Discoloration comes in many forms and colors and therefore it is not always easy to determine the cause. The most common form of discoloration are the gross color changes in large parts of the concrete. These color changes are usually caused by changes in the concrete mixture due to an insufficient mixing time, inconsistent use of additives or low quality materials used in the mixture. 

Another very important cause is water. When water is added and mixed incorrectly, variations in the water-cement ratio cause light discoloration or concrete stains on the concrete surface. 

Efflorescence is formed when soluble salts and other water dispersible materials come to the surface through capillary voids in the concrete. Very often as a result of temperature changes, moisture migration, condensation, rain, dew or too much water in the concrete mix and in the form of white or off-white discoloration of the concrete. The salts in the concrete can come from cement, aggregates, water or additives. 

Any product containing cement is susceptible to efflorescence. It occurs when calcium hydroxide (lime) is formed in the cement hydration reaction and is transported to the surface by water through capillaries in the concrete. On the concrete surface, it will combine with the carbon dioxide present in the air to produce calcium carbonate and water. Efflorescence can also be caused by hydroxides and sulfates of sodium or potassium. These are more soluble in water than calcium and will form it even faster than calcium hydroxide. Calcium Chloride in combination with incorrect mixing will cause severe discoloration in the form of dark spots on the concrete. 

It might be difficult to avoid efflorescence completely, but there are effective ways to prevent or minimize efflorescence formation by controlling the three contributing factors:

• Soluble salts
• Moisture
• Migrating channels

A release agent always leaves a mark on the concrete in terms of color. Therefore it is also very clear that overuse or an irregular distribution of the release agent on the moulds can cause concrete stains on the surface.