Tricalcium phosphate?

[Joekovar]

I made my own structure with a skeleton of PVC pipe and pink high density insulation foam. This was coated in layers of Portland cement and aragonite. Coating was done with very little water where I sprayed it with water, dusted it with cement until it stopped changing color, sprayed it, coated it in aragonite, sprayed it, dusted with cement, and repeated until my coating was about 1/4" thick. It's thicker near bases of columns where the extra material from dusting collected.

I continued to mist it with tap water and keep it moist for the weekend. I misted it daily for about a week after that until it stayed fairly light in color after misting.

I then put it in the tank and filled with tap water, expecting to wait a decent amount of time for phosphate to leech out of the cement. It's been submerged for a few weeks now. (No salt) There's good flow in the tank.

The pH has swung between 8.5 and 9.5, recently the swing has narrowed to 8.5-9.0. I add roughly 3/4 gallon of tap water and 20-25 ml of typical white vinegar daily.

I've started to get this white buildup on the structure, which inadvertently has helped me visualize the flow in my tank when I brush it away and particles move around the tank. They swirl around for awhile, then go away. Hopefully getting filtered out by the polyester filter media below.

I'm guessing it's tricalcium phosphate from the tap water and phosphate leeching from the cement, but I'm not really sure.

 

[Randy Holmes-Farley Verified Community Expert]

If seawater is the liquid, I’d guess it is at least partly calcium carbonate. Might be calcium phosphate as well, or even mixed together.

 

[Joekovar]

I tested for phosphate in a water sample from the top of the tank, then scrubbed off some of the growth on the bottom and sampled that (scrubs off itself easy, much harder to scrub off the cement structure) and there's no noticable difference in the reacted water samples colors.

I'm second guessing it being phosphate-ish.

Still haven't added salt yet, I'm at 3 weeks with fresh water this weekend. A friend spooked me about the cement and sand leeching phosphate so I'm taking things slow.

Going to do some more research on chemical reactions and testing this weekend.

 

[Randy Holmes-Farley Verified Community Expert]

if you are only using fresh water, the solids must be entirely leaching from the rock. I’d just manually remove it.

 

[Joekovar]

Hitting it with a brush breaks it up and it seems to get caught by the polyester quilt stuffing (no fire retardant, mold prohibitors, etc) I have below.

I added salt last night as the pH has been stable for a few days, and that knocked the pH down a point from 8.9 to 7.9.

I've noticed the growth has been concentrated where dry cement piled up as I was coating and misting it, which leads me to believe that it's a product of unhydrated cement becoming hydrated.

The area's I was able to control the coating better have no such growth.