When calculating the Saturation Index (SI), does a single low water parameter such as calcium hardness (CH), or a low carbonate alkalinity (C-ALK), make the water automatically aggressive to pool plaster, even when other water parameters are high enough to balance the SI?
To answer that question, an experiment was conducted. Two quality pool plaster coupons were made and cured in balanced water for 90 days. At that point, plaster coupon #1 was placed into SI balanced water, but with a low CH of 90 ppm. The C-ALK was maintained at 110 ppm and the pH from 7.9 to 8.2, which off-sets the low CH and achieves a balanced SI of -0.1 to +0.2.
Coupon #2 was also placed into SI balanced water that had only 45 ppm of C-ALK. The CH of this water was 360 ppm and the pH was maintained between 7.7 and 8.0, off-setting the low C-ALK and achieving a balanced SI of -0.1 to +0.2.
After six months in the water, the coupons were removed, and the water they were in was tested for the calcium content to determine if any dissolution or etching of plaster surface material occurred. Of course, any increase in calcium from the submersion water starting point would indicate that calcium had been dissolved from the coupon, which was the only available source of additional calcium.
The result? There was no increase of calcium in either water container.
Therefore, these results indicate that if the calcium hardness or the carbonate alkalinity is low, but the water is still determined to be SI balanced, the water is not aggressive. This indicates that the SI is applicable for plaster swimming pools.
This experiment was conducted because some in the pool industry claim that pool water with a low CH or a low C-ALK is automatically aggressive despite what the SI actually is. Often, when a new pool plaster surface has undergone severe gray mottling, white spotting, streaking discolorations, flaking, nodules, or other defects actually attributable to plastering errors, the finger is pointed at aggressive water chemistry instead. Specifically, a CH or C-ALK below the APSP Ideal minimum is blamed, even in cases where the actual LSI is balanced.
It is interesting to note that the current National Plasterers Council (NPC) 7th Edition Technical Manual states that if any isolated, individual water parameter (pH, C-ALK, or CH) is lower than the Ideal range as defined by the APSP, the water is aggressive. Therefore, the NPC is (incorrectly) stating that when the C-ALK is less than 80 ppm (while the total alkalinity may be above 80 ppm), or when the CH is less than 200 ppm, or when the pH is below 7.4, (which are the lower ends of the APSP Ideal Standard), the water is automatically aggressive and considered to be detrimental to the plaster finish (even though the SI may be balanced).
This is a complete departure from the NPC text on this topic in their 5th Edition of the Tech Manual. It stated that pool water needed to be within the acceptable tolerance range as established by the APSP, which set the âminimumâ for C-ALK at 60 ppm, CH at 150 ppm, and a pH of 7.2. It is apparent that the water balance requirement by the NPC is more restrictive now. Why did the NPC make that change? Where is the study documenting that a low CH or C-ALK is automatically aggressive when the LSI is 0.0 or higher? Has the National Pool Industry Research Center at Cal Poly (NPIRC) proved that? No.
Now, we are not necessarily promoting the concept that pool water should be maintained on a regular basis with the pH, C-ALK or CH below the minimum APSP targets. We are not even suggesting that the ranges below the ideal are always the optimum. We would recommend that pH and C-ALK levels be maintained above the ideals, not below, whenever possible. However, we understand two things: that there are times when lower pH, CH or C-ALK are unavoidable, and when that chemistry is SI balanced, it means exactly that, it is not a tool for blaming plaster defects on water chemistry.
As the above plaster study demonstrated, pool water with a single low water parameter (even below the APSP Ideal or Minimum standard) can be non-aggressive if the SI is balanced.
For proper plastering practices, see this post: ten-guidelines-for-quality-pool-plaster-t42957.html