mixing concrete in a heavy duty plastic bag? Answered
I learned to mix concrete in heavy duty plastic bags (40 in dog food, Fertilizer, even concrete bags).
This size batch would work well for the balloon project. start with water, add concrete and aggregate, then roll and knead the mixture. This can be done with getting your hands dirty. Concrete can be pored, troweled or handed out. When all is used, let's the bag dry, then shake it out to reuse the bag for the next project. Try it, you'll like it?