Green Cement?

A California entrepreneur wants to start turning straw into gold by bubbling power-plant fumes through seawater. Industry sources have written about it since 2007, but this month the the story reached Scientific American and, in a less rigorous treatment, the San Francisco Chronicle yesterday.

The process is designed to turn carbon dioxide from the power plant into carbonate in the seawater, which then combines with magnesium and calcium in the seawater to make unspecified carbonate compounds. But those compounds aren't cement, and they can't be turned into cement without driving off at least some of the carbon. So something very important is missing, because the inventor of the process, Stanford prof Brent Constantz, claims that his process pulls half its weight worth of CO₂ from the atmosphere. More precisely, half of the product's mass would be carbon and oxygen in 1-to-2 proportion. Such a material is not cement; it's filler.

Other blogs have written about this, and nobody understands what Constantz is claiming. Add me to them. But Constantz is not a lightweight and deeply understands carbonates as well as how living things deal with them. He may have a secret biocatalyst in the patent-pending process. His firm, Calera, is apparently named after the local Paleocene limestone unit, which crops out in Morgan Hill and Daly City and elsewhere in the Santa Cruz Mountains.

I see a possible waste problem. Bubbling clean flue gas through seawater won't exactly pollute it, but it will charge the water with carbonic acid. This is a major, underappreciated risk associated with CO₂ emissions generally—high CO₂ upsets seawater chemistry so that corals and shellfish and plankton can't build shells. Returning large quantities of carbonated seawater to the ocean will basically bypass the atmosphere and mainline CO₂ directly into the planet's bloodstream. On the other hand, reporters say that heat in the flue gases dries up the water. That's a LOT of water—and what about the salt? Salt is death to concrete.

Basics of Cement and Concrete