Masonry Magazine September 2008 Page. 35
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.
Masonry procedures
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.
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September 2008
Masonry 33