Roads, bridges, buildings, runways, homes, dams, canals and more are all built with concrete. The coarse, gray building material has been so ubiquitous throughout history that even the nearly 2,000-year-old Roman Colosseum was constructed with an ancient concoction of concrete. Despite all its benefits of strength and durability, there is a downside. Production of cement, primary ingredient in concrete, is responsible for a whopping 5 percent of human-generated carbon dioxide emissions. The good news: Some industry newcomers are creating cleaner versions of the versatile building material.