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Plaster Dust Should Be and Can Be Prevented

January 11, 2011

Reprinted with permission from Kim Skinner of OnBalance:

Plaster Dust Should Be and Can Be Prevented
For over fifty years, most (if not all) builders, plasterers, and service techs have assumed that plaster dust (that often develops during the first two weeks after plastering) is normal, unavoidable, and acceptable.  But recent research has shown that plaster dust should be and can be prevented.

Although plaster dust is normal (meaning that it is commonly seen in new plaster pools), it is not ideal (meaning that we can do better – that preventing plaster dust results in better plaster). What is plaster dust made of?  It is formed from dissolved calcium from the new plaster surface. Therefore, by definition, if there is any plaster dust, there was a loss of material from the once-smooth and dense troweled plaster. And when material is lost from the surface, that surface is not as dense, smooth and durable as it once was, and as it could have remained.

Our research documents that three factors promote more plaster dust: poor plaster, filling a pool too soon, and aggressive fill water. Any one negative factor can produce plaster dust, and the more a factor is abused, or when multiple negative factors are present, the more dust is formed. This calcium loss means that the plaster is more rough (even though that may not be visible to the naked eye or by touch), more porous, and more susceptible to deterioration and staining as time passes.

We have also demonstrated through our research and through field demonstrations that strong plaster, sufficient time before filling, and appropriate fill water chemistry will prevent plaster dust formation – meaning more dense, smooth, protected pool surfaces.

In the concrete/cement industry, “dusting” of a concrete surface is known to be a sign of a weak and porous surface, and that improper workmanship practices lead to that condition. ACI and PCA literature cites high water/cement ratios, adding calcium chloride to the mix, adding water to a surface while troweling, and finishing during very hot and dry temperatures as factors leading to a weak surface, and dusting or efflorescence may result on cement flatwork.

Following that lead, through a series of recent experiments and studies, onBalance has identified certain improper practices for new pool plaster that will lead to having plaster dust develop as the pool is filling with water.  The critical factors are as follows.

1. A high water/cement ratio of the plaster mix
2. Adding calcium chloride to the plaster mix
3. Adding water and working it into the plaster surface while troweling
4. Plastering in extreme temperatures (hot and dry, or very cold)
5. Starting and filling the pool with water too soon
6. Aggressive tap water (any water that has a negative Saturation Index)

Plastering Practices – when plastering, the following steps are recommended; use a low water/cement ratio, limit calcium chloride additions to 1% or less to the weight of cement, decrease or avoid calcium chloride use as ambient temperatures rise, do not trowel water back into the plaster surface, avoid overly late hard troweling, and plaster in moderate temperatures (or tent the pool).

Fill Delay – allow the plaster to harden for at least 6 hours before submerging in water. (More time may be needed for certain environmental conditions.)

Fill Water Chemistry – ensure that positive saturation water is used for filling (+0.5 is recommended with a pH under 8.2).  The “Bicarb Startup” process is the ideal pre-treatment chemical startup procedure, which creates positive saturation water.

By following the above recommendations, virtually no plaster dust will result and the calcium level of the pool water won’t increase. The balancing of the pool water afterwards will be much easier, the pool filters won’t get clogged up with scale, and even dark colored plaster jobs stay dark and don’t lighten or whiten in color.

Greg Garrett and Randy Dukes, members and consultants for the NPC, suggest that when aggressive tap water is used to fill a pool; it needs to be balanced after the pool is full of water.  No, that is too late; balancing of the aggressive tap water needs to be done before it is used to fill the pool.  Essentially, it is being suggested that aggressive tap water doesn’t start dissolving calcium from a plaster surface until several days after the pool is filled.  The fact is that by the time the pool is full of water, the original (aggressive) tap water has already changed in its composition (during filling) and may be balanced; because that is what aggressive water will do, especially in contact with new plaster.  The aggressive tap water immediately dissolves calcium and alkali off of the new plaster surface (producing plaster dust) within the first 24 hours, and thus, the damage has already been done!  Very little bicarb or calcium may be needed for balancing by the time the service tech shows up at poolside.

Additionally, the NPC or their consultants have blamed low calcium or aggressive water for a myriad of ills, including gray darkened and blotchy plaster, white spotting, nodules, spalling, and who knows – maybe even birth defects in pool cleaners. The fact of the matter is that aggressive water causes mild etching – which slightly roughens and may lighten the surface somewhat, but not the problems mentioned above. They are improper workmanship and material issues.

The presence of plaster dust – even though it has become accepted as “normal” – is a primary indicator of a less-than-ideal pool surface. It may have become accepted as normal, but a pool that dusts is certainly not as good as it could be. For those who prefer producing a better than normal product, who pride themselves in doing their best, following ideal practices, and eliminating plaster dust is the goal. Proper plastering, adequate fill delay, and properly prepared fill water chemistry ensure that the pool owner will have a quality and durable pool plaster finish without discolorations.

onBalance June 2010

Mike the Poolman
Pool Service & Repair in Folsom, CA since 1995

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