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This article provides the basis and evidence for the recommendations to wait at least 6 hours before filling new plaster pools, using a low water/cement ratio for a plaster mix¸ and reducing the amount of calcium chloride added (one percent or less). Builders, Service Techs and Pool Plasterers can work together to ensure a quality plaster job that will last a long time and be stain resistant. Pool owners want and pay for a quality product.

Several experiments were performed, and first studied were the effects on the length of time one waits after a new plaster pool’s final troweling until the time water is turned on to fill the pool.

For this experiment, plaster was mixed, and coupons were formed and placed in water at various time delays (30 minutes, 2 hours, 6 hours, and 24 hours) after the final hardening.

Each plaster coupon was placed separately into a five-gallon tank of balanced water, a starting pH of 7.7, alkalinity of 110 ppm, calcium hardness of 180 ppm, and a temperature of 70 degrees.

Since this water is balanced according to the Langelier Saturation Index, does this mean nothing from the plaster surface should dissolve into the water?

Of course, we all know that “plaster dust” often forms in the water, and on the walls and floors of pools soon after newly finished plaster pools are filled. And the primary source of this “plaster dust” is, well, the plaster! But why does the plaster come off the walls when the water is balanced to begin with? And why do some pools have more “dust” than others? What can be done to reduce this problem? Those questions will be answered below.

If the chemistry changed in the water after the plaster coupons were placed in the water tanks, it would be assumed as a result of plaster components dissolving and influencing the water chemistry. We also wanted to know if those changes varied according to the “fill delay”.

The first thing noticed was that the pH of the water rose (in 24 hours) in all four tanks. But the pH rose to 8.5 in one tank, and went as high as 9.2 in another. We also noticed some “plaster dust” settling at the bottom of the tanks.

Why did the pH rise so high?

Why did the pH rise to different levels in the different tanks?

Why didn’t the pH stop increasing once the pH reached 8.4, as it would normally?

Since at a pH of 8.4 the calculated Saturation Index of the water would be +0.7, the water isn’t aggressive to the plaster surface, yet the pH continued to go even higher! Where did the pH-increasing influence come from?

The reason for the continuing pH rise in this experiment is the compound calcium hydroxide, a component of hardening plaster, which has an extremely high pH – in the 12 range. It is somewhat soluble even in what we would consider balanced water, since water balance and the Saturation Index is relative to calcium carbonate, a lower-pH compound.

This experiment demonstrates that calcium hydroxide can be dissolved by “balanced” water from the surface of relatively hard plaster. However, calcium carbonate, a major component of a plaster surface, requires aggressive (SI) water to dissolve. The Saturation Index indicates when water has the ability to dissolve calcium carbonate, not calcium hydroxide.

After placing plaster coupons into water at different time intervals after final hardening, we tested the pH of each tank 24 hours after placement. We found that the lowest pH increase (to about 8.5 from a beginning pH of 7.7) was the tank with the coupon that had hardened the longest (24 hours) before placement in water.

The six hour coupons (placed in water after six hours) resulted in a pH increase to about 8.7.

The coupons placed in water after two hours resulted in a pH of about 9.0.

The highest pH increase (to about 9.2) was recorded in the water tanks that received the first plaster coupon in the shortest time, after only 30 minutes after final hardening.

CONCLUSION #1 – The rising pH levels indicated the loss of calcium hydroxide from the plaster surface and into the water.

CONCLUSION #2 – Since the water’s pH change varied progressively from tank to tank, from fill delay to fill delay, we can conclude that the sooner the plaster is placed under water, the more plaster material is dissolved off the surface and into the pool water – even positive saturation water.

It should be reasonable to assume that when plaster loses some of its material (such as calcium hydroxide) from the surface, this loss creates porosity in the plaster surface. Increasing porosity reduces the long-term durability of the plaster surface, since it allows greater water penetration and movement. Greater porosity also increases the likelihood, severity, and permanence of staining discolorations.

Unfortunately, there is no current pool industry standard or guideline limiting how soon water can be started to fill a new plastered swimming pool. The filling of some pools is often started before the finishing crew has even left the job site. This is an area where onBalance thinks scientifically-derived standards could improve the consumer product.

The plaster coupons used for the first phase of the study were made with a water-to-cement ratio (w/cm) of .44. This means that .44 pounds of water was used for every pound of cement. The aggregate in plaster is not counted as part of the w/cm.  A .44 ratio would be considered a low w/cm for the pool industry, and results in a beneficial and desired thick mix for plastering pools.

For the second phase of our experiment, we formed plaster coupons using a higher w/cm of .56, which results in a more watery mix. This scenario can sometimes happen if the plaster mixer person adds too much water to the mix.

We placed these newly formed plaster coupons in water at the same time intervals, or fill delays as in our first phase. The pH was tested again after 24 hours of coupons being placed into the water. As before, the pH rose in all tanks, but this time it rose even slightly higher than before. The water with the 24 hour coupon rose to a pH of 8.7, slightly higher than the pH of 8.5 recorded in the first phase. The 30 minute coupon rose to 9.5, also slightly higher than the pH of 9.2 that was recorded in the first phase.

CONCLUSION #3 – The wetter the original mix, the more plaster material is dissolved off the surface and into the pool water. This is an additional factor to the fill delay issue itself. Again, the loss of material from the plaster surface creates porosity and will shorten its life-expectancy. A high water/cement ratio decreasing a cement-based product’s durability is established, fundamental cement science.

Unfortunately, there is no current pool industry standard or guideline limiting how much water can be used in a plaster mix. This is another area where onBalance thinks scientifically-derived standards could improve the consumer product.

As a third variable in this study, the process was repeated using various amounts (1%, 2%, 3%, and 4%) of calcium chloride (for hardening acceleration which plasterers use) in the making of the plaster coupons. Testing the water for chloride just five days after the coupons were submerged in water showed that some of the calcium chloride from the plaster coupons was dissolving into the water. We learned that about 10% to 25% of all calcium chloride that was added to a plaster mix dissolved out of the plaster paste and into the swimming pool water.

The highest amount of calcium chloride loss came from the coupon with the highest w/cm and placed into water the shortest time (30 minutes). And of course, the least calcium chloride loss was from the most durable coupons – with the lower w/cm in the plaster mix and with the longer hardening time before submersion in water.

Losing yet another paste component, this time calcium chloride, from the plaster surface into the water leads to yet more porosity. In combination with calcium hydroxide loss, a significant level of additional porosity can be created in a plaster surface.

Past industry standards recommended a limit of 2% of calcium chloride (CC) in plaster mixes. Unfortunately, it appears that there is an effort by the NPC to allow greater levels as a result of (inaccurate) conclusions from the NPIRC research. This potential increase in the standard is contrary to existing cement science, and contrary to our research applying known science specifically to pool plaster.

After being in the water for 7 days, all of the plaster coupons were removed from the water tanks. Remember the plaster dust in bottom of the water tanks?  Could dissolving this dust and measuring the calcium increase from the starting point be another way of evaluating the effects of fill delay, of water-to-cement ratios, and CC additions?

Sufficient acid was added to the water tank to dissolve all of the precipitated “plaster dust” at the bottom. Then the water was tested to determine the total amount of calcium that had dissolved out of the plaster coupons and into the water.

The water that contained the coupons with the lower w/cm of .44 and the longest fill delay of 24 hours was tested. The calcium increase was 10 to 20 ppm. Since the ratio of plaster surface to water volume in this experiment was similar to that of a pool, this can be equated to about 2 to 3 pounds of calcium carbonate dissolved away from the surface of a 20,000 gallon swimming pool.

Next tested was the water that contained the plaster coupons with the higher w/cm of .56, the shortest fill delay of only 30 minutes, and 4% CC. The calcium level in these tanks increased about 180 ppm in just 7 days!  This 180 ppm of calcium increase is the equivalent of 30 pounds of calcium carbonate lost from a new plaster pool surface into the water of a 20,000 gallon swimming pool! Quite a difference!

We learned from this study that although calcium hydroxide and CC can be dissolved by balanced water, proper construction and curing practices can reduce the amount of this material that is dissolved away from the surface – resulting in a denser and better product.

The results of this pool plaster durability study indicate a need for the plastering industry to set scientific standards and limits for:

• minimum times for hardened plaster to set and dry before filling the pool with water
• maximum water-to-cement ratio
• maximum calcium chloride content
• Ensuring that tap water contains sufficient calcium and alkalinity (positive LSI) before using for filling pool. (This was actually proved by a different experiment, and the Bicarb start-up program should be recommended)

The American Concrete Institute’s Guide to Durable Concrete (201.2R-01) “describes specific types of concrete deterioration” and “contains a discussion of the mechanisms involved and the recommended requirements for individual components of concrete, quality considerations for concrete mixtures, construction procedures, and influences of the exposure environment, all important considerations to ensure concrete durability”.

This guide provides the following recommendations for obtaining abrasion resistant concrete surfaces and resistance to mild acid attacks:

• Use a low water-to-cement ratio at the surface which will reduce permeability. Tests indicated that w/cm of .40 provided significantly better protection than w/cm of .50 and .60, and a w/cm of .62 provided little protection. (A high water/cement ratio also causes shrinkage and craze cracking).
• Avoid the use of supplemental water when troweling. Do not finish concrete with standing water because this will radically reduce the compressive strength at the surface by increasing the surface water/cement ratio. (Also causes craze cracking of surface)

Other documents from the ACI and Portland Cement Association advise against using more than 2% of calcium chloride to the weight of cement due to the detrimental effects of drying shrinkage and discoloration.

All of these recommendations are consistent with the results of our study.