The low-carbon cement start-up Terra CO2 Technologies has raised $46 million in a series A investment round led by Bill Gates’s Breakthrough Energy Ventures. Terra had previously raised $15 million in seed funding. The firm makes silicate materials that can partially replace Portland cement, the most common binding agent in concrete. Terra says its materials have a carbon footprint 70% smaller than that of Portland cement. Breakthrough recently put $55 million into Brimstone Energy, which is developing a carbon-negative cement.