Water penetration problems are occurring in the dark-colored mortar joints of a masonry building we designed. Is this because the joints are colored? Does water penetrate colored mortars more than it does uncolored mortar? Is this because the pigments add more noncementitious material to the mix, reducing mortar strength?
From the information you give, we can't tell you what's causing the water penetration, but it's probably not because the mortar is colored. Color pigments do not cause problems unless too much of them are added. The maximum pigment dosage recommended by most pigment manufacturers is 10% by weight of cement. Pigments that contain carbon black should be added in dosages no greater than 2% by weight of cement. All the pigment manufacturers we contacted said that color pigments tend to fill the voids in the mortar, making it less permeable. Compressive strength is greater, and tensile strength is unaffected. Pigments do add more noncementitious material to the mix, but the relative amount is small. Typically, 5 pounds of pigment are added to a mortar batch containing 200 to 250 pounds of sand--about a 2% increase in noncementitious material. Given the relatively inaccurate way mortars are made on the jobsite, this increase in noncementitious material is meaningless. Good mortar color can be obtained with recommended dosages if the pigment is added in the proper manner. Generally, all or a part of the aggregate should be added to the mortar mixer first. Then the pigment and some water is added. The mixer is run for a few minutes, the cement and the rest of the aggregate and water are added, and then all the materials are mixed for at least another 5 minutes. These mixing rules don't apply if precolored masonry cement is used. But in either case each batch of mortar should be batched in exactly the same way and mixed for the same amount of time. Batching inconsistencies produce variations in mortar color. Also, colored mortar should not be retempered. The extra water can change the color of the mortar. Problems can arise if excessive amounts of color are added. The excess amount of fine pigment particles demand more water, which increases the water-cement ratio of the mix and decreases strength. Lower strength mortar deteriorates quicker, and thus is more susceptible to water penetration. But unless the contractor added more than 15% pigment by weight of cement (against manufacturer recommendations), color is not the cause of your problems.