Concrete Solutions
Research from X suggests revenue positive method to sequester carbon via concrete recycling
For the last few years, my team and I have been exploring new ways to recycle and process resources like metals and concrete. Concrete produces a staggering amount of excess waste per year—3.4 billion tons. Much of this goes directly to landfills; the remainder is “downcycled” into recycled concrete aggregate (RCA), which is restricted to low-strength applications like sidewalks and drainage layers. A key challenge in recycling concrete is removing mortar—a porous layer that weakens the material and makes it unsuitable for structural reuse—in a low-energy, low-cost way.
As we shared in a paper published earlier this year, our team developed a breakthrough method to separate mortar from RCA using hydrochloric acid and a specialized, slow-moving drum reactor. The result is “upcycled” concrete aggregate that can be used for structural purposes, like bridges or buildings.
This shows how acid produced from an electrochemical ocean alkalinity enhancement processes can be used for concrete upcycling, and how these waste streams can become revenue streams.
Rather than spending precious time and resources creating and refining our own acid, we collaborated with Ebb to use their acid byproduct. Founded by two X alumni, Ebb is harnessing the power of the ocean to store excess carbon dioxide (CO2) from the air. Their technology adds alkalinity to seawater via an electrochemical process, allowing the ocean to absorb more CO2 while counteracting ocean acidification. While electrochemical ocean alkalinity enhancement (E-OAE) is a promising carbon removal technology, it generates an acid stream that needs to be processed safely and economically.
Turning waste streams into a revenue stream
As we outline in a new paper published today, there is a significant economic opportunity for carbon sequestration companies like Ebb to pair E-OAE with the concrete upcycling process and transform two waste streams into a revenue stream. For example, an E-OAE company could be paid by a demolition company to take their excess material. Then, after combining the concrete waste and acid byproduct to create the new, stronger aggregate, they could sell it to ready-mix concrete companies and aggregate suppliers. This approach has the potential to support approximately 100 million tonnes of CO2 removal annually via ocean alkalinity enhancement, while advancing the circular economy in construction materials.
Our team developed a range of prototypes to find the most efficient, low-cost way to remove mortar from recycled concrete aggregate so it can be re-used for structural purposes. Our initial prototype column reactor design (on the left) was a packed bed acid-aggregate reactor, which we used to understand how the acid reacted with the mortar. This resulted in the simplified, more efficient slow rotating drum reactor design (on the right).
Our research presents an exciting opportunity to remove carbon dioxide from the atmosphere while also diverting gigatons of construction and demolition waste from landfills. It also outlines a rare situation where waste creates a revenue opportunity. We hope that the deep-dive analysis and parameters we discuss in the paper, and which we’ve made publicly available in The Concrete Collection, provide the industry with a blueprint for harvesting the value from these untapped waste streams.