Calera, a Silicon Valley startup, has found a way to take carbon emissions from coal and place them into cement, an idea that has garnered at least $65 million from investors. It's unclear whether the cement will be durable and carbon-neutral, and scientists such as Ken Caldeira of Stanford University's Carnegie Institution for Science, remain unconvinced. "People have been looking for ways to do this for 15 years," Caldeira said. "The idea that they're going to come up with something that's both economic and scalable? I'm highly skeptical."