Q.: A job calls for 200 cubic yards of black concrete for driveways. We have been told that this would require 10 percent black pigment by weight of cement. If we use lampblack as pigment will it knock out the entrained air? What can we do to be sure the job will be colorfast?
A.: For a black surface made with integral pigment the best material to use would be a black admixture based on black mineral oxide and containing a water-reducing agent to disperse the color thoroughly. Such an admixture will not knock out the entrained air, but you should of course check the trial mixes for proper air content. All pigments tend to become partially obscured on flatwork in time due to a small amount of lime moving to the surface and being carbonated there. The effect can be minimized by applying a black colored wax after the concrete has hardened. This should be available from the color admixture supplier.