I've never made a pool like you're talking about doing but I'm a floorcovering installer (ceramic, hardwood, carpet, the works) and I build showers (shower beds, etc...).

Tile grout isn't the way to go, it'll crack in a heartbeat because its composition is designed for small areas (grout joints), not large.

A hydraulic portland cement would work and so would a fortified gray thinset. An hpc can be applied thicker than the thinset but for what you're doing all you need is a coating for traction & looks. Thinset can be applied no more than 1/2" without the possibility of it cracking. Depending on the dips & doos of the pool you'll probably only have about 3/8" in the thickest part.

An hpc would probably be a bit stronger overall than thinset. If the cage wasn't going to be moved much after the pool is formed I wouldn't worry too much about it, the thinset should hold up just as good.

If you go with thinset, they make either gray or white. The white is for use with light colored grout to avoid bleed-through, which doesn't concern your goal. Get the "fortified gray." It has the bonding agent already mixed in so you don't have to buy a gallon of it & mix it with water & thinset. (Acrylic polymer, liquid latex, are a few types of bonding agents)

The plain, regular gray thinset costs about $5 per bag, the fortified gray thinset costs about $15. I don't remember what the hpc costs.

Check out these links:
Concret Tank

HPC

Have a good one!
Mike
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It is said that 1 out of every 4 people are mentally unbalanced. Think of your 3 closest friends, if they're normal, then it's you.

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