ich McEachran writes in the Guardian that, in the process of surfacing a road, layers of asphalt – which is composed mostly of bitumen (a byproduct of crude oil distillation) — are poured over an aggregate of crushed stone and sand. The asphalt acts as a glue, binding the mixture together to form asphalt concrete. It’s not complicated, though maintaining the roads is costly.
Simon Hesp, a professor and chemical engineer at Queen’s University in Ontario, believes standard industry asphalt is not sustainable. “The problem with the composition is that it’s poorly controlled … it uses materials with poor performances.”
Mr. Hesp says the presence of certain oil residues lowers the quality of the concrete and is a key reason why roads are failing and many potholes need to be filled and cracks fixed.
But there’s not just a maintenance cost. Asphalt, dependent as it is on the oil industry, is resource- and energy-intensive, which is why the race is on to develop a greener alternative.
Bruno Bujoli, director of research at the Centre National de la Recherche Scientifique, recently played a key role in developing a bio-asphalt from microalgae. It uses hydrothermal liquefaction to convert waste biomass, including wood and sewage, into bio-crude oil. The chemical composition of the microalgae bio-asphalt differs from petroleum-derived asphalt, but initial tests have concluded that it also bears similar viscous properties and can bind aggregates together efficiently, as well as being able to cope with loads such as vehicles.
Mr. Bujoli suggests that microalgae is a greener and more appropriate solution than agricultural oils. The latter, he says, should be kept for food production. “The benefits of microalgae over other sources include low competition for arable land, high per hectare biomass yields and large harvesting turnovers. There is also the opportunity to recycle wastewater and carbon dioxide as a way of contributing to sustainable development,” he said.
Technology such as this is still in its infancy, said Heather Dylla, director of sustainable engineering at the National Asphalt Pavement Association, a US trade organization for the paving industry. “A lot of interesting work is being done in this area, looking at everything from algae, to swine waste, to byproducts from paper making. It’s worth exploring these alternatives, but we need to be sure they provide equivalent or improved engineering properties. We need to understand how they affect the recyclability of asphalt pavement mixtures,” she says.
Microalgae could yet put the paving industry on the road to a greener future. For now though, there are plenty of challenges – from price to scalability – for Mr. Bujoli and his team to address if the bio-asphalt is to be commercialized.
“This is our research focus for the near future. Our current laboratory equipment works in a batch mode,” said Mr. Bujoli. “Scaling up the process will require the design of a large-volume reactor that can operate under continuous flow conditions.”