For all its incredibly diverse array of uses, concrete is a problematic material when it comes to disposal at the end of useful life of a building or other structure. However, the addition of wood fibers during its recycling might give a new lease of life to waste material from demolished buildings and civil engineering structures including bridges, according to research from The University of Tokyo.

The team at the University's Institute of Industrial Science has demonstrated that with the appropriate proportions of waste concrete and discarded wood it is possible to make a new material with a bending strength superior to that of the original concrete. Given the initial environmental costs of manufacturing cement and making concrete, methods of recycling waste concrete are an important part of any plans for construction sustainability.

Concrete has been used in construction for decades, it builds bridges and reaches for the sky. Its production uses considerable amounts of energy and generates large volumes of carbon dioxide. There have been efforts to "green" the production process with varying degrees of success. Of course, concrete has two main components - aggregate, which is commonly gravel and crushed stone, sand, and the cement that binds it all together and allows the mixture to be poured into moulds or on surfaces after which it hardens into the tough structural materials with which the world is very familiar.

Li Liang, first author of new work reported by the team at The Sixth International Conference on Construction Materials (ConMat'20), points out that simply reusing aggregate from old concrete does not improve sustainability as it is the production of new cement that is driving climate change emissions. Even reusing the aggregate will require new cement. A new, environmentally friendly approach is now needed to close the circle on concrete economics.

The researchers have now optimized their method by adjusting the mixture proportion, pressure, temperature, pressing duration, and water content to make a new type of concrete. Rigidity comes from highly cross-linked organic polymers in wood, lignin as it fills the gaps in the concrete and functions as a binding agent when waste concrete powder is heated.

"Most of the recycled products we made exhibited better bending strength than that of ordinary concrete," adds senior author Yuya Sakai. "These findings can promote a move toward a greener, more economical construction industry that not only reduces the stores of waste concrete and wood, but also helps address the issue of climate change."

The new material has another putative advantage over conventional recycled concrete - it could be biodegradable, at least in the sense that bio-degrading agents might ultimately be used to break down the wood-containing materials, which might then find another use.

The team suggests that their method might be extended to using other discarded plant matter, rather than wood, to optimize the properties for other different applications.