Masonry is a versatile medium for architectural expression. Fluted, split-faced, ground-faced, and custom prefaced architectural concrete masonry units are available in a myriad of textures and colors. Fired clay masonry units are manufactured in numerous color and texture options. White or colored mortars can provide a color contrast or harmony between masonry units and joints, further expanding the range of visual statements the designer can make when using masonry.
White mortar is made with white masonry cement, or with white portland cement and lime, and white sand. White cements for masonry should meet the same American Society for Testing and Materials (ASTM) specifications as their gray counterparts. Masonry cement should meet the requirements of ASTM C91 for a Type N, S, or M masonry cement. Portland cement should meet the requirements of ASTM C150 for Type I portland cement, although Type II or Type III cements are sometimes used in mortar.
As with gray or colored mortar, white mortar should meet the requirements of ASTM C270 for the type (N, S, or M) designated. Sand should meet the requirements of ASTM C144 and be free of silt or clay fines. Buff or brown sands will impart an undertone of color to the mortar. This sand color may become more pronounced over time, or with cleaning, as sand particles are exposed on the surface of the mortar joint due to the erosion of the white mortar paste. To assure whitest mortar color, use only white sand.
For colored mortars, the use of white masonry cement or white portland cement instead of the normal gray cements, not only produces cleaner, brighter colors, but is essential for making pastel colors such as buff, cream, ivory, pink, and rose. Integrally colored mortar may be obtained through the use of pigments, colored masonry cements, or colored sand. Mortar color is so sensitive to pigment content that proportioning is best accomplished by using pre-pigmented cements or pre-weighed pigment packages. The color of the mortar joints will depend not only on the pigment, but also on the cementitious materials, sand, water-cement ratio, and tooling.
Mineral oxide pigments (usually natural or synthetic iron oxides) are recommended for use in mortar, since they are compatible with cement and lime and provide color stability in the finished mortar joint. Pigments should conform to ASTM C979, Specification for Pigments for Integrally Colored Concrete, and their addition rate to mortar should not exceed 5 percent by weight of the masonry cement content. Mortars made using carbon black pigments exhibit poor color stability in masonry mortar exposed to weather. Their use in exterior masonry should be avoided. If used in interior masonry, carbon black pigments should not exceed 1% by weight of masonry cement content of mortar.
Selecting White or Colored Mortar Materials
Initial selection of mortar color may be made on the basis of color charts or mortar samples that are typically available from manufacturers of white and colored cements and pigments. However, final selection of mortar color should be determined on the basis of evaluation of a sample panel or mockup constructed using the units, mortar materials, mixing procedures, workmanship, tooling technique, and cleaning procedure that is proposed for use in the final project. The purchaser (owner) and provider (mason) should review the sample panel together and establish a clear understanding of expectations of masonry and mortar appearance. The sample panel should be retained for reference until the masonry is completed and accepted by the owner.
Controlling Mortar Color
To assure consistent color on a project, the variables that determine mortar color (materials, materials preparation, installation, and cleaning) must be controlled.
The same brands of masonry cement, mortar cement, portland cement, hydrated lime, or pigments should be used throughout construction of a project. All sand should be from the same source. Care must be taken to assure that the sand is not contaminated with clay or dirt at the job site. White and light-colored mortars are particularly sensitive to such contamination.
Proper and consistent proportioning and mixing procedures need to be established. Masonry cement, mortar cement, portland cement, and hydrated lime are typically delivered in pre-weighed packages. Thus, consistent proportioning is assured by adding these ingredients to the mixer in bag increments, or sometimes in increments of a simple fraction of a bag. Some positive control should be established to assure that the proper amount of sand is used. This can be accomplished by using a container of known volume to measure sand.
Mechanical mixing of mortar is recommended. Batching procedures will vary with individual preferences. Experience has shown that good results can be obtained when about three-fourths of the required water, one-half the sand, and all the pigments and cementitious materials are briefly mixed together. The balance of the sand and the remaining water are then added to bring the mortar to optimum working consistency. The amount of water added should be the maximum that is consistent with satisfactory workability. After all batched materials are together, they should be mixed for three to five minutes. For pigmented mortars, mixing the full five minutes is advisable.
Mortar production should be scheduled to keep pace with the progress of construction. Mortar that has been mixed but not used immediately tends to dry out and stiffen. Avoid producing mortar too far ahead of expected use to minimize retempering requirements. Retemper white or colored mortar cautiously to avoid color changes. Water content and stiffness of mortar during tooling affect color.
Different joint finishes can be selected to provide a desired appearance in the completed masonry. Concave or V-Joints are preferred for optimum weather resistance. Special care must be taken with white or very light-colored mortars to assure that joint surfaces are not discolored with metallic deposits from the jointer. The mason should use ceramic, stainless steel, or plastic jointers when working with such mortars.
When it comes to cleaning masonry, less is better. Careful workmanship includes implementing skills that minimize mortar dropping and smears on the face of the masonry. Covering the tops of walls at the end of each working day will prevent rain from entering walls and help reduce the possibility of efflorescence.
Use the least aggressive cleaning technique possible. First, make certain that the cleaning procedure is consistent with the recommendations of the manufacturer of the units, and second, pre-qualify the procedure on the sample panel and a small test area of the building. When acid based cleaning solutions are used, the mortar should be allowed to cure at moderate temperatures for about a week prior to cleaning. Always pre-wet the masonry before applying the cleaning solution. After cleaning a trial area, allow the area to dry, and closely examine mortar joint surfaces to ascertain that the procedure has not etched the surface.
White cements and colored masonry cements or pigments expand the range of available mortar colors. Coupled with the variety of masonry unit colors and textures available, white and colored mortars afford the designer virtually unlimited creative options. With selection of the right mortar materials, verification of mortar color in field panels, and observance of proper construction practices, the creative vision of the designer can be made a reality.
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