Foam / Cement cave pt. 1

The 1st coat is finished. Everything with the cement work has gone relatively well. The cement dries so slowly that I can work at a snails pace. I sprayed it with water every few hours to keep it from cracking. It’s cured for 24 hours now.
I think I used a good cement/sand mix, which again I used 2 cups cement,1 cup sand, 1 cup water, 1 Tbls cement color. The cave seems real strong and sturdy; as the bare foam cave was quite weak.
I still have one problem though; the cave still floats in the water. I put a 5 # weight on it and it floated a little, put a 10# weight on it and it sunk. Also, after doing a little research on density/buoyancy of cement and foam, I calculated that I’ll need about 13#’s of cement to sink the cave. So far, the cave weights 7.8 #’s.

Attached is what the outside looks like after the 1st coat.
 
Cave with no foam, just cement

I had some extra cement after the 2nd inside coat so I decided to try the idea that somefinnfishy had, to make a cave without using foam as the base. Although the Quickwall cement is reinforced with glass fibers, it might not have the tensile strength to hold together. The cave I made is quite small, so it might work.

In the attachment is a pic of the small cave using no foam.
 
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I built another small cave, without using foam as the base. I let it dry for 16 hours, and then part of it did break off as I was smoothing out the rough parts. I think I can silicone the broken piece back on though. Its definitely not as strong as I would have liked, but it shouldn't matter much after I fix it.

Attached is a pic of how it looked with the balloons.
 
Attached is another pic of this cave.

I'll probably build alot more small caves without foam, seeing if I can build one thats real nice. I'd like an elaborate cave that is 2 or 3 stories high and has multiple linking pieces.
 
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Attached is a current photo of the Cement Cave with Foam. The 2nd coat is finished. It now weights 12.4 #’s and it still floats. Its still needs about 2 #'s of cement before it will sink.
 
I read at Garf.org in the "Making sand molded AragocreteTM live rocks" news article, that they soaked their live rocks in white vinagar for 12 hours and then rinsed them in fresh water before using them.
I wonder why they do this? Does this lower the ph properties of the rock or does it just kill off bacteria?

One thing I worry about is that these cement caves will up my ph alot. Don't want that at all. If I want to play it safe, I figure I'll have to soak them in water for about 6 weeks before I can use them.
 
vinegar maybe to speed up the cureing prosess.In SW they add extra stuff to drive up the PH
My plan is to let it sit in SW in a kiddie pool in my basement for 6 to 9 months then cycle it out.
 
I’m done cementing the foam cave. I finally got it to sink. It took 4 or 5 or 6 coats, I lost count. It now weights about 20#’s. Next time I try this I will use much less foam. Right now all my caves are curing underwater in a Rubbermaid storage container. Here’s a pic of all my caves. (Hard to fit them all in one pic.)
 
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