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We started our field experiments in 2021 to answer the question: ‘can surplus concrete capture carbon if we weather it in fields?’ An unlikely pairing of a farmer turned carbon finance devotee, a low-temperature geochemist with some spare lysimeters, and two farmers willing to lend us a field led to our first - and we believe the first ever - enhanced weathering field trial using returned concrete. About 1-4% of the 20-30 billion tonnes of concrete made each year is returned unused to where it was produced due to, among other things, over ordering. The early results were so promising, which are shortly to be published, that our weekend carbon removal pursuits became much bigger than a research question. We created Silicate - the world’s first carbon removal company leveraging the massive potential of surplus concrete to remove excess CO2 from the atmosphere.
I’ve always been crazy about agriculture. While studying marine biology at the University of St Andrews I worked part time on large arable farms in Fife - that was my first time working with big machinery, having spent my school summer holidays on my uncle’s farm in Donegal driving small Massey Fergusons. I spent the three years following my undergraduate degree harvesting potatoes in Essex, lambing sheep in Scotland, harvesting and planting cereals in Western Australia, and trying to corral thousands of sheep in Tasmania for shearing (see picture below). Farming was something I could not shake. But neither was my awareness of climate change.
When I came back to Europe in 2018, I decided it was climate or bust. Animal welfare and an awareness of our individual impact on the planet made me vegan long before, and carbon finance sounded like a great place to start my climate career, so I went to Edinburgh. The MSc Carbon Finance programme (now Masters in Climate Change Finance and Investment) was a seminal juncture in my journey to carbon removal. Understanding carbon accounting and additionality from first principles has been a vital grounding for me as we develop our methodology and scrutinise the economics of our process. Consequential carbon accounting, which is the system-wide assessment of impacts caused by an intervention, and the approach all carbon removal projects should be using (!), can be a tricky thing to understand in comparison to the seemingly more digestible attributional approach - stacking emissions sources and sinks. (More on this in a later blog!)
From there, it was a fortuitous reading of two papers (Beerling et al., 2020 and Renforth, 2019 - for a deep dive on the literature, check out our science page's reading list), and meeting a lot of very encouraging, generous and willing people, like the amazing and inimitable Professor Frank McDermott, that led me to Silicate, and to making carbon removal my resolute focus.
As a low-temperature geochemist based in University College Dublin, I’ve long been interested in the process by which chemical weathering acts as a thermostat for Earth’s climate over long geological timescales. We are currently in a climate emergency - Earth’s atmosphere has more CO2 than at any time in the past three million years, and I am interested in how weathering processes can be accelerated to draw down some of this excess anthropogenic CO2 . Previously I have carried out field and laboratory trials using basalt, but an exciting finding from my ongoing research collaboration with Silicate is that crushed returned concrete gives a much clearer signal of carbon capture (as dissolved bicarbonate) than any of the other materials I’ve investigated previously. My collaboration with Silicate has given me the opportunity to carry out multiple, well-replicated field trials and access to working farms and infrastructure on a scale that would be very difficult, if not impossible, to achieve without industry collaboration. Enhanced weathering is a complex biogeochemical process. I find it intellectually stimulating to combine multiple new field, laboratory and modelling approaches to provide robust estimates of weathering rates that will enable us to optimise the process for CO2 removal. Ultimately, my goal is to publish the results of the ongoing field trials in peer reviewed scientific journals with colleagues in UCD and Silicate.
Concrete is amazing at capturing CO2. Why? Because of its congruent dissolution! Easily weathered calcium- and magnesium-rich carbonate and silicate rocks are often used as the aggregate component in concrete mixes. But what really makes it a superstar material for terrestrial enhanced weathering is the cementitious part. Cement manufacturing involves driving off CO2 from limestone (CaCO3) to form CaO. About 8% of global CO2 emissions come from the heating and calcination of limestone to make cement. (For the record, we are completely behind decarbonising cement manufacturing, technologies like Sublime and Brimstone seem very promising, and would be fully compatible with our solution.) The CaO becomes hydrated to CaOH when it is mixed with water, sand and aggregate to form concrete, and over time it tries to return to its original CaCO3 state. We help it on its way by milling it (increasing surface area) and putting it in a carbon dioxide-rich environment (soil often has 10 times as much CO2 as the atmosphere), so that it can capture lots more CO2 than it would have been able to without our intervention. Critically, as well as re-carbonating CaOH, we are also weathering the silicate, hydroxide and carbonate minerals in our material, meaning our removal process has two distinct steps, and huge - measurable - carbon removal potential.
Secondly, concrete is everywhere. The key bottleneck for any terrestrial enhanced weathering process is transporting the material to the field. Milling/crushing the material is actually pretty efficient and can be done using electricity, but transporting it to farms means diesel trucks carrying a maximum of ~30 tonnes of material per load. If transport distances can be minimised, net carbon removal can be increased. Concrete is hands down the best material for proximity to farms in enhanced weathering. Another reason why we ❤️ concrete.
And, finally, concrete is safe and quick to weather. Counterintuitively, the white/grey man-made material is actually very good for the soil - it can improve crop productivity through optimising soil pH, enhance plants’ natural resistance to herbivory through the release of bioavailable silica, and it can provide vital nutrients to the soil. Most importantly, our analyses have shown that its heavy metal content is well within the regulations for ground limestone and is perfectly safe to apply to agricultural land. It also weathers in about 1/20th the time of basalt, so, we can measure what is going on in our fields more easily than if we were using basalt. Which is the topic of blog #2 - stay tuned!
