Concrete is essentially a mixture of cement, sand, gravel and water. Properly mixed and installed, it is a strong, durable, and pest-and fire-resistant material. It is also a versatile material whose many forms include structural blocks; ornamental imitation stone; precast beams, piles and roof tiles; and poured-in-place reinforced concrete. It is this latter combination of metal bars (primarily steel) and concrete in reinforced concrete that has most shaped the industrial landscape in the form of skyscrapers, bridges, dams and highways. Environmentally, concrete can offer some advantages, because of its durability and its capacity for heat storage in solar buildings. But, as it is conventionally produced and used today, it is a destructive material, primarily because of the energy-intensity of Portland cement, which constitutes about 12 percent of most concrete. For every ton of Portland cement produced, approximately a ton of CO2 is added to the atmosphere. The cement industry is one of the top two manufacturing industry sources of carbon dioxide emissions, in itself generating about 8 percent of the worlds CO2 emissions (Mehta, 1998b; van Oss & Padovani, 2002). The CO2 emissions embodied in an average size North American house is roughly equivalent to 100,000 vehicle miles driven (Shell, 1998). There are other problems, notably in dust generation (in cement production and transport) and water pollution (in the ready-mix concrete industry). Emissions from cement production include sulfur dioxide (SO2) and nitrous oxides (NOx). Cement production is also responsible for an estimated 4 percent of US dioxin emissions. And in the mid-nineties, it was estimated that concrete represented 67 percent by weight and 53 percent by volume of construction and demolition (C&D) waste, with only 5 percent being recycled (Demkin, 1998). These local problems pale in comparison, however, to