the pigment is required to coat with pigment particles.
A stable, high-quality pigment should be utilized to assure a long-lasting mortar color shade. Color pigments with a proven history of longevity in masonry are made from natural and synthetic iron oxides (red, black and yellow), Cobalt (blue), titanium oxide (white) and Chromium oxide (green). Blends of the red, black and yellow natural and synthetic iron oxides will produce buffs, browns, tans, coppers, oranges, chocolates and other colors needed to compliment the vast colors found in brick, block and stone. These pigments exceed the requirements set forth by ASTM-C979 “Pigments for Integrally Colored Concrete.” The pigments are water-wetable, lightfast, alkali resistant, stable under curing, and will not adversely affect the strength of cement when added at less than 10 percent of the weight of cementitious ingredients.
Pigments made of carbon black have a high tint strength value and are listed under ASTM-C979, but are generally not accepted by pigment manufacturers who supply pigments to the masonry industry. Exterior masonry mortars colored with carbon black have a history of losing color from the surface of the mortar joint within six months after installation. While carbon black is sun-fast, it does not bond well to cement. It eventually disappears through weather exposure, resulting in a gradual loss of color on the surface area of the mortar joint.
The pigments used to color mortar have a high tint strength value and are added to the mortar mix based on the weight of all cementitious materials in the mix (Portland cement, lime or masonry cement). It’s best to use a minimum of 1 percent color, based on the total amount of cementitious material to minimize the potential for color variations. Normally, the amount of color used in a mortar mix falls between 1 percent and 7 percent of the cementitious material weight. Generally, the 7 percent dosage rate is the color saturation point; adding color will not noticeably improve the intensity of the mortar color shade.
As with all variables listed, mason contractors impact the colored mortar appearance of the masonry structure. Mortar joints should always be tooled only after the mortar joint has achieved a “thumb print” consistency. Tooling the joints too soon brings moisture from the mortar to the surface and creates masonry smear lighter in color. Conversely, tooling a mortar joint that has become too dry tends to burn or scrape the joint, due to lack of moisture. The result is a mortar joint darker in color.
To reduce the amount of cleaning required, clean deposits of colored mortar from the face of the masonry units during construction using a moist towel and stiff brush before the mortar cures and rigidly bonds to the masonry unit. Cleaning new masonry should be undertaken after the colored mortar has sufficiently cured (depending on weather conditions, generally seven to 14 days after the masonry installation). A commercially prepared “proprietary” masonry cleaner should be applied at manufacture’s suggested dilution concentration. Cleaning too quickly and/or using muriatic acid or a highly concentrated masonry cleaner will caused degradation of the colored mortar with the consequential release of color pigments from their cement bond, and with the result of leaving a porous exposed sand surface. Insufficient or irregular washing can produce streaky or blotched areas on the masonry wall. For the well being of all involved, sample mock-up panels should be tested with job cleaning procedures and approved by the owner or architect.
Discoloration by efflorescence (a white coloration film of calcium and soluble salts) on the surface of the colored mortar joint is a common problem and more noticeable with colored mortar. Although color is often blamed for efflorescence, pigments do not contribute to the formation of efflorescence. To reduce the occurrence of efflorescence during construction, masonry walls should remain dry by covering with a strong waterproof tarp at the day’s end. The design of the wall should include proper flashing to prevent penetration of wind-driven rains and allow weep holes for proper drainage. Masonry walls, which have moisture trapped, perpetuate the occurrence and reoccurrence of efflorescence.
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The water-to-cement ratio of the mixed mortar will dictate the color shade of the mortar joint. A mortar mix with more water will lighten the color of the mortar joint, due to the pigment particles being spread farther with the extra water. Conversely, a mortar mix with less water will appear darker in color. Re-tempering of colored mortar with the addition of water should be avoided.
The rate of hydration affects the final shade of the mortar color. Cold weather masonry work has a slower cure cycle verses warmer masonry work. Therefore, jobs started at one time of the year and finished at another time can experience a difference in the final color, due to the change in the rate of hydration. During colder temperatures, the occurrence of efflorescence is more common due to the rate of hydration being slower.
The use of mortar color is not difficult, but it takes care and consistency to assure a uniform job. Fortunately, the masonry industry has experience using mortar color and for the most part, a job well done. Designers and specifiers must have realistic expectations concerning acceptable mortar color variations. When faced with a non-standard specified mortar color, rely on the expertise of pigment manufacturers and cement manufactures to assist with the match work and mix design.
Good to Know
The following criteria should be considered when specifying and producing a particular mortar shade:
- Cement shade
- Mortar mix design
- Color pigments
- Masonry procedures
Rich Solomon is president of Solomon Colors Inc.
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