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Plaster surface discoloration debate rages on

By Marcelle Dibrell

It’s hard to imagine a drier or dustier topic than the subject of concrete.

The old saying “like watching paint dry” could be easily replaced with “like watching concrete set” and something could be gained for the change; paint dries over the course of hours; for concrete, setting takes place over the course of years.

It’s also hard to imagine that such a discussion could inspire interest, let alone strong feelings. Yet, for over 30 years, in the building and scientific communities alike, this topic has aroused deep controversy and is an ongoing subject of debate.

Today, the specific topic under debate is the discoloration of plaster surfaces. Itis an issue with a long history.

In 1966, The Research and Development Laboratories of the Portland Cement Association published their findings concerning “Surface Discoloration of Concrete Flatwork.” The article seeks to explain the causes of the deterioration in the appearance of concrete. The article was not targeted specifically to the swimming pool industry.

Nonetheless, the scientific findings concerning mottling discoloration of these surfaces are relevant to our field, given that we are discussing the same medium, namely, concrete. The discussion here is not related to external staining caused by dirt, metals, or foreign matter in contact with the concrete.

The following is a synopsis of their conclusions:

“In these studies, no single factor seemed to cause discoloration. However, combinations of factors caused very severe discoloration. Factors found to influence discoloration were calcium chloride admixtures, cement alkalis, hard-troweled surfaces, inadequate or inappropriate curing, concreting practices and finishing procedures that cause surface variation of water-cement ratio, and changes in the concrete mix.”

This study was not, however, conducted for concrete under swimming pool conditions. As we all know, the special water chemistry that can affect concrete surfaces certainly should not be ignored. Variations in pH, for example, can drastically alter the appearance of plaster surfaces.

In 1988, the National Spa and Pool Institute Florida Council formed the Plaster/Marcite Committee to “seek out the problems that were besieging the industry and come up with a solution.”

The results were published in 1991 in an article entitled, “A Study of Marcite (Plaster) Deterioration in Swimming Pools.”

The study hoped to examine a number of problems involving blemishing, etching, mottling and discoloration of plaster in pools. The stated purpose of the study was to examine “corrosion related surface deterioration of swimming pool marcite.”

The conclusions of the study were that the deterioration of plaster is primarily due to leaching of calcium hydroxide from Portland cement paste, mainly due to improperly balanced water.

What we have are two independent scientific studies: One aimed to determine causes for discoloration in plaster, unfortunately not located in swimming pools; the other aimed to discuss the problems associated with corrosion-related deterioration.

Here’s the rub: Where is the study that examines surface discoloration of swimming pools without the stated implicit belief that the problem is corrosion-related in the first place?

The debate rages on.

The crux of the debate is whether plastering practices cause the mottling and discoloration or whether it is due to the subsequent water chemistry imbalance after the plaster has been laid.

On either side of the debate, we have the plasterer, who is hopefully following plastering industry standards, and the pool service technician, who is hopefully keeping the pool water perfectly balanced.

In most cases, we cannot know that swimming pool water is perfectly balanced 100 percent of the time.

We can only know that the water is perfectly balanced at the time of measurement. Therefore, it seems easy to blame bad water chemistry for discolorations in plaster that was laid according to plastering industry standards.

On the other hand, when water is measured to be perfectly balanced at all

times of measurement, it seems negligent not to look at plastering practices.

In this special edition of Service Industry News, we provide a platform for the current opposing views on the subject of white spot discoloration of plaster surfaces. These views are presented by onBalance and the National Plasterers Council (NPC).

For those unfamiliar with these establishments, onBalance is a team of service technicians that seek to dispel some popular myths in the swimming pool industry as well as recommend scientifically backed service practices.

The National Plasterers Council is a trade association focused on studying plaster surface problems and the writing of technically sound practices for the industry.

The facts and opinions presented here are those of onBalance and the NPC, and we leave it to discerning readers to formulate their own conclusions.

And once those conclusions are formulated, we urge our readers to share with the writers of Service Industry News.

Why? Because this discussion concerns the pool & spa industry at large. The readers of Service Industry News makeup a fundamental portion of this industry and therefore their opinion and thoughts matter very much.

And so, with that said, email your letter to the editor today:

And to read the entire March 15 issue — complete with give-and-take discussion from onBalance and the NPC, as well as start-up tips, charts and more — we welcome you to subscribe to Service Industry News today. Just $12 for 24 issues or $22 for 48 issues. Call us at 949-916-0292 to get started.

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