A Critique of the PBL Research Report
"An Assessment towards the Phenomenon of Discoloration and Deterioration of Plaster Surface Materials within the Recreational Water industry & The Carbonate System"
Discussion given by Don Hafer at the National Plasterers Council Conference 2001
By Que Hales, Doug Latta, & Kim Skinner
INTRODUCTION
PBL has released a 1996 study on plaster deterioration which concluded that "the most probable cause" of discoloration, delamination, nodules, spot etching, corrosion and overall degradation to pool cement (plaster) finishes, is high levels of aggressive carbon dioxide. The author and senior analyst of PBL, Don Hafer, made this conclusion after observing some plaster coupons (which were placed in five water tanks with five different balance programs) reportedly suffer these abnormalities.
Essentially, Mr. Hafer claims that when balancing pool water using the "carbonate system" (adding strong acids and strong bases), the amount of carbon dioxide (expressed somehow as carbonic acid) increases and will often exceed the appropriate compensating levels necessary to stabilize the "carbonate equilibrium" (calcium bicarbonate). This, he claims, results in compromising the pool plaster finish and creating the above mentioned detrimental results. Additionally, he suggests that even if water is balanced according to the Langelier Index, this condition of high carbon dioxide (CO2) levels could still result and lead to overall plaster degradation.
EXAMINATION OF CO2 LEVELS
We first examined whether PBL accurately obtained the high levels of carbon dioxide indicated in its report. Immediately noted was that a significantly high content of CO2 was claimed with a pH reading above 8.3. This is in contradiction to proven chemical science. The following is an example of a chemical analysis by PBL: the tap water used in the experiment was recorded to have a pH of 8.47, total alkalinity 120 ppm, calcium hardness 320 ppm, temperature 19.7 degrees C, TDS 544ppm, and a CO2 content of 40 ppm.
According to Standard Methods 18th Edition, and Water Chemistry by Snoeyink & Jenkins, (both of which PBL cites as references), and the NALCO Water Handbook 2nd Edition, the end-point for CO2 is at a pH of 8.3. In other words, CO2 ceases to exist in measurable quantities above a pH of 8.3. When using the titration method (as PBL claims to have used), the above tap water would result in a reading of "0" ppm for CO2. Using the reliable nomograph method as outlined in Standard Methods, it would be less than 1 ppm of CO2. This contradicts the claim of 40 ppm of CO2 by PBL. Interestingly, on page 26 of the PBL report, a reference indicates that "tritrating to a phenolphthalein end point pH of 8.3 corresponds to the neutralization of carbonic acid to bicarbonate. " It should also be pointed out that a titration test kit for CO2 should not be used for swimming pools due to several chemical interferences (as listed in Standard Methods and the Hach manual) that are normally contained in pool water.
Every CO2 analysis performed by PBL has the ppm amounts of CO2 far exceeding the results that would be given by the nomograph or calculation methods. One example from each tank is as follows: The listed PBL readings are followed by the nomograph reading and calculation result for CO2.
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As one can see, PBL's claimed results for CO2 do not coincide with the results given with the reliable nomograph and calculation methods, and are in fact, many times higher than would be in actuality. This claim by PBL is simply not plausible.
EXAMINATION OF OBSERVED RESULTS
The next aspect of the report we examined was the observations and apparent conclusions drawn from the "plaster coupons" in the various water tanks. PBL states that each of their five water tanks had a different water balance maintenance program. They are listed as aggressive, alkali, neutral, control, and zero degree hardness. Placed in each tank were 40 plaster coupons for approximately 45 days. Summaries of the observed results are as follows:
Pictures of plaster coupons were shown by PBL at the 2001 National Plasterers
Council Conference in Reno. Mr. Hafer directed our attention to certain areas on
these photographs claimed to be spot etching. The images we observed did not
appear to be any type of spot etching we are familiar with. Additionally, these
photographs were supposedly performed with a 400X magnification, which means
that the pictures should contain an area about the size of a grain of sand.
Thus, we are uncertain whether any coupon "spot etched" at all.
From these
observations, PBL concluded that the high levels of aggressive CO2 (again, for some reason, expressed as carbonic acid) in tanks
numbers 1, 2, & 3, led to discoloration, spot etching, nodules, and overall
degradation of these affected plaster coupons. PBL observed that the slightly
lower levels of CO2 in tank #4 resulted in only slight
discoloration on all coupons and slight spot etching on four coupons. Finally,
PBL observed that its "special" non-chemical program in tank #5 led to low
CO2 amounts where no discoloration or degradation was
observed on any plaster coupon.
ASSUMING HIGH LEVELS OF CO2 DID
EXIST
Although it has been demonstrated that excessively high levels of
CO2 are not plausible in any of the PBL water tanks, let us
accept for a moment for the sake of argument that those high levels of CO2 did exist in these water tanks. We still question, why water
chemistry balance (including high CO2 levels) would be
assumed to cause the above results? For example, why did only 30 coupons out of
160 (the total coupons in tanks 1, 2, 3, & 4) result in spot etching and/or
nodule growth? If CO2 in water is homogenous and consistent
throughout its volume (and this was circulating water), logic would dictate that
all the coupons would be affected equally. Not only should water that contains
high CO2 affect all of the plaster coupons equally and
similarly, but also affect each coupon surface equally and uniformly (and result
in uniform etching), not in individual spots or locations. PBL failed to explain
this inconsistency.
As mentioned above, the coupons in tank #5, reportedly
did not exhibit any plaster phenomenon such as spot etch, nodules, or
discoloration, and PBL claimed the reason for this was the low amount of CO2 in tank #5. However, the chart on page 76 for tank #5 indicates
a period of time that the CO2 reading was at 30ppm
(according to PBL's test method). It appeared that this condition could have
existed for as long as one month. By PBL own standards and claimed readings,
this amount of CO2 is too high and aggressive.
Tank #4
had recorded results for CO2 of 22 ppm or less for a period
of 50 days, with the exception of only one recorded reading of 40 ppm, which
lasted only 4 days. Yet, these coupons suffered some discoloration and some spot
etching, while tank #5 coupons did not. PBL does not address this
discrepancy.
The PBL study also claimed that the increase in calcium hardness
levels in tank numbers 1,2, &3 was due to high CO2
levels above the compensating amount, which dissolves calcium from the plaster
finish. PBL does correctly state that when the carbonate hardness of water is
low, the compensating amount of CO2 is also low. On page
67, PBL claims that only about 3 ppm of CO2 is necessary
for about 100 ppm of CaCO3, and when the calcium hardness
is lower than 100 ppm, then even less CO2 would be required
for maintaining the calcium bicarbonate equilibrium. However, the chart on page
76 (under the data for the "zero degree hardness" tank 5), shows PBL's own
listed readings of CO2 always exceeding 3 mg/l - several
fold - the highest being 30 ppm as mentioned above. The calcium hardness in tank
#5 started out at zero and eventually increased to 100 ppm.
Additionally, on
page 67, PBL writes, "It would appear that the use of soft water without the use
of chemicals normally prescribed by the recreational water industry was
beneficial to these plaster coupons." But tank #5 had an increase in the calcium
level of 100 ppm without any acknowledgment or explanation by PBL. Where did the
calcium come from if not from the plaster coupons? Additionally, the above
statement by PBL contradicts studies, which demonstrate that soft water (low
calcium, low alkalinity) is aggressive against cementitious products.
DISREGARDING CLAIMED HIGH CO2 LEVELS AND USING
LANGELIER INDEX
As is stated above, it is not plausible for the CO2 levels to have been as high as claimed by PBL. So, we will now
disregard the claimed high CO2 levels and compare the
results of five different water tanks using the Langelier Saturation Index. Tank
#1 water was to be maintained with a negative index, but only ended up negative
temporarily. Interestingly, these coupons apparently did not exhibit any spot
etching, although some in the swimming pool industry claim that aggressive water
is the actual cause of such. Tank #2 water was maintained with a positive index
(base, alkali, or scale forming water), and reportedly resulted in six coupons
spot etching. PBL attempted to maintain Tank #3 with a balanced index, but was
actually slightly positive overall. These coupons apparently suffered the worst,
supposedly resulting in 13 coupons spot etching. Tank #4 also was maintained
with a slight positive index, but had higher alkalinity levels than tank #3.
These coupons apparently had minimal spot etching on four coupons. Tank #5 water
was maintained with similar alkalinity levels as tanks #2 & #4, but with a
slightly higher pH, and started out with a zero calcium hardness content,
resulting in a slightly negative Index also temporarily. Curiously, these
coupons apparently showed no visible signs of degradation or other effects.
Interestingly, an analysis of these particular reported results seem to have
indicated that the most aggressive water (as in tank #1 & 5) had the least
aggressive effect observed on plaster coupons. This is opposite of what the
Saturation Index would predict. The coupons in the alkali tank #2 showed signs
of etching, also opposite of Index predictions. The neutral tank #3's coupon
seemed to show the most problems. Finally, tank #4's coupons, with higher
alkalinity levels than tank #3 (but similar to tank #2) showed very little
effects of carbonation and spottiness. Essentially, these results are
inconsistent and generally in opposition to the Langelier Saturation Index
predictions.
EXAMINATION OF THE CAUSES OF DISCOLORATION & NODULES
In
regards to the claimed discoloration caused by reportedly high CO2 levels of all coupons observed in the first four tanks, it
appears that PBL overlooked an obvious factor as to the cause of discoloration.
PBL's own test results indicated that copper was present in the tap water used
to fill the first four water tanks, but unfortunately they did not test for
iron. Well water, which is used to supplement Lake Mead's water for the Las
Vegas area (where PBL is located), is likely to contain iron. It is far more
probable that the discoloration noted on coupons was caused by the presence of
these minerals, rather than supposedly high CO2 levels.
Furthermore, as for the coupons in tank #5 that did not discolor, PBL filtered
and softened the tap water that entered this tank. PBL performed this process to
remove the calcium for its "zero degree hardness" program. However, this
softening process is also known to remove other minerals such as copper and
iron! Therefore, no copper or iron was available to discolor the tank #5
coupons.
Despite these facts, PBL concluded that the plaster coupons
discolored due to aggressive CO2, and interestingly, did
not explain the mechanism or process of how CO2 would cause
plaster coupons to turn gray or brown. In contrast, it is known that copper can
precipitate out as copper carbonate, copper sulfate, or copper oxide and stain
plaster finishes. Iron can be oxidized and precipitate out as iron oxide onto
the plaster as a brown deposit.
PBL infers that the nodules formed on a few
coupons were also the result of aggressive CO2. This claim
does not follow proper chemical rationale. The formation of a nodule is the
process of precipitating calcium carbonate, or in other words, calcium becomes
insoluble. When CO2 levels are high enough to be aggressive
towards calcium carbonate, then these conditions (high CO2)
would actually prevent calcium carbonate nodules from forming, and instead, make
and keep calcium in a soluble form. It is already well known that delamination
of plaster from the underlying base can cause calcium nodules to form. PBL
purposely created conditions whereby water could seep down the side of a few
petri dishes (used to form the plaster coupons), and create a void area
simulating delaminations. Therefore, it is not surprising that some nodules
formed.
EXAMINATION OF CHEMICAL FORMULAS
Other discrepancies in the
report include PBL's assertion that adding carbonates increases the CO2 and/or carbonic acid in water. PBL provided a chemical formula
to advance this concept: CO3 + H20 =
H2CO3 + O. Obviously, the above claim
is erroneous and does not follow proper chemical rationale. Instead, adding a
carbonate (such as soda ash) to water does not increase the content of CO2/H2CO3 in water; in
fact it will decrease the amount of CO2/H2CO3 in water if it is present. The
following formula illustrates: Na2CO3
+ H2CO3 = 2NaHCO3. Sodium carbonate plus carbonic acid equals sodium bicarbonate.
Carbonic acid has been neutralized and the amount decreased.
PBL
additionally asserts that adding acid to water to neutralize carbonates (i.e.,
extreme high pH) would also increase the content of carbonic acid. PBL provided
the following formula to illustrate: H + CO3 + H2O = H2CO3 + H. Again,
this formula is erroneous. The correct formula is H + CO3 +
H2O = HCO3 + H2O.
As one can see, the acid added reacts with carbonate to form bicarbonate. No
carbonic acid is created in this reaction.
OMISSIONS BY PBL
In laying the foundation that high levels of
CO2 are aggressive towards plaster (cement), PBL quoted
from the sixth section in the third chapter of Concrete Corrosion & Concrete
Protection by Biczok, 1964 (although PBL listed it as Biczolis, 1967). Mr. Hafer
transferred most of the Biczok section into his work, but without quotation
marks, indents, or other such standard indicators to cue the reader to
transitions from Biczok's words to Hafer's. In the process, he also omitted
critical, key paragraphs, which would have shown Hafer's own positions to be
incorrect. For instance: "It will be readily perceived now that water containing
carbonic acid is not aggressive to concrete unless two conditions are fulfilled
simultaneously: pH value lower than 7.0 and presence of aggressive carbonic
acid." PBL also did not refer to the information preceding the above quote,
which laid the groundwork for the conclusion made by Biczok. The main
significance of this is that PBL cited water balance problems even though the pH
of the water in all of their tanks was consistently above 7.3. (There was one
test in one tank that resulted in a pH of 6.34 for a few days, but those coupons
in tank #1 weren't observed as having any etching problems.)
Finally, we
question how PBL could rule out the effects of application and finishing
techniques (troweling) on coupons when only 30 out of 160 coupons suffered some
degradation. If troweling is performed on coupons individually, (whereas water
is universal) and the PBL study indicated "that various finishing techniques
were used to fabricate imperfections on plaster surfaces," how then can
finishing techniques be ruled out as a possible cause to the few affected
coupons? Also, if Hafer is going to refer to concrete industry texts as
authoritative on the subject of pool plaster deterioration, why does he omit key
sections and disregard material from those sources that cite high porosity, and
other improper practices and conditions as causes of corrosion, leaching, and
excess carbonation?
ADDITIONAL COMMENTS
The PBL 1996 research report seemed to promote
a chemical program consisting of sanitation by Hydroxyl and Oxygen Radicals as a
preferred method. At Mr. Hafer's presentation for the NPC, the concept of
Hydrogen Peroxide as an oxidizer and PHMB as a residual sanitizer was
recommended as the best method as to not cause degradation to swimming pool
plaster finishes. In both cases, sufficient problems with the report,
methodology, and testing exist to cause doubt that any accurate conclusions
could be reached. What is certain is that the promoted chemical regimens have
not been demonstrated to be either superior or inferior to other methods
relative to etching, staining, discoloration, etc.
SUMMARY
It is well documented in concrete and water chemistry textbooks that a high content of carbon dioxide (CO2) can be aggressive towards cement products. However, it is also well documented that high levels of CO2 generally do not exist when the pH is above 7.0, and that CO2 (or even carbonic acid) levels are not considered aggressive when the pH is above 7.0. PBL cited water balance problems even though the pH of the water in their tanks was consistently above 7.3. The Langelier Saturation Index is often used in the swimming pool industry to predict if water is aggressive towards cement or is scale forming. The observed results by PBL however, are in contrast to Langelier Saturation Index predictions. Some chemical formulas and concepts provided by PBL are fundamentally incorrect, which can lead to such false conclusions.
In recognition of this, it becomes obvious that the PBL study is
fundamentally flawed. The CO2 levels as claimed by PBL
could not have been as high as were recorded, and were in fact, a fraction of
the recorded amounts. The assumptions that carbonic acid in the water led to the
plaster phenomena problems on a few affected plaster coupons leap beyond reason,
logic, and the limitations of the experimental data. As mentioned in the body of
our critique, there is serious doubt that any plaster coupons actually spot
etched. Notwithstanding, the supposed results from the effects of the different
water conditions on the plaster coupons did not follow a consistent or logical
pattern. Some results were similar where they should have been different, and
some results that were different should have been similar.
Not only did PBL
not prove or document spot etching causes, they did not describe how or why
spots would appear as etched areas as opposed to uniform etching or why some
areas of the plaster coupons were unaffected. There is no legitimate basis given
for discarding "finishing techniques" as possible contributing factors to its
study, especially when concrete chemistry books cite that high porosity can lead
to corrosion, leaching, and excess carbonation of the cement surface.
Additionally, the presence of copper and likely presence of iron in the tap
water was ignored as probable contributors to any noted discoloration on
coupons.
Therefore, the authors of this critique paper reject the PBL study
and suggest that the conclusions promoted by PBL are without foundation,
unwarranted, and have no basis in fact.