Tech-Notes on Mortar & Masonry
There are several factors that affect the ultimate color of any colored concrete product. Variations in raw materials, mixing, curing, and finishing can all result in inconsistent color in the finished product. Many procedures that help produce consistent and uni-form colored products seem to be common knowledge. It is, however, important to be aware of them when working with color.
Consistency during the curing process is crucial when producing colored concrete masonry units. It is very important that the rate at which the water leaves the masonry is constant from unit to unit and within each unit. If a preset is being used before placing units in the curing chamber, it should be for a constant amount of time. Once in the kiln, the units should all remain there for the same amount of time. This means that leaving blocks in the kiln over a weekend could cause color variation.
In addition, the conditions inside of the kiln are extremely important. The temperature at all places in the kiln must be uniform. If not, the units will be receiving different amounts of heat, and therefore water will be leaving the units at different rates.
As far as finishing for concrete masonry, there are a few important points to consider. It is extremely difficult to manufacture consistently colored smooth face units. If at all possible, try to manufacture only colored units that have some surface treatment. The most common surface treatment is a split face of some sort This will help tremendously in producing consistent colored units. Also, it is extremely difficult to judge the finish color of a unit until it is completely cured. This takes as long as several months in some cases. The buffs in particular are extremely hard to judge when first removed from the kiln.
Prior to discussion of curing and finishing, there are two important factors involved in laying mortar that need to be addressed. When laying colored mortar, absolutely no retempering can be tolerated. Retempering completely eliminates all control of the water/cement ratio and would change the end color. Also, the temperature at which the masonry is laid can affect the speed of curing and the finished color.
When discussing mortar, curing and finishing go hand in hand. The amount of curing that takes place before strikeoff must be consistent throughout the project. This can partially be judged by time, but joint hardness must also be considered. Also the type of tooling employed, unless for architectural effect, must remain the same. Different tooling will make the color appear different. A raked joint will make a color appear darker in comparison to a struck joint. If the same color is desired throughout the project, the same finishing technique must be employed throughout the job. It is generally true that if these common good masonry practices are used, consistent mortar color will be achieved.