In the first floor area of an office building I would like to place battered base shear walls with an exposed aggregate finish utilizing local gray aggregate, and would also like to integrally color the mix so as to produce a color that is at least dark or even black. Is as simple an admixture as lampblack a solution to this problem?
Lampblack or emulsified carbon black or black iron oxide have been successfully used to produce dark-gray to black concrete. If lampblack or carbon black is used it should be a product prepared specifically for use in concrete. From 2 to 5 pounds of pigment per bag of cement is an accepted dosage and will not affect the quality of the concrete. For best effect use dark (igneous) aggregates, both coarse and fine. When using any pigment, special care must be taken in maintaining uniform proportions of all ingredients; changes in water content of a mix affect color and so does early exposure to drying out.