Driving on coffee

The human race consumes around 9 billion kilograms of coffee annually, and demand is rising by about 2.5% each year.

While spent coffee grounds can be used in compost, concerns about coffee’s acid levels inhibiting plant growth means that much of it is simply thrown away. That got one Australian researcher thinking: why not look at this as an engineering material?

Professor Arul Arulrajah from Swinburne University of Technology specialises in recycling construction materials and also happens to be an avid coffee drinker. When he noticed the piles and piles of spent coffee grounds being thrown out by his local cafe, he decided to investigate potential uses for the organic waste.

With the help of PhD candidate Teck-Ang Kua, Professor Arulrajah started collecting used coffee grounds from local cafes, drying them out and testing them as a construction material. You may soon be driving over his results.

Professor Arulrajah’s team from the Centre for Sustainable Infrastructure mixed the dried spent coffee grounds with slag and fly ash, waste products from the steel and coal industries, to create compressed cylindrical blocks strong enough to use as the subgrade material in road construction.

The cylindrical blocks were stringently tested and found to comply with the structural strength requirements for subgrade materials demanded by a variety of road-building authorities.

Professor Arulrajah estimates that “the coffee grounds from Melbourne’s cafes could be used to build 5 km of road per year”. When scaled up to include global consumption, there could be many miles of blacktop built from coffee waste.

The spent coffee grounds used as the base material comprised 70% of the total, with the remaining 30% made up of ground granulated blast furnace slag leftover from steel production and fly ash, a waste byproduct from the combustion of coal in power plants. To bind the three main waste products into a usable construction material, Professor Arulrajah’s team used a mixture of sodium silicate solution and sodium hydroxide solution as an alkaline liquid activator.

The geopolymerisation of spent coffee grounds, slag and fly ash as an alternative cement for use in road construction has the potential to transform the road construction industry through sustainable usage of waste byproducts.

Published in Construction and Building Materials, this research was undertaken in collaboration with Shanghai Jiao Tong University, Nanjing’s Southeast University and Thailand’s Suranaree University of Technology.

Image credit: ©Africa Studio/Dollar Photo Club