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News — Cement is widely used as a binder to strengthen soil on infra construction sites where soft clay challenges the durability of foundations. However, cement is the world's third largest carbon emitter after the dairy and fossil fuel industries, and pressure is mounting to reduce its use in the construction industry.

Researchers at Aalto University and VTT Technical Research Centre of Finland have a bio-based binder material that can significantly reduce carbon emissions from infrastructure construction.

"Biochar is a carbon-negative material that is a by-product of the biofuel industry. Its use as a binder in ground improvement instead of cement can reduce emissions from binder manufacturing by up to 75% –– without compromising the durability of structures," says Sanandam Bordoloi, Assistant Professor of Geotechnical Engineering at Aalto University.

The researchers aim to go even further, making ground improvement itself carbon-negative by using infrastructure as carbon sinks. In their latest study, the team managed to bind CO2 gas in a stable, solid carbonate form within the cementitious clay layer. The technology has implications for the development of next-generation cementitious building materials for the construction industry.

"Until now, no one had succeeded in binding carbon dioxide in the weak clay that is problematic for construction,” explains Professor Leena Korkiala-Tanttu, Senior Advisor at Aalto. “Our technology could be a paradigm shift in reducing the need for cement — especially for ground improvement applications wherein clay is stabilised with cement. Now, instead of cement, we can turn our attention to biochar and other similar recycled industrial by-products, which work very well both technically and in terms of reducing emissions."

Infrastructure, such as roads, streets, tunnels, bridges and buildings, can only be built where the soil is sufficiently stable. This is achieved by deep stabilisation: a process of ground improvement in which the soil is hardened by means of a binder. Soft clay stratum is common throughout Nordic countries and can also be found in parts of Asia and North and South America, meaning that the biochar binder has broad industrial potential. 

Next, researchers plan to test the carbon sequestration technology in field conditions, with plans to extend the technology to other construction materials. The group also has a patent pending for an innovative method of using clay waste from construction — traditionally landfilled — as raw material for various applications on a construction site itself, such as in non-structural backfill material, rammed earth or noise barriers. 

"The construction industry is often quite conservative, and rightly so, as structures are expected to function for more than 30-100 years. However, radical creativity is needed to reduce emissions from the construction industry,” says Bordoloi. “We invite industry partners to join us in taking our research from the laboratory to the construction sites."

FACT BOX: What is biochar?

Biochar results from heating organic materials at very high heats in an oxygen-limited environment (pyrolysis). The liquid and gaseous products emanate as biofuel and biogas, which is then used for energy generation, while the solid by-product — essentially carbon and ash — is the biochar. A key attribute is that it is carbon negative, and amongst other purposes, it can be used to aerate soil, reduce soil emissions of greenhouse gases and reduce nutrient leaching and soil acidity. 

Despite myriad possibilities, an underdeveloped biochar market leads to large volumes being burned as waste each year, not only squandering the carbon-negative potential of the material, but also generating CO2.