Scientists strengthen concrete by 30 percent with used coffee grounds

Humans produce around 4.4 billion tons of concrete every year. That process consumes around 8 billion tons of sand (out of the 40-50 billion tons used annually) which has, in part, led to acute shortages of the building commodity in recent years. At the same time, we generate about 10 billion kilograms of used coffee grounds over the same span — coffee grounds which a team of researchers from RMIT University in Australia have discovered can be used as a silica substitute in the concrete production process that, in the proper proportions, yields a significantly stronger chemical bond than sand alone. 

“The disposal of organic waste poses an environmental challenge as it emits large amounts of greenhouse gases including methane and carbon dioxide, which contribute to climate change,” lead author of the study, Dr Rajeev Roychand of RMIT's School of Engineering, said in a recent release. He notes that Australia alone produces 75 million kilograms of used coffee grounds each year, most of which ends up in landfills. 

Coffee grounds can't simply be mixed in raw with standard concrete as they won't bind with the other materials due to their organic content, Dr. Roychand explained. In order to make the grounds more compatible, the team experimented with pyrolyzing the materials at 350 and 500 degrees C, then substituting them in for sand in 5, 10, 15 and 20 percentages (by volume) for standard concrete mixtures. 

The team found that at 350 degrees is perfect temperature, producing a "29.3 percent enhancement in the compressive strength of the composite concrete blended with coffee biochar," per the team's study, published in the September issue of Journal of Cleaner Production. "In addition to reducing emissions and making a stronger concrete, we're reducing the impact of continuous mining of natural resources like sand," Dr. Roychand said. 

"The concrete industry has the potential to contribute significantly to increasing the recycling of organic waste such as used coffee," added study co-author Dr Shannon Kilmartin-Lynch, a Vice-Chancellor’s Indigenous Postdoctoral Research Fellow at RMIT. "Our research is in the early stages, but these exciting findings offer an innovative way to greatly reduce the amount of organic waste that goes to landfill,” where its decomposition would generate large amounts of methane, a greenhouse gas 21 times more potent than carbon dioxide.