Gray Mottling Discoloration #3

Gray (or grey) mottling discoloration which occasionally occurs in plaster swimming pools may also occasionally occur on other cement flatwork, including things like driveways and sidewalks. According to the American Concrete Institute and the Portland Cement Association, adding calcium chloride and troweling concrete too late when the surface is hard are two factors that can cause graying or dark mottling of the surface.

In the first picture below of a concrete sidewalk, for example, note the gray or darkened discoloration where the concrete has been troweled to a smooth surface. This mottling may have likely been caused by late troweling to smooth the area along both sides of the edging and expansion joint.

In the second picture, note that the smooth, edge-troweled border on one side of the expansion joint in this sidewalk is gray and the edge-troweled border on the other side is not gray or discolored. It appears that the non-discolored side was troweled at the proper time, but the discolored side was not.

Let’s also understand that this two year old concrete sidewalk has been subjected to rain water, which is similar to putting distilled (very aggressive) water on the surface. Note that the rain water did not cause gray discoloration of the entire concrete sidewalk surface, nor did it remove the existing gray. On what basis do National Plasterers Council (NPC) representatives claim that slightly aggressive pool water causes gray mottling discoloration in swimming pools?

We have also received reports that NPC representatives are (incorrectly) claiming that there are no similarities between pool plaster and concrete, and therefore, concrete studies and results should be disregarded. Yet, the NPC Tech manual references ACI and PCA literature numerous times as applicable to their product. They can’t have it both ways and obviously can’t refute the facts.

Let’s discuss why concrete materials and pool plaster are similar, and how improper concrete flatwork and improper pool plastering practices are similar.

Both concrete and pool plaster are made with Type 1 Portland cement. Cement is the material that reacts with water and calcium chloride which can result in a gray or darker discoloration, and hard troweling, especially late in the sequence, also causes a darkening and mottling of the surface. While concrete contains very large aggregate, relative to pool plaster, after tamping, floating and troweling is performed, the surface usually only contains small sand particles along with cement material, which would be similar to pool plaster. Issues of improper troweling, temperatures, humidity, water-to-cement ratios, porosity and density, are similar during placement for both pool plastering and concrete flatwork, and concrete/cement authorities agree (CTL/RJ Lee). And since pool plaster is only about a half-inch thick and troweled to a very smooth finish, we contend that proper workmanship, hydration, and curing, is even more critical to the durability and aesthetic appearance of pool plaster, certainly not less.

Although service techs need to be professionally responsible for such things as scaling and etching that are chemically caused, don’t let an NPC representative fool you or intimidate you into accepting responsibility for a plaster defect.

For more detailed information on concrete discoloration, click on this link below, and go to pages 3, 4, & 5.

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  • Quartz is certainly a good product to improve the durability of plaster mixes.  It also doesn't discolor.  But if cement is still part of the mix, then discoloration is still possible from improper workmanship practices and the addition of calcium chloride if added. 

  • I have been reading and developing what I consider the solution to all the problems concerning Swimming Pool Plaster and have developed a 100% Pure Quartz Plaster using no Oxides as the base color, the color is provided by using the same Quartz that is Ceramic Coating applied.

    This Plaster is mixed using a 2 parts to 1 part of White Portland Cement, this is added on the site unlike some of the pre-mixed plasters being sold, these days I consider that because of the shelf life of cement in these products they could have already have started to hydrate, which can cause the pools surface to become rougher over time.

    Tempeture is an important factor when plastering a pool, firstly the shell below 16 degrees C and you can expect problems also the same with the air tempeture, sure adding warm water can help but the if the shell is to cold beware.

    Sorry but my Plaster is not available in the USA only New Zealand if you wish go to my website and you will see some pictures of the product that does not fade or bleach over time.

    Another aspect of Quartz is that I have discovered is that it is a wonderful Filter Media filtering down to 3 microns it even removed the rust out of the water into a 25m x 9m pool that I recently built is Shanghai, other tests seem to indicate that it may be the best media you can use.

    Norman Tyree 

  • Good question. Yes, using warm or hot water will increase the rate of hydration and the cement would set up quicker. The benefit is that using warm water would not cause a graying discoloration. Also, making the plaster mix thicker (using less water) also would help set the cement faster.
  • Note - there are non-chloride accelerators that do not have the down-side effects of chloride. Google "non-chloride accelerator" and it will return a plethora of products and manufaturers. They are not only compatible with steel (and hence recommended and/or required in some specs for reinforced concrete) but are also compatible with integrally colored concretes, which chloride is not...
  • Kim,
    As the fall and winter approach the increased use of calcium chloride is a necessary evil. you add calcium to water and it gets hot. Is there also a continued chemical reaction from the calcium which caues the continued
    hydration in colder weather or could the same results be achieved by the use of just hot water in the plaster mix? Would the hot water eliminate the curing discoloration caused by adding calcium?
  • Kim,
    I also think it important to mention as well that swimming pool plaster surfaces are generally between 1/2 inch and 3/4 inch thick, and have a richer cement mix than other concrete flatwork.
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