Christine:

Will your walls be weight bearing, or are you just giving a nice resurfacing?

You need not resurface existing stone with tufa. You can just try something like this:

Dena's how to make a fake rock.

What I would do is create semi-stones (only the parts that will be exposed in the wall, and bond them together when they're done. You can do this by laying them down face up on a melamine box form, and pouring the slurry around the rocks to bond them together. See the following on how to create the form:

Making a flat mold

Now, note, I don't know how this technique would work on a wall-sized project given weight constraints. These are just a couple of ideas being whooped together into something that will hopefully work out for you :-)

Can you post a picture of your retaining walls?

I'm interested too. I've thought about trying a veneer of 'tufa over blue styrofoam cut in a variety of squares and rectangles to face a section of (ugh) vinyl fence. Weight would be a big problem there.

You might look up patterns like "Random Asher" which would allow you to make multiples of a variety of shapes in dimensions that would fit both together and leave no waste if you can figure a way to make thin "stones" for veneer.

I'm thinking a mixture of Portland, sand and vermiculte with a larger proportion of vermiculite than one might typically use would be light weight. It wouldn't be as strong, but for a veneer that might be ok. You could experiment with adding colors and maybe even doing some layers of two colors (or tones or shades of one color) to make some striated "rocks". The FAQ section has various recipes.

Vermiculite has some natural silica stone reflective qualities in some of the pieces. Hope you get lots of suggestions and work something out.

David