I am building a garage, is there a cheap, green substance to take the place of concrete for the floor?
For your garage to be a permanent structure, you will need to dig and pour concrete footings that reach below the frost line. These footings can then support the walls and roof of the garage. The garage floor is independent of this process and does not need to be poured concrete.
Concrete itself is a necessary evil in the construction business. The embodied energy in a truck full of concrete as it arrives on your site is a big contributor to global warming. The energy (usually electric) to superheat limestone to make Portland cement makes concrete a not-very-green material. We need to use substitutes for concrete whenever we can. Here are a couple of ideas that you may want to consider.
I came upon some products recently that I want to try. One is called Grasspave2 and the other is called Gravelpave2. The systems consist of a matrix of plastic cylinders several inches high. This mat is laid over a level surface, filled with gravel and screeded to level. The Gravelpave cylinders keep the gravel stable and provide load bearing ability. This system would result in a level, low maintenance floor for the garage—but the texture may be unsuitable for your needs.
Another possibility to try goes back to the pioneer days when a dirt floor provided a smooth hard surface (even for a living room). For today's applications I would contact a local excavator and ask if he had any resources for a high clay content soil. A truck load of this clay could be spread out, tamped and rolled into a hard durable surface that would provide many years of service. I would put a vapor barrier under the clay to allow it to dry out. In the old days they would refresh the floor by throwing some water on the floor and smoothing it out. You could do the same.