The gravel in regular concrete is heavy and costs money. I decided to replace it with free plastic bottles and pieces of Styrofoam trash. Sand and cement were the only purchased materials.
Although the air cavities from the bottles don't have the compression strength of solid cement, the spaces between them create interior columns, walls and arches which are load bearing. The top layer was more solid, with pieces of foam and cement to help spread out the weight. The patio only needs to support foot traffic, and it does that just fine.
Step 1: Collecting the Trash
Step 2: Basic Structure
Bottles went down first, with richer amounts of Styrofoam toward the top. I already knew that Styrofoam cement makes a pretty non-compressive layer for walking on. That layer helps spread one's weight out over the foam-like bottle and cement layer below. Foot pressure is not concentrated on a small area.
It supports my weight well now, and I am expecting no problems.
Step 3: Mixing the Cement
Mix the sand and cement dry first. Make a hole in the middle of the pile and add water. It is better to add less water at first and creep up on the proper consistency little-by-little. Mix with the shovel, trying to not let water escape the ring of dry material. If you get it too soupy, the bottles will float in it and the cement doesn't behave well as mortar between the bottles. If you make a ball of mortar, it may sag some, but it should not run.
Use rubber gloves to protect your hands. Cement is caustic to skin. Use a trowel for smoothing the cement.