Concrete is composed of two main ingredients, cement paste and sand/aggregate (S/A). The properties of the cement paste are affected by the presence of the S/A, specifically during the short period of time when blending in the mixer. When water is added, cement particles are dispersed and begin the chemical conversion. However, the cement particles are in fact suspended in water, even in low and zero slump concrete, and cannot pack together as efficiently when they are in the presence of the much larger S/A. This particle packing is commonly known as the wall effect. During mixing, which can be from two to six plus minutes, the shearing effect of the S/A can cause the water to separate from the cement particles. The result is a very narrow region around each S/A with fewer cement particles and thus more water. This is called the interfacial transition zone or the ITZ.
The ITZ is a region around each S/A with a higher water to cement content ratio and therefore, higher porosity than the bulk paste. The ITZ varies in thickness from point to point around each S/A. Because of the larger pores, the ITZ is characterized by a higher presence of larger crystals, calcium hydroxide. The ITZ has important effects on the properties of the concrete, especially long term surface durability performance and color fading.
Evidence of faded colored concrete is found all over the world.
The original color is just not as vibrant after a few years in the field. Paste removal exposing sand and aggregates is the common cause (see below). The pigment/paste layer is eroded and removed over time with the underlying sand and aggregates exposed. The result is a lower intensity, lighter, different shade of concrete than the original. The pigment, which is almost always Iron Oxide, has not changed chemically but has simply been removed at the ITZ region. When other performance pigments like Blue and Green are used, the contrast in color between pigment and S/A is even greater. This is why complete dispersion is critical when making concrete.
With pigment particles added into the ITZ equation, things get more complicated. Pigment is a combination of three forms - primary particles, loosely bound agglomerates of primary particles and hard fused aggregates of primary particles. Granular and liquid colors contain differing ratios of these three forms but in general, once the pigment hits the mixer and begins dispersing, the following occurs. As the cement chemical conversion begins in the first 60 seconds in the mixer, the primary pigment particles wet. There is no bond at this point because chemically the cement is just beginning it's reaction and conversion.
The surface area of the loosely bounded agglomerates of pigment particles is a different story. Think of this as pyramid stacks of micro ping pong balls with a space in between each one. The single ping pong ball is the primary particle and the stack is the agglomerate. The developing ITZ area is not going to be the same around each pigment stack. More details can be found on the Part B Micro Color pigments page.
When using color for exterior product, the objective of any cement and concrete producer is to keep the pigment particles attached to the binder. The cement is the binder or the glue keeping everything attached to each other. The ITZ plays a very important role in how the pigment is kept attached within the binder. If excess water is present, the ITZ is more pronounced and a weaker link is formed. The amount of water added should only be enough to chemically convert 100% of the cement. With limited mix times and production rates required by most producers, this is not an easy task. Water should never be thought of as the lubricant to make concrete flow or pack. Excess water is why concrete cracks, degrades and falls apart over time.
Water is both a necessity and enemy of the cement concrete system, especially in colored concrete. The higher the water content, the lighter the color and the weaker the cement paste.
Why does more water create a lighter color? Try this: pour a soda or a cold beer into a glass and watch what happens. The foam, which is the same fluid, is much lighter in color. The small air bubbles in the foam are scattering the incoming light, therefore the lighter color. This same phenomenon happens with excess batch water in concrete. Tiny air bubbles are created, scattering the light and giving the appearance of a lighter color. The objective should always be to use as little water as possible when making any concrete. More details can be found in the admixture section.