I wanted to extend a hillside patio area. Hauling in dirt fill by hand was more work than I wanted to do, and it would probably settle over time. Cement was better.
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
I don't drink soda pop, or use vast quantities of Chlorox. Going against the normal flow of things, I went to our city's recycling department and was given bottles that they had collected. I also put the word out locally and helped clean up our barrio of locally generated bottles.