Concrete is a time-tested construction standby, but the production of the key ingredient, cement, creates a massive amount of CO2 emissions.
Cement manufacturers in recent years have begun to offset cement's high electric and coal-fired consumption with other, bio-based sources. Plants are using from 5% to 20% alternative fuels, for example.
Some cement makers are innovating production to make it more sustainable. For instance, Calera has developed a process to collect 50% of the CO2 emissions from the cement-production process and use it to produce a new type of cement.
Some manufacturers have replaced some of the limestone in the cement mix with supplementary cementitious materials such as fly ash produced from burning coal and unheated limestone. Reducing the amount of heated limestone in the cement reduces the amount of energy needed in production.