The Challenge
Before the industrial revolution, the amount of carbon dioxide in the air was 280 parts per million (ppm). Nowadays, it's 420 ppm—a level that hasn't been seen in over three million years. Back then, the temperature was about 3 degrees Celsius higher than it is now. The concentration of carbon dioxide in the atmosphere continues to rise.
Limiting global warming to 1.5°C (we are currently at 1.1°C) requires not just urgent decarbonisation of the global economy, but also permanent removal of 20-660 billion tonnes of carbon dioxide from the atmosphere by 21002.
What is wathering?
Weathering is the process by which rocks and minerals break down at the Earth’s surface. It is a natural process that acts over millions of years to help regulate the planet's temperature. When carbon dioxide levels are high in the atmosphere, rainfall dissolves this gas to form carbonic acid (H2CO3), which contributes to weathering.
Weathering is the process by which rocks and minerals break down at the Earth’s surface. It is a natural process that acts over millions of years to help regulate the planet's temperature. When carbon dioxide levels are high in the atmosphere, rainfall dissolves this gas to form carbonic acid (H2CO3), which contributes to weathering.
Although weathering helps to regulate the Earth's climate over long timescales, it's not fast enough to offset the excess carbon dioxide caused by human activities. Currently, natural weathering processes capture only about 3% of annual anthropogenic carbon dioxide emissions - equivalent to about 870 million tonnes of CO2 per year. This means that much of the carbon dioxide we emit into the atmosphere remains there, contributing to global warming and climate change.
Enhanced weathering
Enhanced weathering is a strategy that accelerates the natural process of weathering to remove atmospheric CO2 and mitigate the climate crisis. The method involves crushing rocks and minerals that are rich in calcium and magnesium, which increases their surface area. The crushed minerals are applied to agricultural soils, reacting with carbon dioxide at higher rates due to the soil's concentration of CO2, which is up to 10 times higher than in the atmosphere.
Enhanced weathering materials with abundant calcium or magnesium cations (Ca2+, Mg2+) have a high capacity for removing carbon. Carbon removal can be significantly accelerated by increasing the amount of these materials available to react with carbonic acid (rainfall), allowing for carbon to be removed in a matter of decades, rather than over millennia.
Why concrete?
Crushed returned concrete is an ideal enhanced weathering material because it:
has a high CO2 capturing capacity
reacts quickly with carbonic acid, and
is widely distributed which minimises transport costs and energy
The composition of concrete varies depending on how it is made, but it always contains two principal components: cement and aggregate. Cement, the liquid part of concrete, contains high calcium concentrations that are primarily hosted in fast-weathering minerals such as portlandite (reaction 1) and amorphous calcium silicates – perfect for rapid carbon drawdown.
The aggregate (chunks of rock) that makes up the rest of the concrete is determined by the local rock sources. Carbonate rocks (limestone) and silicate rocks, like basalt, are commonly used.
Just like cement, both limestone (reaction 2) and basalt will remove atmospheric carbon dioxide when weathered, although their carbon removal capacities and weathering rates differ.
As weathering gradually removes carbon dioxide from the atmosphere, it helps to lower the Earth's temperature. But there's an important aspect to this process: a negative feedback loop that acts like a thermostat. This means that when there is excess CO2 in the atmosphere (which can happen during warmer conditions), weathering reactions increase to draw down more atmospheric CO2, eventually restoring equilibrium. In other words, the process of weathering acts as a self-regulating mechanism, helping to stabilize the Earth's climate over long periods of time.
1. Cement (portlandite weathering)
2H2CO3 + Ca(OH)2 → Ca2+ + 2HCO3- + 2H2O
2. Limestone aggregate (calcium carbonate weathering)
H2CO3 + CaCO3 → Ca2+ + 2HCO3
3. Silicate aggregate (e.g. olivine in basalt)
4H2CO3 + Mg2SiO4 → 2Mg2+ + 4HCO3 + H4SiO4
Our process
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Our measurement approach
To show and measure carbon removal, we conduct rigorous geochemical measurements. Our measurement protocol involves three independent geochemical measurements to constrain weathering rates and carbon removal at the site of concrete application.
By tracking carbon through the soil, soil-water, and gas phases, we are gaining a comprehensive understanding of how our material performs across a range of different environments. The data we derive from these detailed studies will allow us both to quantify our carbon drawdown and optimise for carbon removal at scale.
Soil
By tracking soil chemistry over time, we can monitor the breakdown of our concrete amendment. We can determine the initial amount of calcium added to the soil and measure how much is lost due to weathering, which indicates the speed of carbon drawdown. The faster the weathering, the quicker the carbon drawdown.We also monitor changes in soil pH over time, which is a major co-benefit of enhanced weathering and an important indicator of weathering progress.
Soil-water
Changes in soil-water chemistry are the most reliable indicator of carbon removal because bicarbonate ion (HCO3-), the main carbon storage molecule, is soluble in water and is the product of carbon removal. Higher bicarbonate concentrations in the soil waters at our amended sites relative to our control plots provide a clear signal that we are removing carbon dioxide from the atmosphere.The absence of increased bicarbonate in our water samples could indicate the presence of other, undesired, weathering acids in the soil.
Greenhouse gas
We can validate carbon removal independently by measuring the reduction of CO2 gas flux from soil to the atmosphere. This measurement is not affected by other indicators and provides external validation of our enhanced weathering approach. Our expectation is that amended sites will show a decrease in soil-air CO2 flux compared to control sites, consistent with increased dissolved bicarbonate levels in the soil-water.
By increasing the pH level of the soil, it is possible to decrease the emission of N2O, which is a potent greenhouse gas. In addition to monitoring CO2 levels, WE also tracks N2O emissions to gain a comprehensive understanding of greenhouse gas fluxes from the soil to the atmosphere, providing independent verification of the other measurements used to assess carbon removal.
Want to learn more? Dig into some of the key literature below.
The Literature
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