Concrete color consistency, from batch to batch and from job to job, is possible, but it takes discipline, attention to detail and a basic knowledge of good concrete practices.

Most color problems arise from careless mixing and batching practices. Consistent batch-to-batch color can be achieved by careful and accurate weighing and dosing of all ingredients, especially the mix water.

Only by accurate weighing and by making each batch identical can color consistency be hoped for. Electronic scales that weigh in fractions of a gram help to achieve consistent pigment dosages, and larger scales that weigh in fractions of a pound (or kilogram) help to achieve consistent cement, aggregate and water dosages.

“Eyeballing” mix water addition, especially without any way of knowing the exact amount of water added, is the number one reason color inconsistency occurs. Water is known to have a strong influence on color, so if the exact amount used isn’t known, then each batch of concrete will turn out a little different. Other reasons stem from poor curing practices and from changes in ingredients from one batch to another. Different cement brands, different colored aggregates or different pigment brands will all ensure the final color of one batch won’t match another.

Everything that goes into making concrete has some effect on its appearance. Variability in the ingredients themselves can cause inconsistency. Pigment is the most consistent ingredient because manufacturers exert the highest quality control measures, but changing brands can cause inconsistency, as can switching mid stream from a dry pigment to a liquid pigment.

Cement is less consistent, especially gray cement. White cement is an architectural grade ingredient, so it conforms to tighter appearance standards than gray cement. Gray cement can vary from manufacturer to manufacturer, from lot to lot and even from bag to bag, depending on where it was made and what it was made of. It’s a good idea to use the same cement manufacturer all the time so that the variability in color is minimized as much as possible.

Pozzolans will affect the color of the concrete too, except in certain circumstances. White cement blended with metakaolin looks almost identical to plain white cement, so the blend would not affect color to any significant degree. However adding metakaolin to gray cement would lighten it, affecting the color. Other pozzolans like fly ash, slag or silica fume each have different colors and will affect the concrete color differently. It’s important to stay consistent with the ingredient and the amount of each ingredient if color consistency is desired.

Admixtures like superplasticizers can influence color, not because they add color, but because they can act like dispersants, aiding in cement and pigment dispersion and the resulting color strength. Other admixtures usually have little effect on the color, like fibers, accelerators or retarders, however calcium chloride accelerators are an exception and should be avoided for this and other reasons.

Thorough and complete blending of all of the concrete ingredients is very important to achieving a uniform and consistent color. All of the pigment added to the mixer should be uniformly blended. If pigment is stuck to the sides of the mixer or in lumps or streaks then the resulting concrete will not be consistent with other batches nor will the color of that batch be uniform.

Adding liquid pigment to the mix water before adding the now pigmented mix water to the concrete can cause color inconsistency because a significant amount of the pigment usually remains in the bucket. Or if the residual pigment is washed out, extra mix water is added thereby altering the ingredient proportions.

Liquid pigments are really ultra fine pigment particles suspended in a liquid. If the liquid pigment is added to mix water, the pigment particles will quickly settle out because the suspension fluid is now greatly diluted by the mix water. No amount of stirring will suspend all of the pigment particles, so much of the color remains in the bucket rather than going into the mixer.

When done carefully, it is possible to add the liquid pigment to a portion of the mix water and then use the remaining mix water to rinse out any pigment residue.  But this involves extra work and requires extra attention. A simpler process would be to add the liquid pigment directly to the mixer and then to rinse out the pigment container with the mix water as it is added.

Curing the concrete has an effect on concrete color. Curing the concrete after casting helps “lock in” the color. If some slabs are allowed to wet cure for longer than others, the slabs that dry out sooner will appear lighter.

Forms for fluid concrete mixes must be watertight in order to achieve consistent colors, and form materials themselves (the texture and porosity) can affect the color of the concrete too.