Water Gardening: NEED HELP!, 0 by

wrote:
When you make your mortar slurry, you will use part water and part bonding agent to do the mixing. The more bonding agent you use, the thicker the mix is likely to be, but there's no actual rule to how much you add. You want the slurry to be about the consistancy of cake batter or a little bit runnier. Use only enough liquid to make your slurry runny enough to paint on. Too much water will cause it to be weaker after it cures. Before you brush the mix onto your styrofoam, you paint some bonding agent full strength directly onto the styrofoam. Use a big paintbrush. Then, let it get 'tacky', but not dry, and use the same brush to paint your slurry onto the same area. You don't want the bonding agent to dry completely before brushing on your slurry, so do a small area at a time until you are familiar with the materials.
This will start to set up and then you can paint another layer of slurry onto the first before the first layer is totally dry. If you wait until the first layer is totally dry, you'll have to paint more full strength bonding agent onto your structure before painting on more slurry.
Do several layers in order to have a good base to work from. Then, just like you do with hypertufa, I'd spray it lightly with water and cover it with plastic - like a big garbage bag or tarp. Let it cure and keep it moist during the curing process. Keeping it moist will help the mortar create a good hard bond.
I was thinking that if you did this, you would then be able to add more cement to the structure and it would stick. Using the bonding agent as an additive to your concrete mixture, and painting it onto the structure full strength as you add concrete will make the new concrete stick to the old and form a tight bond.

Anna said she used stucco to do the same thing and it worked for her. The mortar mix is a little like stucco but I think has less lime, or something. I use the mortar mix to finish my garden walls (made with recycled concrete) because it makes a nice smooth finish like stucco. I always use the bonding agent in the mixture, and paint it on the wall itself before adding the mortar mix, which I trowel on and then smooth. The bonding agent makes a huge difference in getting the slurry to stick to old concrete or vertical surfaces. Before I 'discovered' it, I was getting mighty frustrated trying to finish my garden walls.

Here's one of my walls before I planted sedums on top of it. The mosaic keeps an area free for sitting. The finish on the wall is mortar mix with the bonding agent and water.

Hope this helps. I've never used this to cover styrofoam, but I don't see why it wouldn't work. YOu could always test it on a smaller piece before taking the plunge. The bonding agent is a little pricey, about 18$ a gallon, but it is well worth it.