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Surface Discoloration on Concrete Slabs

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Although nobody really asked,  here is some information on surface discoloration on concrete slabs.


Surface Discoloration on Concrete Slabs

Surface discoloration can appear as light or dark blotches or as color changes over large areas. Although discoloration is typically a cosmetic problem, if it is caused by poor finishing practices, serviceability may be impaired.

One of the most common causes of discoloration is local variations in the w/cm of the concrete. A high w/cm will usually produce a light-colored concrete, a low w/cm will produce a darker color. The difference in color is a function of the hydration of the ferrite (iron) compounds in the cement.  Ferrite compounds become lighter as they hydrate; if the w/cm ratio is low, the ferrite compounds may remain unhydrated, resulting in a darker concrete.  

Repeated steel troweling of areas that have already set reduces the w/cm at the surface, darkening its color; this is typically done intentionally, to create a "burnished" finish.  However, dark areas may also be the result of dry cement that was troweled into the surface to absorb excess bleed water.  The darker color indicates that the cement was not fully hydrated and thus the surface may be weak. Low spots where bleed water remains longer before evaporating may be lighter because the cement is more completely hydrated.  Light-colored areas may also be the result of overworking of excessively wet concrete or finishing while there is bleed water on the surface; these practices are likely to produce a weak surface.

When curing is done with waterproof paper or plastic sheets, there are often dark blotches in places where the sheet is not in contact with the surface. Wrinkles and bubbles in the paper or plastic allow water to evaporate from the surface and there may be less complete hydration of the ferrite compounds at these locations. Although this discoloration may be aesthetically objectionable, it has no effect on the quality of the concrete.  The discoloration is aggravated when the concrete contains calcium chloride; calcium chloride accelerates the overall cement hydration process, but has a retarding effect on the hydration of the ferrite compounds.

Light-colored areas can also be caused by efflorescence, a crystalline deposit that occasionally develops on the surface of hardened concrete.  Efflorescence occurs when chemicals in hardened concrete are carried to the surface by moisture.  Calcium hydroxide is the most common source of efflorescence; calcium hydroxide itself is water-soluble, but when carried to the surface, it reacts with carbon dioxide in the air and precipitates out as calcium carbonate. Using a pozzolan such as fly ash or slag helps to reduce problems with efflorescence. Pozzolans react with calcium hydroxide to form an insoluble calcium-silicate hydrate; they also tend to reduce concrete permeability.

Uneven application of dry-shake materials, such as mineral or metallic hardeners can cause local color variations.  Changes in the mix design, including changes in the source or type of cement can cause large-scale color changes. Different brands and types of cement may have different iron oxide and alkali contents. Changes in the amount, source, and chemistry of a mineral admixture also affect discoloration. Silica fume gives concrete a dark-gray tint.  Fly ash can be tan to dark gray or black, depending on its constituents.  Tan and light colors are typically associated with high lime content whereas a brownish color is associated with a high iron content; a dark gray to black color typically indicates a high unburned carbon content.

Gail Kelley