Calcium scale on new pebblecrete?

[tomfrh]

I pebblecreted our spa a month ago. DIY. Pebblesheen and white cement.
The rest of the pool remains marble plaster.

In that time the pebbles have developed tiny scale deposits on them. Appearance wise it looks like table salt stuck to the pebbles. Like you shook a salt shaker on the pebbles. You can only see it when the spa is drained.

acid burns them off.

there are a few calcium nodules in the marble pool, but none of these regular sprinkley deposits that affecting the pebbles

any hints?


levels
FC 5-10
CC 0
ph 7.8 (I drop it to 7.2 but it comes back quickly)
CYA 50
TA 50-60
Calcium 300
Salt 3500

 

[Texas Splash]

It sounds as though you've confirmed that is calcium. Your numbers aren't bad, in fact they provide a slightly negative CSI (assuming a pool water temp of about 30 C) which would tend to negate the formation of scale. However now we are speaking primarily of the spa. Once that water temp kicks-up (i.e. ~104), along with the pH rise from aeration and new plaster, I suspect it's an easy target for calcium to precipitate. But I wouldn't try to lower the pH any further than about 7.6. Going too low can actually have a negative effect, possibly creating more outgassing/aeration and therefore pH rise. Plus you'll drive the TA even lower with all the acid to the point you have to raise it again and go back & forth.

This is a connected spa to pool yes? @JoyfulNoise, any other thoughts?

 

[tomfrh]

Yes the spa is plumbed with the pool, and overflows via weir.

it has separate returns and drain. They all mix together in general usage


The spa gets hotter than pool, because the spa fills up quickly with the heated water. We’ve had the spa up to 40 degrees (cycling solar thru the spa only) since these pebbles went in.

 

[JoyfulNoise]

You say your did the spa plastering yourself. Can you be more specific on the materials you used? Was the "white cement" designed for swimming pools? Also, what startup method did you use? Did you add any calcium chloride accelerant to the mix or did the cement contain CC as part of the mix? What is the nature of the aggregate you used?

 

[tomfrh]

Blue circle White Portland cement plus sky pebble pebblesheen in 2:1 ratio of pebbles:concrete.

No accelerant calcium based or otherwise.

no particular start up procedure