One would think that when mixing white cement with white limestone aggregate, the final pool plaster product would always be white. But that is not always the case.
Unfortunately, white pool plaster sometimes turns gray (or grey) either immediately or a few months after the pool is filled with water. So what causes that to happen?
The concrete/cement industry has documented that late hard troweling can cause a cement surface to darken, mottle, and spot. Also, adding a high dosage of calcium chloride (CC) to a plaster mix causes a slight darkened (graying) hue to develop. The combination of the two issues is really problematic. The pictures below are plaster coupons from an experiment conducted by onBalance and demonstrates how white pool plaster can darken and mottle.
Picture above: Top left coupon: no CC added, properly troweled. Top right coupon: no CC added, late hard troweled. Bottom left coupon: 2% CC added, properly troweled. Bottom right coupon: 2% CC added, late hard troweled. All were placed in balanced water for one year.
Note the darker color of bottom left coupon that had 2% CC added in comparison to top left coupon which had no CC added. Note the darker color of bottom right coupon that had 2% CC added and received only minimal late hard troweling in comparison to the top right coupon, which had no CC added, and received four times the amount of late hard troweling.
Picture above: Note that the top coupon is the whitest. That coupon had zero CC added and was properly troweled. The middle coupon had 2% CC added, received some late hard troweling. The bottom coupon had 2% CC added, received some late hard troweling, and placed in balanced water for one year. Note how the bottom coupon darkened even more after being submerged in water.
Pool owners pay good money for uniform white plaster, not gray or blotchy plaster.