Pigments are fine dry powder or slurry or aqueous suspension that added to the concrete ingredients to produce colors, other than the normal grey color. It is added during batching as a liquid or finely divided colored inert powder.
Pigments can be natural or synthetic, organic or inorganic. Synthetic pigments are more stable and color fast. Examples of pigments are iron oxide (black, bround, red, yellow of varying particle size and shape), chromix oxide (green), titanium dioxide (white), phthalocyanine (blue, green).
Wetness and alkalinity of concrete can be aggressive environment to pigments. That is why it is very important to use pigments designed for use in concrete.
Plasticizers may be introduced to concrete mixture in which pigments are used in order to reduce water demand and other constituents that reduce the occurrence of surface efflorescence and minimize color fade caused by UV light.
Types of pigments
1. Iron Oxide PigmentsÂ
Iron oxide pigments are widely utilized but it can be costly. Different colors are browns, reds, blacks, and dirty yellows.
2. Special Metal OxidesÂ
Special metal oxides include some iron oxides are purply red, blue or yellow; chromium oxide is green, and titanium oxide is bright white.
3. Synthetic PigmentsÂ
Synthetic pigments are more expensive than previous pigments but provide the desired colors which are unobtainable from metal oxides. For instance, brilliant violet, pure red, and canary yellow.
4. Powdered PigmentsÂ
Powdered pigments can be either loose or granulated. The former is dustless since its particles are adhere together. The latter is designed for use in mixer trucks where the coarse aggregate and lengthy, vigorous mixing action break the particles up.
5. Liquid PigmentsÂ
Liquid pigments are solid pigment particles suspended in a liquid. The liquid helps to keep the pigment suspended for the duration of the measuring and dosing.
Pigment Dosage
According to ASTM C 979-10, the maximum pigment dosage is equal to or less than 10 % mass of cement. However, a normal range is between 3% and 6%.
When two types of pigments are blended to achieve certain color, the same maximum dosage of single pigments for the total amount of both pigments used.
Effects on Concrete
- The color of pigment becomes an integral part of the concrete.
- Pigments decrease concrete strength, but the maximum allowable dosage rate shall not cause a strength decline of less than 90% of unpigmented concrete strength at 28 days.
- When maximum 10% of pigment dosage is used, it should neither accelerate the initial or final set by more than 1 h nor retard the initial or final set by more than 1.5 h, compare with unpigmented concrete control mixture.
- Pigments influence concrete air content, but it shall not change the air content by more than 1.0 %, as compared to the unpigmented concrete mixture, when maximum 10% pigment is added.
- Higher dosage rates could affect the water/cement ratio of concrete.
- For lighter shades a white base cement is preferred.
- Reductions in strength experienced are usually as a result of using higher water content for a given workability as the pigment is around 10 times finer than cement. This is the case for example using goethite (a yellow iron oxide pigment) due to fine needle-shape particles.
Where to use Pigments?
- Exterior flatwork, floors, walls and countertops.
- Used to obtain uniform tones with no variations.
- It also works well as a contrasting base shade for creating layers of color.
- Pigment is a good option to shake-on color hardeners if mess and cleanup are concerns.
Advantages
- Color of pigmented concrete extends throughout the entire concrete. It is not affected by surface abrasion; the color would not wear away.
- Exposure to heat or ultraviolet light would not fade away the color of pigmented color.
Disadvantages
It can be costly