Plaster pool has a 'natural' pH maybe?

[aquarium]

I used the aeration method to lower alkalinity from 230ppm to 90ppm. Although the amount of acid now consumed has dropped a lot, the pH insists on rising to 7.6 and holding there unless I add more acid. But if I add acid the next day it's back to 7.6.

Is it possible that the plaster in the pool is causing the pool to set its own 'natural' pH?

It's an older pool that came with the house. Perhaps coincidentally, or not, some small patches have broken loose from the surface of the topmost step. It may just be age, but I'm also wondering if my attempts to run the pH down at 7.2-7.4 is having an effect on the plaster.