Brent Constantz's last startup, Skeletal Kinetics, created a bone-fracture cement that costs $200 per gram and helps orthopedic surgeons heal their patients.

His new startup aims to churn out billions of tons of market-price construction cement that Constantz says can help heal planet Earth by embedding billions of tons of greenhouse gases into concrete. And it will deliver desalinated water as a byproduct.

Constantz is founder and CEO of Calera, a 3-year-old Los Gatos-based startup with a demonstration operation at a Moss Landing facility that once helped make incendiary bombs during World War II. His move from the medical device field illustrates how Silicon Valley's dynamic, risk-taking business culture has quickly turned a region best known for computer technology and life sciences into America's top incubator of clean, green innovations.

Reinvention is at the heart of cleantech, and several of the sector's leading entrepreneurs have transformed themselves to pursue these massive new market opportunities.

Elon Musk, the CEO of electric carmaker Tesla Motors and chairman of Solar City, first prospered as a young dot-com mogul whose credits include PayPal. Better Place, a buzz-making electric car services startup in Palo Alto, was founded by Shai Agassi, who previously had been a rising executive at software giant SAP. Bloom Energy, an innovator in fuel cells, is led by K.R. Sridhar, formerly a University of Arizona professor who helped NASA explore the potential of life-sustaining technologies for Mars.

Marc Porat had been a key player at Apple and an e-commerce entrepreneur before launching three green building materials startups — Serious Materials, Zeta Communities and CalStar Products. Kevin Surace, the CEO of Serious Materials, had previously led an e-commerce company.

As Surace tells it, he did not have great expectations when he accepted Porat's invitation in 2002 to build a business around a polymer patent: "It started out as a hobby," he said.

Today, Serious Materials is producing energy-saving glass, window and drywall products at five manufacturing facilities in California, Colorado, Illinois and Pennsylvania. The company has raised more than $120 million in venture capital and won praise from the Obama administration for creating green jobs amid environmental and economic troubles.

Several valley entrepreneurs see potential in reinventing "the built environment." Conventional means of producing cement, bricks and drywall are energy-intensive and give off vast amounts of carbon dioxide, which scientists view as the chief accelerant of climate change. Surace and Porat cite a Department of Energy report that found

the full life-cycle of buildings, roads and bridges — the production of materials, the construction and operation — accounts for 51 percent of the nation's energy use.

Constantz, who is also a consulting professor at Stanford, said it was at Stanford's Woods Institute for the Environment that he first became aware that the cement industry produces 5 percent of the world's carbon dioxide emission, ranking it a third leading cause behind transportation and power plants. Constantz developed a radically different chemical process that eliminates carbon dioxide production.

Then he placed a call to venture capitalist Vinod Khosla, whom he had known since the 1980s, when Khosla was among the founders of Sun Microsystems. Khosla, known for investing in experimental technologies aimed at big markets, quickly embraced the project and has provided an undisclosed amount of funding.

Calera was founded in 2007 to bring Constantz's brainstorm to market. While conventional cement production requires kilns that heat limestone to 1,400 degrees Celsius, Calera's process recycles power plant emissions, scrubbing the carbon dioxide with alkaline water to create a raw material for cement. The result, Constantz says, is a "negative carbon" product because it both cleanses power plant emissions and eliminates carbon dioxide in cement production.

At its demo operation beside Dynegy's natural gas-burning power plant at Moss Landing, Calera is fed by two old pipelines with seawater and a new pipeline from the power plant that redirects 10 percent of its flue gases. (The goal is to eventually use it all.) The chemical process, which Constantz says is akin to converting milk into powdered milk, produces cement powder. The byproduct of desalinated water is sold to the Pajaro Valley Water Management Agency or returned to the ocean.

Calera also has pilot projects in Australia, using underground brine water, and in Dubai, using seawater. In those locales, Constantz said, the water byproduct is a much-coveted bonus. Calera has inked a partnership with Bechtel, the San Francisco-based construction giant, to bring the technology to global market. Power plant operators facing pressure to curb carbon emissions may do so by getting into the cement business.

Calera also spread the word by delivering a set of benches made from its green concrete to the plaza outside the recent Copenhagen climate summit. Viewed from above, the benches spell: HOPE.

Contact Scott Duke Harris at 408-920-2704.