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Misc Comments on Concrete Discoloration

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Surface Discoloration on Concrete Slabs
Surface discoloration can appear as light and dark blotches, or as color changes over large areas. Although discoloration is typically a cosmetic problem, if it is caused by poor finishing practices, the serviceability of the slab may be impaired.  One of the most common causes of discoloration is variation in the w/cm ratio of the concrete. A high w/cm ratio will usually produce a light-colored concrete; a low w/cm ratio 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, some of the ferrite may remain unhydrated. 
When curing is done with polyethylene sheets, there are often dark blotches in places where the sheet is not in contact with the surface. Wrinkles and bubbles in the plastic allow water to evaporate from the surface and there may not be 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 exacerbated when the concrete contains calcium chloride; calcium chloride accelerates the overall hydration process, but has a retarding effect on the hydration of the ferrite compounds.
Repeated hard-steel troweling of concrete that has already set reduces the w/cm ratio, 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.
Light-colored areas are sometimes caused by efflorescence, a crystalline deposit that occasionally develops on the surface of hardened concrete.  Efflorescence occurs when calcium hydroxide is carried to the surface by moisture.  Although calcium hydroxide is water-soluble, when it is 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 the concrete's permeability.
Light-colored areas may also be the result of overworking excessively wet concrete or finishing while there is bleed water on the surface; low spots where bleed water collects may be a lighter color because the cement is more completely hydrated.
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 cement can cause large-scale color changes; different brands of cement may have different iron oxide and alkali contents. Changes in the amount, source, and chemistry of a mineral admixture can also affect the concrete color. Fly ash can be tan to dark gray or black, depending on its constituents.  Tan and light colors typically indicates a high lime content, whereas a brownish color indicates a high iron content; a dark gray to black color typically indicates a high unburned carbon content. Silica fume also gives concrete a dark gray tint.