New York Times: It seems like alchemy: a Silicon Valley start-up says it has found a way to capture the carbon dioxide emissions from coal and gas power plants and lock them into cement. If it works on a mass scale, the company, Calera, could turn that carbon into gold. Cement production is a large source of carbon emissions in the United States, and coal-fired electricity plants are the biggest source. As nations around the world press companies to curb their greenhouse-gas emissions, a …