Hi Sharon,
Some differences between mortar and concrete: mortar is generally a mixture of "cement" (a specific type used for those applications) and sand whereas concrete is mixture of "cement" ( different type), sand and aggregate (think a variety of small stones maybe as large as 3/4" and smaller). Concrete is stronger for sure but I don’t think you are talking about an application where strength is the issue. I’ve enclosed the term “cement” in quotes because there are significant differences between different types of cement (see the Wikipedia URL listed below for a bit more information...it appears to be reasonably correct). For example, two main differences are “hydraulic cement” and “Portland cement”. Hydraulic cement doesn’t have as much early strength, sets up quite rapidly (lower working time - on the order of 5-15 minutes) but will harden underwater. Portland cement is the typical cement used in many industrial, commercial and home applications. It requires several hours to set up and won’t really develop full strength for approximately 30 days.

So what? Some of the types of cement and specially mortars will have more lime (both calcium carbonate - not a problem - and calcium hydroxide - this could become a problem). I would stay away from using mortar since it is likely to create a bit higher pH if you’re not careful; I would use a general purpose Portland cement and mix it with sand to make your bonding material to fill in around the rocks. When laying the rocks and cementing them in place with your cement/sand mix I would do areas you can handle within the set up time...don’t try to do your whole pond at once. I would also recommend that you use as much stone as possible to reduce the amount of exposed cement. For example, you could use wider, flatter stones on the bottom surfaces or for vertical areas; you can build the equivalent of an underwater rock wall. After laying the stones and wiping out as much of the cement mix as possible wait a few days for it to set up. Then you will want to wash it down with an acid to help rinse it off the rock surfaces (this will help stabilize the pH later on). What to use for an acid wash??? This is where I am sure some folks may come out and start yelling that you are dealing with poison; you’re going to kill your fish, etc. I’d recommend muriatic acid. This is nothing more than a 10% solution of hydrochloric acid. If you go to a store selling masonry/stonework products you may see some so-called “environmentally friendly” stone-wash products but if you read the product label you will often find they contain hydrochloric acid. You could use vinegar (acetic acid)...it works, but very slowly and it may be necessary to apply it two or three times to get the same effect as muriatic acid. You’ll want to wear rubber gloves, safety glasses and rubber boots. I’d use a plastic garden sprayer to spray it on - the joints and the surface of the rocks (some of your limestone-containing rocks will foam a bit). Spray the rocks and using one of those plastic scrub brushes on a long handle quickly scrub the surfaces. Immediately rinse with water. And now the thing you must do: dump in baking soda (it may take many pounds so buy BIG bags) to neutralize all the liquid that is now sitting at the low point in your pond (hope your remembered to stopper off the drain!). Once you’ve neutralized it it’s safe to pump out of your pond ... you’ve converted all that acid to sodium chloride...it’s nothing more than a salt solution at this point and is no worse than running a water softener which periodically recharges and dumps the brine solution usually into a septic tank or sewer. You then reapply a solution of dissolved baking soda to the rocks and cement to make sure you’ve neutralized everything. At this point you can fill the pond. Check on the pH over the next couple of weeks and it shouldn’t jump much above 8.5 or so (make sure you buffer it to have plenty of alkalinity). I would move whatever plants you intend to have in there back before I put the fish in ... at this point you’ll dealing with all the typical problems of “new pond syndrome”. Hope some of this helps.
---David
P.S. Sorry it took so long to respond but you know me...it's almost hibernation time!

Here is a link that might be useful: Wikipedia write up on what is Cement

No need for apologies, old friend! I'm a huge hibernator myself.

Thanks for all the great info.

I've already paid for what they call "six inch minus round stone", which sells for a mere $40 per yard. I think the average stone is about 3". Now I'm afraid I've made a mistake, regarding your advice to maximize stone/minimize cement. But the larger, flatter rocks I used for my edging and waterfall cost several times as much, and are not nearly as smooth. I thought the rocks I ordered would mimic the effect achieved by "Waterbug Guy" in the picture below:

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(photo borrowed from http://www.waterbugdesign.com/pond/rock_bottom_pond.html )

Is it a mistake to attempt this with the smaller stones?

It's not a mistake to do it with smaller stones, but I'd suggest a mix of sizes. Put in large ones then insert smaller ones in between and even smaller ones in between those. The more stone you've got showing on the surface the less cement-line you will have exposed. If you look at water bug's layout I can see lots of places extra stones could have been inserted (hind sight is ALWAYS 20:20!!! and in no way should this be taken as criticism of what is done). Of course you'll have to temper that with "time & effort". One could doodle around for hours inserting smaller and smaller stones. Plus the fact the project might look a bit weird if you had all flat stones on the bottom and rounded ones everywhere else. I'd probably pick stones I could set into the cement sufficiently to have it hold on rather than very small ones which might not have sufficient adhesion to hold them in place over time. One question which comes to mind: what is the potential impact of frost heave? Your liner will give with dimensional changes as the soil around the pond freezes... I don't know what will happen with the stones in/on the walls if they are cemented in place. On the other hand concrete/gunite swimming pools will withstand freezing water in them so not sure if this is even an issue for you. The bottom line is: whatever works...it doesn't matter what I think or what anyone else thinks, for that matter.
---David

Good points, David. Thanks. I was thinking I'd use the sizes mixed as you said. It will look nice that way, too, I think. I was also wondering about cracking with frost, and certainly wouldn't attempt this without a liner. But as you said, there are lots of gunite pools around here and they do ok.

It may also be a blessing in disguise that I'm stuck re-doing. This time I've got a person who does roofing laying it in there, and he talked me into 60 mil EPDM and talked me out of laying it over the old liner, because of the risk of dry rot. I think I actually found some of that starting between the old liner and the underlayment -- the stuff sold commercially for the purpose. They were sticking together and the white residue felt sort of gummy. Once it's covered with tons of concrete and rocks, the last thing I want is for the rubber to fail!

One more quick question: When doing the acid wash, do you mean use one of those hose-end sprayers?

Nope, use a plastic pump sprayer.
---David

Oky-doke. Thanks, David.

Alternatively, as we discussed offline, I may follow these instructions:

Increasing pH trends can be minimized by an initial pretreatment or curing of a new concrete pond. Fill the pond with water (no fish), add Muriatic acid (swimming pool acid) as necessary to adjust the pH to about 5. Circulate continuously and test daily, adding additional Muriatic acid to maintain the pH level until no additional acid is needed. This normally takes 2-3 days. After draining, the pretreatment cure is complete and the pond is ready to be filled for use (now you can put in some fish). A properly treated concrete pond will usually reach an equilibrium state where the production of compounds which reduce the alkalinity is matched by the components being leached out of the concrete.

from http://www.aquariumlife.net/articles/pond-care/89.asp

BTW, I didn't mean to overlook your contribution to the discussion, Serenae. I do appreciate it. The author of the Waterbug Designs site referenced above says he doesn't seal his mortared ponds so I'm leaning toward doing without sealer. I am prepared to move the 8-foot kiddie pool and the fish into my basement if necessary, but with about 150 fish (mostly small), I sure hope it won't be! (Yes, this is a BIG lily pond.)

FYI _ I used cement and no seal and it's worked out fine. My pond does tend to stay green which may be the result of high Ph (Guessing, no idea what the Ph is) but the goldfish, plecostamous and others seem to have no issue with it. I did need to use wire screen in the sides to help hold the cement in place as it would tend to slide down the walls (with the rocks) and it helped to give it something to adhere to. I have had no issue with freezing cracks (my cement is 2 - 4 inches thick) but i did slope all the walls outwards the thought being that any ice would be able to lift upwards rather than push outwards.

Poppa

Hey, Poppa, good to see you again. Thanks for the tips. Did you also use cement over rubber liner? I also have nicely sloping walls so am hopeful my cement will stay in place and not crack. So much work ahead!