Now the experimentation is more elaborate, designed to tailor the concrete to the need. Increasingly, that need includes the environment. Aesthetic considerations aside, concrete is environmentally ugly. The manufacturing of Portland cement is responsible for about 5 percent of human-caused emissions of the greenhouse gas carbon dioxide. “The new twist over the last 10 years has been to try to avoid materials that generate CO2,” said Kevin A. MacDonald, vice president for engineering services of the Cemstone Products Company.