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.