By Kim Skinner‘
As familiar as it may seem, plaster is far more complicated than most people think.
As a consequence of that complexity, it has for years been the subject of discussion, controversy and a great volume of research and analysis. All of that has been dedicated to reaching a better understanding of plaster‘s performance, including the causes and mechanisms of problems ranging from etching, scaling and nodule formation to delaminations, spotting, staining and general surface deteriorations.
In the process, we‘ve learned something unsurprising — that known principles of concrete/cement science actually apply to pool plaster. This recognition has in turn led to development of a list of ten basic recommendations that reliably result in the creation of durable plaster.
Kim Skinner has extensively updated this article. To see the revised version, please click here.
The best cement-to-aggregate ratio is about 1 part cement to 1.5 to 1.75 parts aggregate (marble sand) — with quartz finishes being a little richer in cement. This ratio results in a higher-quality pool plaster finish than either richer (more cement) or leaner (more sand) mixes. Plaster that is too rich tends to shrink and crack, while plaster that is too lean is unworkable and isn‘t durable. Further, the cement and aggregate (and admixtures, for that matter) should be selected with care: All need to be of good quality and consistency and of an appropriate grade for use in pool plaster.
A water-to-cement ratio of .48 or less is best when mixing plaster. The American Concrete Institute (ACI) and the Portland Cement Association (PCA) have established that lower water-to-cement ratios (that is, less than .50) yield better quality cement that can resist occasional exposure to mild acids.
Lower water-to-cement ratios also increase density and reduce permeability, porosity, shrinkage (craze cracking) and movement of water within the cement product, while higher water-to-cement ratios can result in a finished product that does not offer adequate protection or long-term durability against the natural effects of water and the environment.
Plaster should contain as little calcium chloride set-accelerant as possible — and never more than two percent of the amount of white cement. (Colored plaster, of course, should not contain any calcium chloride.) It has been documented by PCA and cement researchers that using more than this amount increases the chances of gray discoloration, mottling and cement shrinkage. Helpfully, alternatives to calcium chloride that lack this downside potential are available.
A plaster mix should be mixed thoroughly, but not for too long. In fact, it is recommended that if the plaster has been mixed for more than 90 minutes, the batch should be discarded.
Water should not be added to plaster surfaces during troweling because of the strong risk potential. It has been documented by both ACI and PCA that doing so (that is, “skewing the surface water-to-cement ratio”) increases porosity, shrinkage, streaking, spotting and variable discoloration.
That said, small amounts of water used for trowel lubrication don‘t seem to have a significant adverse effect on the plaster surface. Most visible problems, which take time to become exposed, require a combination of water finishing, chloride abuse and other improper practices.
The important thing is not to “work” or force additional water into the plaster surface while troweling, because this particular practice can weaken the surface and reveal deterioration and discoloration (or color variation) much sooner than usual. (Dark-colored plaster is even more susceptible to discolorations from water additions than white plaster.)
Well-timed hard troweling can help produce a good, dense plaster finish. If, however, the plaster surface has hardened too much before a smooth surface is obtained, PCA has documented that this late hard troweling can cause dark discoloration and color variation, especially when calcium chloride and extra water are also used.
Plastering in extreme weather conditions can lead to problems. ACI and PCA both mention that applying cement-based products on days or in conditions that are either too cold or too hot will adversely affect both quality and durability. These problems can be avoided by “tenting” the pool, thus protecting the plaster surface (and the plasterers!) from temperature extremes. Tenting a pool in extreme dry and high temperatures, for example — and perhaps even directing air from an evaporative cooler under the tent — can help the plaster retain its moisture and thereby properly cure and harden.
Once the pool has been plastered and is finished, the filling of the pool with water should not be started too early. Conditions vary, but as a general rule the water should not be turned on for at least six hours after completion of the finish troweling. This allows the plaster to harden properly before it is submerged in water — a key point because even balanced tap water can dissolve certain components in an insufficiently hardened plaster surface, creating increased porosity and early deterioration.
Steps should be taken to ensure that fill water is appropriate for filling a new plaster pool. Studies have shown that 75 percent of the damage done to new plaster surfaces by aggressive water occurs within the first 24 hours, although this deterioration is uniform. (Other new plaster surface discolorations — drips, splotches, spotting, trowel marks and hand and footmarks — are caused by finishing errors.)
Surfaces can be further damaged by using aggressive (acid) start-up techniques that can increase the amount of uniform surface loss, while baking soda startups will, by contrast, both neutralize aggressive fill water and also promote a superior plaster surface.
Once a pool is filled, it‘s best to balance the water and keep it that way. Although this does not serve to “create” long-lasting, durable plaster in the same way the practices listed above will do, balanced water chemistry helps preserve the surface for the long run.
Indeed, long experience has shown that aggressive water uniformly etches plaster, while overly saturated water scales it. The Langelier Saturation Index is a good guide: The water should have a saturation index value in the range of -0.3 to +0.5 — a range that does not actively promote either scaling or etching.
Keeping It in Perspective
White plaster is a good and relatively inexpensive surface that makes many pool owners happy. Indeed, properly applied and braced by reasonably good maintenance, a white plaster surface will last about 20 years.
These finishes are not inherently weak, nor are they unable to stand up to the “real world” environment of pools exposed to the vagaries of chemistry and maintenance. This is evidence that many pools are plastered correctly by good plasterers. And this is true despite the suggestion in recent years that pozzolans, blended cements, quartz and pebble aggregates can provide even better results that white plaster. It is important to note that none of those alternatives negate the need for proper workmanship!
In the real world, of course, we all recognize that some variables fall beyond the control of even the most professional tradespeople: Weather happens, materials that are assumed to be consistent may vary, and well-intentioned but unknowledgeable outsiders can intervene. It‘s also true that pool plaster is a hand-crafted product, and nothing in nature — especially where humans are involved — is perfect.
But a plaster finish can also be poorly made. If only one minor abuse of the above guidelines occurs, it probably won‘t manifest its effects in any visible way early-on. If more than one of the abovementioned practices is significantly abused, however, then far quicker deterioration can take place, even in balanced water. When problems arise, I would suggest that those involved should investigate whether these guidelines for providing a quality, durable plaster surface were followed.
Today‘s technology provides us with the ability to analyze plaster for most of the abovementioned issues, including such tell-tales as the water-to-cement ratio, the cement-to-aggregate ratio and the calcium chloride content. Technology can also determine whether the surface is more porous than the interior matrix as a result of water additions, and whether the plaster has been etched by aggressive water or has simply deteriorated as a result of its poor quality.
In this context, adhering to good plastering practices and standards and then following accepted water-balance practices will eliminate almost all pool plaster problems. Ultimately, pool owners will be happy — and our industry will prosper.
Kim Skinner has been part of the pool industry for 40 years as a service technician and plasterer and as president of Pool Chlor, a chemical service firm. He has conducted laboratory and field research on pool water chemistry and on the relationships between water chemistry and pool plaster surfaces; he also developed the bicarbonate start-up method for new plaster pools. Currently, he is with onBalance, a consulting group that also includes Que Hales and Doug Latta. For more information, go to www.poolhelp.com.