Sometimes color provides an esthetic emotion like the specialty “warm tone” portland cements of some years ago and today's proprietary colored mortar cements. Today, there are pigments of all sorts—mineral and chemical—and colored concrete is becoming more popular.
Such is the case with a bank building where, anchored at each concrete floor level, are rectangular granite units laid end to end. The granite is splotched with olive-colored minerals so at a distance it looks like dollar bills end to end girdling each floor—a delight to its bank firm owner.
Most of the time color variances are ignored, and some are even desired. But sometimes they are so frustrating, resulting in litigation.
How about a brick-masonry building where mortar joint color is embarrassingly different than an approved mock-up; a pink concrete abutment face, a surprise when forms were removed; driveway and floor slab surfaces mottled various shades of gray; unacceptable variegated greenish-blue concrete flatwork surfaces; concrete block walls spotted with various gray shades and deemed an architectural dilemma. You can probably add to the list, however, the causes for these usually are explainable.
Portland cement and fly ash are fine powders and act like pigments—a good portion of the particles are in the submicron size range. Portland cement's ferrite mineral phase (C4AF, brownmillerite) can be light brown, amber, deep brown, reddish brown, greenish brown, or almost black. Its dicalcium silicate mineral phase (C2S, belite) can be colorless, olive green, green, orange, light brown, medium brown, and deep brown. Its tricalcium silicate mineral phase (C3S, alite), although usually colorless, can sometimes have a gray overtone. White portland cements usually have a distinct green, brown, or blue overtone that sometimes make their manufacturing sources traceable.
Fly ash is also a fine powder and, like portland cement, has different intrinsic particle colors that vary from gray, brown, green, olive, yellow, amber, red, to yellow brown. Aggregate fines are a pigment. Their effects on color vary depending upon color and concentration.
Hydration changes things, particularly when using admixtures. For example, the light yellow tone masonry mock-up joints versus the deeper yellow tone building joints resulted because calcium chloride, although not permitted by specification, was used in the building's jointing mortar.
The different gray concrete block variations resulted because of variable amounts of fly ash contamination, proved with petrographic proof. This point was later acknowledged by the block manufacturer who blamed poor fly ash storage control at his block manufacturing plant, an admission that ended a trial (that should never have started).
The pink concrete surface resulted because of a phenolic-based coating on wood forms. The upgrade was unappreciated until its nonadverse cause was diagnosed and the concrete's future projected. As anticipated, the upgrade was temporary; the gray soon returned when the surface carbonated with no adverse effects. The phenolic coating was like applying phenolphthalein, an indicator of carbonation, to the surface. Phenolphthalein turns uncarbonated paste variable shades of pink.
The greenish-blue flatwork surface, as could best be determined, was due to trace amounts of chrome in the portland cement. The mottled gray flatwork surfaces resulted because of restricted hydration of the portland cement's ferrite phase, a result of finishing manipulations that squeezed water from the immediate concrete surface region, thus resulting in a darker gray color.
Concrete made with portland cement manufactured using iron slag as a component of its raw feed, and concrete made using ground granulated blast-furnace, initially will result in dark bluish-green paste. With time, the color will change to a warm-tone brown that results when an iron sulfide component oxidizes.
Color variations can be related to variable fine porosities of portland cement paste. Such porosity varies with water-cement or water-cementitious materials ratios. For example, light impinging on paste is either absorbed or diffracted depending upon the paste's pore size. If that size is smaller than the wavelength of impinging light, the light will be absorbed and surfaces will appear dark. On the other hand, if the pore size is larger than the wavelength of impinging light, the light rays bounce (or diffract) between surfaces; some will be directed away from the surface and the surface will appear light. That's why wet concrete is darker than dry concrete. Let it dry, it loses its optical continuity and becomes lighter.
Aside from purposeful chemical and mineral pigment additions, concrete color can be influenced by: color intrinsic to cementitious materials; hydration effects on cement and fly ash minerals; chemical admixtures; water-cement and cementitious materials ratios; aggregate fines; and finishing manipulations.
William Hime is a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA over 54 years ago.
Bernard Erlin is president of The Erlin Co. (TEC), Latrobe, Pa., and has been involved with all aspects of concrete for over 48 years.