Listening notes: zero carbon cement on the Volts podcast

I listen to quite a few podcasts, and as an experiment in Jan 2024, I’m going to try to share the notes from ones that stand out for me. This is partly as a way to retain what I learn, and share with others, but get better at writing quickly. The first one is an episode all about of all things, Zero Carbon Cement, by David Roberts in his Volts podcast where he interviews Leah Ellis, of Sublime Systems.

Why write about cement?

In my line of work, I increasingly write about infrastructure – stuff that most of us don’t really notice or pay attention to, but nevertheless shapes our society. Cement is roughly responsible for about 8% of annual global emissions, so 3-5x the digital sector, and most of the time, we don’t think about it.

Where does this massive carbon footprint come from?

Unlike digital, where almost all of the emissions come from energy used to geneerate electricity in some process, the main ways of making cement, even if you had entirely zero carbon heat still emit carbon dioxide. To understand why this is, you need to understand how cement is made, and confront a little bit of chemistry.

As a media graduate who graded terribly in my high school when doing my chemistry A-levels, this took a bit of reading up on, but hopefully this summary should be accessible.

Cement is the binding agent that once water is added, hardens to hold other bricks, or aggregations of stone in place to form concrete. Concrete is the big one for us – after water, it is the second most used product in the world.

So, we have established that we use a lot of cement – how does making it cause emissions? There are two main sources:

1) the heating of the input chemicals up to a high enough temperature to bring about the required chemical reactions that make what we call cement, and

2) the actual chemical reaction itself that makes cement

The second is important, because one of the key inputs for making cement, limestone (mostly calcium carbonate) – contains loads of CO2 – it’s about half the rock’s weight!

You heat up limestone to produce lime (mostly calcium oxide or calcium hydroxide), which you do want, and along the way you end up with a bunch of CO2 released, which for the most part you don’t want.

This CO2 is normally just vented into the sky – which is obviously bad news, climate wise.

Is this the only way to make cement?

For the most part, the way we have made cement for since Roman times has stayed the same. You heat up limestone, and mix up the resulting lime with other building materials to build stuff, because heating up limestone and accepting the massive carbon emissions has been the only option, and because cement or concrete turns out to be really, really useful.

It turns out not to be the only way to make cement though.

A mental model for thinking about highly emitting industrial processes like making cement

In the interview, Leah Ellis introduces a really nice, really visual mental model for thinking about decarbonising highly emitting processes. It’s really memorable, and I can see myself using it when talking about climate in 2024 a lot – it’s called the Leaky Tap model, and it goes a bit like this.

You can think of some important, but polluting processes like having a leaky tap. It’s really handy to have taps, but when they leak, it causes a mess if we’re lucky, and at worst causes significant damage to our surroundings.

We can’t get rid of the tap, so we have three options:

Fix the leak – we can fix the leak, so when we use the tap, it no longer leaks all over the floor. In climate terms, this is a bit like redesigning a process or activity to no longer emit needless carbon.
Put a bucket under the tap – if we don’t want to do that, in the short term, we can put a bucket under the tap to catch the leak. We still need to dispose of the water that builds up, and it doesn’t address the root cause, but if you don’t want to change much about your tap, this is a measure some people take. In climate terms, this is like point source capture on a process that emits carbon – like putting a scrubber on a smokestack, and so on. It’s a bunch of hassle to clean up, but slightly better than the third option.
Keep using the tap, and let it pour all over the floor and then clean up the mess with a mop and bucket – if we don’t want to do either of the options above, one option is to mop up all the water over the floor. This means we change the original activity even less, but now we have to spend loads of time and energy mopping up water and disposing of it, instead of doing useful stuff. In climate terms, this is like Direct Air Capture – now we’re spending loads of energy trying to collect the carbon in a really spread out, diffuse form, before we can even think about disposing of it safely.

I really like this model, because it really highlights how wasteful the second two options are, and that there needs to be someone doing a bunch of wasteful busywork, caused entirely because we are deliberately choosing to avoid fixing the leak. It’s also really easy to picture – almost no-one likes mopping stuff up if it’s avoidable.

It’s also applicable to discussions about how we might get off using fossil fuels in other sectors, like when people talk about replacing the burning of fossil fuels with hydrogen. There you have multiple ways to create hydrogen, which is a key input to loads of processes we rely on. The most common way we create hydrogen right now is taking fossil gas (mostly methane), and carrying out a chemical reaction that splits the gas into hydrogen, and carbon dioxide, which gets vented into the sky just like cement (bad news!) – this is like option 3 above.

Another approach is to try to capture this carbon dioxide, then figure out how to either use the carbon dioxide, or store it somewhere safely (hydrogen made like this is often called blue hydrogen) – this is like option 2 above.

One of the other ways growing in popularity is to avoid using fossil fuels at all, by using electricity to split water into it’s component atoms of oxygen, and hydrogen. Hydrogen made this way when the electricity comes from clean sources is often referred to as green hydrogen, and is a bit like option 1 – redesigning the process.

This is not a new idea – people have used electrolysers to split water like this for nearly a hundred years – but making green hydrogen like this is only possible because renewable energy has become so much cheaper over the last ten years (prices are around 10 times lower per unit of electricty generated than they were 10 years ago).

And this leads us to the exciting idea in this interview – we are seeing new option 1’s in all kinds of industries, like making cement.

Electrochemical Cement

Remember above, where I said that we make cement by turning limestone into lime, by mainly heating it up, and mixing it with other chemicals?

Limestone is mostly a substance called calcium carbonate – which is made up of calcium, carbon, and oxygen atoms. For the most part, it’s the calcium and oxygen that are needed for making cement – the carbon is mainly along for the ride. So just like you can get hydrogen from water using electrolysis instead of fossil gases like methane, it turns out that you can get the calcium and oxygen from other input materials, removing the need for limestone entirely – and removing the need for such massive amounts of industrial heat too.

This is the key thing that the Sublime Systems folks have done – they’ve figured out how to take advantage of the falling costs of renewable energy (and now falling costs of electrolysers) to find a way to make the lime for cement, without relying on burning fossil fuels for energy, nor burning limestone for lime.

Other notes

There were a few other things I learned from this, which aren’t so much about cement, but are more about chemistry in general. The more I learn, the more it feels like magic to me – I wish I understood this at high school!

Electrolysis can also be used to remediate toxic ash and other waste products to reclaim useful minerals – lots of the byproducts from burning coal end up dumped in toxic ponds (one of the terms used was “ponded bottom ash”). These minerals are often useful, but it’s energetically expensive to reclaim them. The availability of cheap renewable electricity changes the maths on this somewhat, like with cement or hydrogen.

With enough heat, even unrecyclable plastics and other hard-to-dispose of materials can be safely broken down into component molecules that can be reclaimed or captured. Some existing cement kilns burn at such hot temperatures that even tyres when thrown in last about 20 seconds before they are entirely combusted into carbon dioxide for example.