A new type of porous concrete that provides plants a place to grow could help promote urban farming, from indoor crop fields to rooftop gardens, and even allow vegetation to spill down the walls of buildings. Scientists at McGill University in Montreal who developed the new concrete believe it is a step toward a construction material that doubles as a substrate for vegetables  and other crops, as well as ornamental plants.

Hydroponic gardeners already have materials that support plant growth. The most popular is Rockwool, a spun fiber widely used as insulation that’s made of limestone and diabase, a type of igneous rock. Rockwool, however, is expensive and not recyclable. Concrete, on the other hand, is affordable and has good mechanical properties. It can be broken up and recycled to make new concrete.

The porous concrete project was a collaboration between bioresource engineer Mark Lefsrud of McGill University and a research group at CEMEX, an international building materials manufacturer based near Monterrey, Mexico. “They want to make huge [non-bearing] structures out of it. You can cover roofs, or make paneling on a building,” says Yasmeen Hitti, a McGill graduate student in bioresource engineering who did the research for her master’s thesis and reported the results in Agricultural Science and Technology, published by the ACS Publications, a division of the American Chemical Society.

First, they had to show they could make concrete cube prototypes that would provide a suitable environment for plant growth. Concrete is an agglomeration of rocks, known as aggregate, held together with a binder, most often Portland cement. Standard concrete reaches pH levels of 13.5 during curing due to reactions of alkaline metals (calcium, sodium, and potassium) in the cement, and it remains highly alkaline over its lifetime. Most plants, in contrast, prefer slightly acidic or neutral conditions, with pH levels between 5 and 7.

To lower pH levels, the researchers replaced the Portland cement with aluminosilicate binders made of slag from copper and steel blast furnaces. This lowered the pH to roughly 9.5. The nutrient solutions fed to the plants were slightly acidic, and they lowered the pH further, to between 7.7 and 10 for the standard solution and to between 4.6 and 9.1 for the 5x concentrated solution. “It wasn't a perfect match at first, but we did grow healthier plants,” Hitti says.

The team chose quartz aggregates between 2 mm and 3.2 mm, which created a continuous network of pores that account for about 30% by volume of the concrete cube prototypes. This gave the concrete the right level of porosity to wick water and nutrients through it and allow plant roots to spread.

Researchers tested the growth of tomatoes, radishes, and romaine lettuce in the porous cubes using three different concentrations of nutrient solution, and they compared the results to the same plants grown in Rockwool with a standard solution. None of the concrete-based plants grew as large as the controls. The radish did the best with a two-fold concentration, reaching 85% of the control mass. Hitti tried the radish. “It didn’t taste any different,” she confirms.

Further research could improve the porous concrete as a growing material, Hitti says. And with scientists projecting that 9.5 billion people will be living in cities by mid-century, food that grows near their homes could become more essential than ever.

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