Concrete is the world's most abundant building material. Rice is one of the world's most abundant food crops. One group of researchers is putting the two together to try to make concrete more environmentally friendly.
More than 600 million metric tons of rice are grown worldwide each year. After the grain is harvested, though, the rice hulls are mostly considered waste. In many places, farmers burn them, along with the stalks, creating thick smoke that can cause breathing problems. In the United States, the rice hull usually "ends up in landfills or is used as poultry or animal litter," says chemist Rajan Vempati at the ChK Group, an engineering firm. "It has no high-tech application."
But Vempati and his colleagues would like to change that. They want to use these rice hulls to reduce the amount of carbon dioxide created by making concrete.
Making a concrete world greener
The modern world is made of concrete. The mixture of crushed stone and sand is the essential building material for everything from skyscrapers to sidewalks. It's held together with something called Portland cement, and that's where the greenhouse gases come from.
"For every ton of Portland cement we manufacture, we release about a ton of carbon dioxide, which is a greenhouse gas," says Clemson University engineering professor Prasad Rangaraju. "So if we can replace or reduce the use of Portland cement, we would definitely take the right step in reducing the carbon footprint of concrete."
With about five billion cubic meters of concrete produced each year, it adds up to about five percent of the world's man-made carbon dioxide production.
But it turns out that when you take Portland cement and add a bit of ash from rice hulls burned in a controlled process, it makes the cement stronger, so you can use less of it. That means producing less carbon dioxide.
New twist on an old technique
The basic idea goes back to Roman times, says Colin Lobo, senior vice president of engineering with the National Ready Mixed Concrete Association. Lobo says the Romans discovered that mixing in volcanic ash from the slopes of Mount Vesuvius made their cement stronger. Today, he says, some builders use slag left over from iron-making or the ash from coal-fired power plants to do the same thing.
"Our modern-day volcanoes are essentially the power plants," he says.
But inventors Vempati and Rangaraju say the advantage of rice hull ash is that, unlike fly ash or slag, it's a renewable product that doesn't generate any additional carbon dioxide. Plus, they say their process makes a light-colored cement that's better at reflecting sunlight, so buildings made with it cost less to air-condition. And this concrete resists corrosion better.
Their success may depend on cost, however. They developed their process with a grant from the National Science Foundation, and it hasn't been commercialized yet. A different rice hull ash product on the market is relatively expensive. And transporting rice hull ash would add to the cost, says concrete industry expert Colin Lobo.
"But if it's in an area where they have [rice] in abundance," it might make more sense, Lobo says. "For example, they grow a lot of rice in India. So, if they do it in India, and they have a way to turn a product that would usually go into a landfill into a beneficial use, then that would be great."
Vempati and Rangaraju would like to develop a small, portable processing unit that could be taken to the many small farms in countries like India or China, where the ash could be used locally.