Q: I am building a 900-gallon concrete fishpond and waterfall, and I am confused as to which type of mortar to use and what I can use as an additive to help waterproof the cement. I will be using a poly liner under the concrete prior to pouring, reinforcing with 3/8" rebar and lathe wire for adhesion. I want to form the mix into rock-like formations around the perimeter and the falls.

I would like to use mortar mix as I have used it before in creating rock-like formations, but this is not completely necessary. Any help you could provide would really be appreciated.

A: The polyliner isn't necessary to waterproof the concrete, but if you must, I would suggest using a product like Thoroseal, which can be found in most construction supply stores. It's a cement-based product and does a good job of weatherproofing. You apply at least two coats to the concrete surface. It can be colored also for decorative effect. Since you are from the San Diego area, your concrete will not require air entrainment.

In terms of using a mortar mix for carving the rockwork, that will be fine. A latex additive will help in terms of flexural strength and curing. Controlling the pH of the water in the pond will be important as time goes by so that the cement surfaces will not be leached out.