Professor Theodore Hanein · Lei Zhang · Anna Simmons · Cecilia Pesce (earth4Earth) · Host: Jackie De BurcaRelease

Jackie De Burca spends this episode inside one company. earth4Earth makes bricks out of soil dug from building sites, bound with a lime its co-founder worked out how to make at room temperature — by accident, on a day the lab furnace wouldn't switch on. Four of the people behind it explain what they have built, what the industry says back to them, and why the most interesting question about a brick might be what happens to it at the end.

In 2017, Professor Theodore Hanein went into a laboratory to heat a material and found the furnace broken. He mixed it cold instead, ran the characterisation anyway, and got results good enough to keep. What he had stumbled into was a way of producing lime — one of the most widely used bulk materials on earth — without the roughly 900°C kiln the process has needed since antiquity. He did not publish until 2021, and an attempt to patent it was turned down.

That binder is now in a brick. The bricks are made from soil excavated at construction sites that would otherwise be landfilled, mixed with the lime-based binder, and compressed rather than fired. Because the lime carbonates as the brick sits in the open air, it keeps absorbing carbon dioxide during its service life. Independent verification by EPD Hub found the company's N10 brick to be carbon negative across its full life cycle, absorbing in the region of 0.178 kg of CO₂ per brick — a real number, and one worth knowing the size of before anyone reaches for the phrase "buildings as carbon sinks". At end of life the bricks can be crushed and remade, or returned to soil to grow things in, which concrete cannot be.

The commercial half of the story belongs to Lei Zhang, a structural engineer of eighteen years who was handed a finished design in 2020 and asked to make it net zero. He went looking for a way to do it and concluded the technology did not exist. Later, calculating embodied carbon for a building on HS2, he found a single structure carrying more than 400 tonnes of CO₂ — a figure he had not expected and could not design away. He had read about Part Z, an industry-drafted proposal for whole-life carbon reporting, and assumed regulation was coming. It still has not. The episode carries a short correction on that point: embodied carbon remains unregulated in the UK, which arguably sharpens his argument rather than weakening it.

The most bracing passage comes from Cecilia Pesce, whose background is in the chemistry of conserving cultural heritage. Asked what the past can teach sustainable building, she gives the expected answer — raw earth is an ancient technique, dismissed as rudimentary, and it works — and then volunteers the opposite case. Traditional lime production is badly incompatible with modern carbon targets: you burn fuel to heat limestone, and the limestone itself releases CO₂ as it breaks down. The old ways are not automatically the wise ones, and the company's entire proposition rests on having replaced one of them.

Then there is the catch. Anna Simmons, who handles business support, is candid about the reception: real enthusiasm, and real scepticism, because when something sounds that good people go looking for the flaw. She does not claim the bricks are cheap. Some prospective customers tell her the price is outside their budget; others are pleasantly surprised. In an episode largely made of people describing their own company, that sentence does more for its credibility than any of the carbon figures.

The range that answers the objection: standard bricks, brick slips and frogged bricks, in colours that follow the soil or...