Mortar.

I need some rock glue too. To glue a big rock to the side of a cement block. Do they make a mortar in a tube or just the mix up kind in a bag?

They don't make mortar in a tube because mortar is like a 2 part epoxy, once mixed with water the reaction starts and it sets up, even if under water.

A 10 lb bag of mortar is about $2 here.

But of course you can use any glue you like. Just a question of cost, mess, how long it lasts, whether it'll hold the rock on a vertical surface while it cures or needs bracing, etc. Everything is a trade off. But for the past 200 years portland cement mortar has been the king. Nothing today even comes close. DIYers do seem prefer more expensive and harder to use glues, but that's just inexperience.

Rub the mortar on the rock and the wall first to to clean the surface and get a good bond. Don't mix the mortar too wet, should be like mashed potatoes. After mixing let the mortar set for 10-15 minutes and then mix once more and it's ready.

One reason mortar is so popular is something DIYers rarely think about, tear down and repair. Mortar doesn't like vibration, so a few taps with a chisel and off it comes. Glue is forever.

If you use have a good base for suport then, I agree mortar is the best, but I've done well with construction adhesive in the tubes - liquid nails, etc. - which I will use to glue river rock pebbles to a pt wood base to look like a rock foundation for my waterwheel feature - but I establish a solid base then build from the base.

Disclaimer: ZERO experience with this stuff here as far as ponds/rocks go (tho I've used it for other applications). That said, some experienced ponders whom I have come to trust over the years have reported good results. See link below - then do a search for *Great Stuff*, limited to this forum, for additional comments/info..

blessings,
~digger

Here is a link that might be useful: rock glue

In addition there is a marine epoxy that will adhere even when items are wet. Whether it would be appropriate for your needs, I don't know.

I have used Great Stuff foam around rocks, etc for my stream and waterfall and you have to work hard to unstick things, but I don't know that you can apply it when the surfaces are wet.

I see people using "waterfall foam", which appears to be an expanding foam, just in a black color. Never used it. I am going to be assembling my first waterfall in the next week, and I'm going with mortar. I have done lots of brick and rocks (out of water), and there's a reason people have been using mortar for over a milennium. It's easy to use, cheap, fast and it works.