Concrete is the most widely used building material for residential and commercial buildings. From its humble origins in Roman times, this mixture of Portland cement, aggregate, water, and chemical additives is now a $35 billion industry in the US alone, employing over two million workers. However, when it comes to greenhouse gases, concrete is believed to be a major culprit. The construction and operation of buildings in the United States accounts for about 40 percent of all greenhouse gas emissions. According to a new study from the Massachusetts Institute of Technology (MIT), certain measures can be taken to drastically reduce and possibly eliminate the carbon footprint of new concrete buildings, and even some older ones.