I have been doing some experiments:
https://postimg.cc/gallery/kVS5zrbThe single layer cement/landscape fabric part is flexible and will probably crack if bent too far. Not very useful by itself. I believe it will be waterproof or at least water resistant.
Three layers of cement/fabric are still somewhat flexible.
Cement/fabric adheres well to foam. At a fabric edge, you can lift it and pull it up with some force to separate the foam and fabric, but if you cover the fabric edges with another layer of fabric, it is not easy to separate.
One layer of cement fabric leaves exposed edges which are vulnerable to separation. The resulting part is stiff and lightweight.
A wrapped piece of foam is stiff, strong and reasonably lightweight. The part in the pictures was wrapped with fabric/cement, then wrapped in plastic, and a weight was put on top of it. The result was an uneven surface which had too much cement on it. A better result would be to do one side at a time and air dry it. Using a squeegee or some type of scraper to limit the amount of cement slurry on the fabric would result in a lighter composite, smoother surface and more visually appealing result.
About 1 cup of portland cement mixed with 1/2 cup water makes a thick slurry which will saturate about 4 square feet of landscape fabric. One square foot of landscape fabric saturated with cement slurry and cured on top of plastic weighs about 70 grams. I also tried adding 1 or 2 teaspoons of latex additive, but did not notice a great deal of difference in the result. The additive fabric/cement sheets were slightly more flexible than sheets without the latex additive.
Recommendations for further experimentation:
Try landscape fabric as a replacement for canvas in a foamie build using paint in the "normal" way.
Add portland cement slurry to a water based latex paint and use it as the foam to fabric adhesive and fabric covering. Try about 1 part paint to 1 part cement slurry.
Make larger foam panels with cement/fabric sheathing, with wrapped edges. Put one layer of fabric/cement on one side of the foam, let it cure with the fabric side up to the air (no plastic), then turn the foam over and repeat on the other side. After both sides are cured, take strips of fabric/cement and wrap them around the edges.
Conclusion:
I don't know enough to compare this method to the paint and canvas method. It is a very low cost way to cover foam. It should be very UV resistant. For reasons of appearance, it may need a covering of paint. The resulting foam/fabric/cement composite is stiff and lightweight.