Researchers have demonstrated a laboratory method for making concrete blocks from crushed demolition concrete and a calcium bicarbonate solution linked to carbon dioxide. The process, called Calcium Carbonate Circulation System for Construction (C4S), forms calcium carbonate that binds the particles. It is a promising research concept—not a commercially available, code-approved brick or a proven way to build a home.
How can demolished concrete become a new block?
C4S is based on using calcium recovered from demolished concrete or other calcium-containing industrial waste alongside bicarbonate derived from carbon dioxide. The calcium and bicarbonate form calcium carbonate, which serves as the binder. In the original 2021 concept, that binder could hold natural-rock aggregate or crushed demolished concrete together. The concept paper describes the approach as an alternative way to make concrete, rather than simply adding recycled aggregate to conventional cement.
The distinction matters: ordinary recycled-aggregate concrete reuses demolition material as aggregate, while C4S proposes calcium carbonate formed from recovered calcium and bicarbonate as the binding material.
What happens in the reported block-making process?
A 2024 laboratory study describes crushing waste concrete, carbonating the fine particles, adding a calcium bicarbonate solution, applying pressure, and drying the resulting block. The researchers attribute hardening to calcium carbonate forming at contact points between the fine particles, a process they describe as cold sintering. The study reports that higher-temperature conditions, longer loading periods, and use of the bicarbonate solution produced higher strength in its experiments.
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Those findings describe experimental conditions and results; they are not a building-product specification or independent certification for structural use. The paper also describes a possible reuse route in which blocks are crushed and repressurized, but that is a proposed method, not evidence of a commercially proven closed-loop system.
Does “made from air” mean the blocks capture CO₂ directly?
Not necessarily. The 2021 C4S concept says its bicarbonate can come from CO₂ collected from air or from industrial emissions. The 2024 block-study abstract does not establish that the CO₂ used for the particular reported block batch came from direct ambient-air capture. “Made from air” is therefore an oversimplification: carbon dioxide is part of the proposed chemistry, but the source of the CO₂ can vary and is not specified for that experimental batch.
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Why reuse demolition concrete?
Construction and demolition debris is a substantial waste stream, but the available U.S. figures are historical rather than a current global measure. The U.S. Environmental Protection Agency’s 2018 estimate puts construction and demolition debris at 600 million tons. More than 90 percent came from demolition; just over 455 million tons went to a next use and just under 145 million tons to landfills. Aggregate was the main next use reported.
EPA notes that this debris can include concrete, bricks, metals, glass, and other materials, and that concrete and rubble are often recycled as aggregate or in new concrete products. That broader recycling context does not mean those products use C4S or have the same performance as the experimental blocks.
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Are C4S blocks ready for a house?
The published evidence establishes a proposed process and laboratory block experiments. It does not establish commercial-scale production, retail availability, building-code approval, or a completed inhabited building made from these blocks. BGR reported in August 2024 that researchers aimed to build a two-story home using the material by 2030; that was a future target, not confirmation that the home was built or approved. BGR’s report identifies the C4S framing behind the “demolished buildings and air” description.
For construction professionals, the next practical questions are not only compressive strength, but also repeatability, durability, water and weather performance, production energy, emissions across the full lifecycle, and compliance with applicable structural and material standards. The cited papers do not provide a complete lifecycle comparison with conventional concrete or a commercial product specification, so they cannot establish that C4S is lower-carbon or suitable for a particular project.
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