Here's another way to make concrete a 'greener' material. Instead of using newly manufactured polypropylene or nylon fibres in the mix, you can now get nylon fibers made entirely from recycled used carpeting.
Chopped plastic strands added to concrete prevent or reduce the amount of shrinkage cracking in the initial set. This leads to greater overall strength in the final concrete. In small-scale concrete artwork, fibres make a more workable mix with less separation, easier to apply to vertical surfaces–more clay-like. The fibres improve tensile strength and in some cases can be used to replace or reduce steel reinforcing.
Nycon in Westerly, RI, has developed the technology of harvesting old carpets. This not only saves carpets from going into landfill, but also means fewer petrochemicals are used. Their brand of recycled nylon fibre is called NyconG. One variety called "ConTrol-G-CounterTop" is recommended for applications including castings such as statuary and countertops.