Same thing that causes calcium scale in a new pool, or any pool for that matter. Calcium scale is usually the result of a "perfect storm" type of water chemistry. If you have high calcium levels, say 400 ppm or better and pH above 7.5 and TA over 120 you are at risk of developing calcium scale. High calcium levels can be managed if you keep the pH and TA on the low end of the acceptable range, say 7.2 and 70ppm respectively.
Bottom line is when the pool is properly balanced there will be no scale formation.
I have the perfect storm in which you are referring to. So sad. I thought my plaster was lightening, but I just realized tonight that it has a coating on it.
Used Leslie's Test Kit 81-330...then raced to get some chemicals, and they tested my water too. Results were similar, but I think mine are more accurate because the guy running the chemicals was sloppy and I was precise! But here they are:
FAC 3 (store said 5)
TAC 3 (store said 5)
CC 0
PH needed 7 drops, so I am adding 1.5 gallons over the next two days. (Store added 5 drops)
TA 190 (Leslie store said 140)
CYA 33 (store said 30)
CH 525 (store said 440)...we both used 44 drops, my kit said multiply by 25, he said as a general rule you multiply by 10
Salt 2500 (going to add salt tomorrow to bring it up to over 3000). Should I wait to add until my PH is good?
Phosphates 150 (from Leslie's)...
I added 1 gallon of liquid stabilizer. It pears I needed 3 gallons, as that one gallon will only raise my CYA by 10 and I need at minimum 50 (from Pentair manual) and 60 according to Pool School.
But the huge issue at hand is the enormous amount of white flakes. While cleaning my tile, I was able to notice that where I thought my plaster was lightening, there were streaks of darker plaster (I have gray plaster) and that the lighter parts were elevated so it makes me think my whole plaster is covered in scale!