Emerging tech

Fixing cement’s carbon problem

New production techniques are helping to reduce emissions in an industry that’s been hard to decarbonize.

November 17, 2022
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Concrete is an incredible material. It’s essentially pourable rock, and we use it in almost every part of the built world. We also use more of it than any other manufactured material in the world — about 3 tons per person annually.

And the secret ingredient in all this concrete? Cement. Think of it as the glue that binds the crushed rocks in concrete together.

But here’s the problem. Making cement emits lots of carbon. The cement industry alone produces 8% of global CO2 emissions.

Why? First, the process happens at 1,500 degrees Celsius, a temperature so hot that companies often burn coal to reach it. Second, the chemical reaction used to produce cement releases carbon dioxide.

So what are the solutions?

In this episode, Shayle talks to Leah Ellis, co-founder and CEO of Sublime Systems, a startup that has developed a novel way to produce cement at room temperature without releasing carbon dioxide. Shayle’s venture capital firm Energy Impact Partners is an investor in Sublime.

Shayle and Leah discuss:

  • The key properties of cement and why we use so much of it.
  • The chemistry of cement and why it releases carbon dioxide.
  • Alternative chemistries to Portland cement, the most common and useful formulation.
  • Ingredients you can add to the mix, known as ​“supplementary cementitious materials,” to offset some of the Portland cement required (such as fly ash from coal-fired power plants).
  • Adopting performance-based standards that allow more flexibility in the materials used in cement.
  • Replacing coal with electrification and alternative fuels in cement kilns.
  • Post-combustion carbon capture for cement kilns.
  • CarbonCure’s technique for injecting carbon dioxide into concrete to increase strength and reduce the amount of cement required.
  • Sublime Systems’ electrochemical technique for manufacturing cement without carbon emissions.

Recommended resources

  • The New York Times: Making the concrete and steel we need doesn’t have to bake the planet
  • Canary Media: Major construction firms team up to get the carbon out of concrete
  • Bloomberg: Breakthroughs are helping even cement and steel go electric
  • E&E News: Congress wagered on ​‘low-carbon’ concrete. Will it pay off?
  • Canary Media: Cement is terrible for the climate. California just passed a law to fix that

Catalyst is supported by Scale Microgrid Solutions, your comprehensive source for all distributed energy financing. Distributed generation can be complex. Scale makes financing it easy. Visit scalecapitalsolutions.com to learn more.

Catalyst is supported by CohnReznick Capital, a trusted source for renewable energy investment banking servicing the U.S. sustainability sector. Visit cohnreznickcapital.com to learn more.

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Speaker 1: From the studios of Postscript Media and Canary Media.

Shayle Kann: I'm Shayle Kann, and this is Catalyst. How big exactly is the average Portland cement plant right now? Give me a sense of scale.

Leah Ellis: They are colossal. So the average small Portland cement plant produces a million tons of cement per year. And on average, just rounding up to big numbers, one ton of cement produces one ton of CO2.

Shayle Kann: It's time to cement your understanding with concrete ideas about how to decarbonize the built environment. Please forgive me.

I'm Shayle Kann. I'm a partner at the venture capital firm Energy Impact Partners. Welcome. Now, normally I jump straight from that into today's topic, but actually I do want to spend just a minute this time talking about my day job, which is what I spend 120% of my time on, and then whatever's left after that goes to sitting behind a microphone talking to you all. It's a big week for me. We just announced this week that we at EIP had done the final close on what we call our Frontier Fund, which is what I've been working on for the past couple of years. It's a $485 million venture capital fund dedicated to investing in revolutionary technologies to enable deep decarbonization. So the idea here being we've got 50 gigatons, 50 billion tons of annual emissions of CO2 equivalent in the world. We're going to have to take that from 50 billion down to net zero by mid-century or earlier.

In order to do that, we're going to have to fundamentally transform numerous industries whose total addressable markets measure in the trillions. This is an enormous challenge and it's going to require some really big swings with really fundamental technology advances to get there in time. That's what we're looking to invest in. Those are the entrepreneurs that we're trying to partner with and we've been at it for a couple of years and love the dozen or so portfolio companies we've already got, but we've got a lot more work to do. So if you're an entrepreneur who's leveraging some fundamental technology advancement to help us get from 50 billion to zero, then please get in touch.

And in the meantime onto today's show. So here's the main thing you need to understand about cement and concrete. We collectively, humans, produce a truly ridiculous amount of it. Cement is in fact the second most consumed material in the world, second only to water. We use around 30 billion tons of concrete every year, and its consumption has been growing. We actually use three times as much per capita today as we did 40 years ago, but we've been making it a very long time, literally for millennia, back to ancient Egypt. But alas, the emissions. It's responsible for something in the order of 6-8% of global greenhouse gas emissions, right up with steel for the largest source of industrial emissions in the world. Big problem obviously, and a bunch of different pathways toward partial or complete solutions in some cases. It's a complex value chain with multiple causes of emissions. So perfect for us to discuss.

My personal favorite entrepreneur and cement decarbonizer is Leah Ellis, the co-founder and CEO of Sublime Systems, which is an incredibly exciting early stage company tackling the heart of the cement decarbonization problem. I'm biased because we at EIP are investors in Sublime, but I suspect you'll find yourself biased once you hear from Leah as well. Here she is. Leah, welcome.

Leah Ellis: Thanks, Shayle, pleasure to be here.

Shayle Kann: Excited to talk cement and concrete with you. We're going to start with some basics and then as usual, we're going to get into some wonky details. But let's start at the highest level. First of all, because I've heard a lot of people confuse these two things, what is cement and what is concrete?

Leah Ellis: So cement is the glue that holds concrete together. So concrete is cement, sand, aggregate and water mixed together to hold everything in place. So cement is essentially rock glue. So if you think of paper mache with paper and glue, so cement is that glue that holds the final form.

Shayle Kann: And cement is like the core building block of concrete, right? Is that the right way to think about it? People talk about how much concrete we use as a society globally, and it's a ridiculously enormous amount, but in the production process of that concrete, most of the industrial part of it is the cement production. Do you think of it that way?

Leah Ellis: Well, concrete is about 90% rocks and 10% cement. But even so, cement is the most massively produced manmade material in the world and second most consumed material besides water. So even if it's just 10% of concrete, it's still produced in tremendous volumes.

Shayle Kann: Just speaks to how much concrete we use. If cement is only 10%, it's still the second most produced thing in the world. All right, let's talk through the production process. So just walk me through end to end today in the standard cement and concrete production process. What do we start with and how do we get to concrete at the end?

Leah Ellis: Yeah, so cement is essentially a calcium silicate. So the way we make cement today, it's made in a long rotary kiln. So it's a big kiln that rotates at a five degree angle. So you put rocks on one end, and as the kiln rotates and spins, the rocks tumble towards the end of the kiln, which is very, very hot. And you get, most basically, two chemical reactions happening as the rocks fall down this kiln and get made into cement. And the first reaction is the conversion of limestone, calcium carbonate into calcium oxide. So calcium carbonate, the major component of cement, it's chemically inert, so calcium bound to CO2, can't react with anything. And then at around 1,000 degrees Celsius, so about halfway down the kiln, the calcium carbonate breaks up into CO2, which escapes as a gas and calcium oxide, which then goes on in a second step to react at the silicates to make Portland cement, which is the cement we largely use today.

Shayle Kann: So we'll come back to where the emissions come from in that process in a minute. But I thought it would be interesting to talk a little bit about the history here. As you said, Portland cement or ordinary Portland cement is, I think most folks will know, pretty ubiquitous. It's most of what we use today. It's not the only cement that is out there, there are others, but how did we end up where we are today where Portland cement is so widely adopted? Is it just the best cement in the world or is it some relic of history?

Leah Ellis: Yeah, there are many types of rock glue that we've been using over the past millennia as humans. And it all started thousands of years ago when people would burn limestone to make lime, the calcium oxide, and then use that as a mortar for brick making or for building pyramids and ancient structures. So that was the first type of construction material was a lime clay mixture. And then the Romans, of course, pioneered a new type of cement. So they started using volcanic ash and they were blessed with large quantities of volcanic ash thanks to their geography. And that has turned out to be quite a fantastic cement. So their cement was one part burnt limestone, lime, and one part volcanic ash, and it's proved to be incredibly durable.

So the Romans came and went, and then following the Romans, cement was still made by burning limestone and mixing it with clay. And then what happened about 200 years ago is someone discovered, almost quite by accident, that if you had a specific chemistry of rock, so if you had the right calcium to silica ratio in your input limestone that you were burning and you heated it to an extra extra hot temperature, so around 1,500 degrees Celsius, you got what's now known as Portland cement. So Portland cement contains a very special phase called tricalcium silicate, which is only stable at around 1,500 degrees Celsius. So as it falls out of the kiln and is quenched, this tricalcium silicate phase is frozen. And that phase is what leads to Portland cement's very high early strength and early set time, which has allowed us to build vertically and build very quickly. So that's been the evolution of cement over the past few thousand years.

Shayle Kann: Good brief history of cement. You mentioned early strength and early set time. This is another thing I wanted to talk through with you. So different formulations of rock glue have different characteristics, and I'm interested to hear what are the ones that you think, or maybe not you, what are the ones that architects and designers and engineers think of as being the most important? If you're going to have a good quality cement, what are the things that you absolutely need to achieve?

Leah Ellis: Yeah, of course all the properties are important, but the main ones are compressive strength. So you want it to be strong. So cement is very strong in compressive strength, not so good in tensile strength. So you need about 30 megapascals after 30 days. There's also early, early age strength and quick set time. So set time is a little bit different from early strength in that you want your cement to gel and harden, and this means that the contractor can go home and doesn't have to stand around a long shift shooing kids off from writing their names in wet concrete.

So what's interesting is that 90% of the total installed cost of concrete is actually labor from the contractor. It's often unionized labor and cement being a commodity material, is so cheap. So Portland cement is around $130 a ton. So basically the most important properties are the ones that minimize labor costs. So having that early strength, early set time, proper flow out of the truck so that it doesn't require too much water. And then of course, durability. So you don't want to have cracking or corrosion or expansive reactions with different minerals in the soils. And so that's very important too.

Shayle Kann: Okay, so back to Portland cement then. So as you said, I mean, we discovered it a couple 100 years ago and it turns out to have these attractive characteristics on early strength and set time. Is it the high watermark, is it the highest performing cement? Or I guess the other way to ask it is just why is it that Portland cement is so ubiquitous?

Leah Ellis: Yeah, I guess it's ubiquitous because it works and it's been a one size fits all cement for a very long time. And of course there are cements that have faster set times and faster early strengths. So those are used for, let's say, patching up highways in the middle of the night so that everyone can drive on them. But for a long time it's overshot the technical requirements for certain applications. So there's thousands of different things that we use cement for and not all of them require the properties that Portland cement brings.

Shayle Kann: All right, so let's talk through the value chain then. Who is doing what today in this massive cement and concrete industry?

Leah Ellis: It all starts at the Portland cement kilns. So Portland cement companies are often very large international companies. So there's a handful of colossal companies that own the majority of the Portland cement kilns all around the world. So they're often located near a limestone quarry because cement’s very bulky and of course half the weight of the limestone gets lost to CO2. So they're located next to a quarry, operate these big kilns, and then ship the final cement powder by train or by boat to a port. And then that's used by ready mix concrete. So in every city and in every town there's a ready mix concrete producer, some of them are vertically integrated with these large cement companies and some of them are mom and pop or family operations or independent companies. And they operate these spinning trucks that we see and they sell to contractors who are told what to buy from a structural engineer who specified a certain cement for a certain purpose, all designed by an architect. And then of course, ultimately, everything is paid for by the building owner. So there's quite a long chain of value all throughout.

Shayle Kann: So the way I think about it, you could tell me if this is right, is that Portland cement production is highly centralized. Those plants, as you said, they're near limestone quarries because you don't want to ship limestone around, as you said, you're going to lose half the weight when you turn it into lime anyway. So that's very centralized and those plants are big. And then what we ship around is the cement powder, and then you get to the ready mix concrete world. And that's actually fairly distributed. Not down to the level of necessarily the individual construction project, but down to the local regional level.

Leah Ellis: Yeah, exactly. So about 80% of ready mix plants are independently owned.

Shayle Kann: How big exactly is the average Portland cement plant right now? Give me a sense of scale.

Leah Ellis: They are colossal. So the average small Portland cement plant produces a million tons of cement per year, and they go all the way up to three or four, maybe even FIVE million tons of cement produced per year, so absolutely huge. And on average, just rounding up to big numbers, one ton of cement produces one ton of CO2.

Shayle Kann: Okay, that's a great segue into the next thing. So one thing I think everybody who's looked at cement understands this, but otherwise people don't necessarily recognize that it's a little bit different about cement production versus other sources of large industrial emissions, is that you've got two separate very big components of emissions. This is part of why cement production is such a huge overall number on emissions. So walk me through where the emissions come from in the cement making process.

Leah Ellis: So yeah, cement being the most massive industry by volume in the world also produces an almost equivalent amount of CO2. And that's, again, speaking in big even numbers, that's roughly half from the fossil fuel needed to get to a 1,000 degrees and 1,500 degrees to decompose the calcium and have it fused with a silicate. And the other half of the CO2 is from the limestone itself as it decomposes into the oxide. And this is what makes cement one of the most difficult things to decarbonize from a technical standpoint is that you need limestone, so you need the calcium oxide and then you also, you need to get to these high temperatures to make the calcium and to make the silicate. So there's almost no way around that except for post combustion carbon capture if you're making Portland cement.

Shayle Kann: So we'll come back to the ways to potentially decarbonize. But yeah, I guess to repeat that point, so you've got one challenge, which is you need to heat a kiln really, really, really hot. Industrial heat is a big challenge across a variety of industries. And this is one that is among the hottest, 1,000 or 1,500 degrees C that we've done historically by combusting fossil fuels basically. So that's a big chunk of the emissions there. And then the other one is in the actual chemical reaction that naturally and unavoidably, if you're turning limestone into lime, the byproduct of that is CO2, and there is no alternative going from limestone to lime then to produce CO2.

Now what you do with that CO2 is a different question, but that is a natural part of how we produce Portland cement at least, and makes it a particular challenge because you could solve the industrial heat problem, you haven't solved the process emissions problem. You could solve the process emissions problem, you haven't solved the industrial heat problem. So if we're seriously going to try to decarbonize, we have to figure out a way to deal with both of those things, right?

Leah Ellis: Exactly. You put it very well.

Shayle Kann: All right, so let's talk about decarbonizing then. One of the things that's interesting about cement in part because it has this multi-stage value chain that involves cement production and involves turning that cement into concrete, there have been a bunch of different ideas proposed for how we can decarbonize part or all of this market. Let's run through a few of them and then I think we'll spend a bunch of time talking about Sublime's approach obviously. But at the high level, what do you think of as the major categories of decarbonization possibilities?

Leah Ellis: Yeah, the major categories are post-combustion carbon capture. So you'll see this in all of the major cement companies that are pathways to net zero by 2050. You'll also see alternative fuels, so burning things other than fossil fuel, be that tires or biomass. You'll also see big cement companies using supplementary cementitious materials. So like I said before, Portland cement has been used, especially in the US, as a one-size-fits-all cement. But there are a number of things that you can blend into cement that actually improve the performance and durability. And so you can tailor the percent of Portland cement you use for each application. And this is done increasingly in Europe where the cement is blended to meet specific performance or CO2 goals.

Shayle Kann: Can we talk about that one for a minute? I think post-combustion carbon capture is fairly straightforward. Everybody understands what carbon capture is. You have a cement plant, you capture the CO2, you do something with the CO2. We can talk about why there are challenges there, but that one's fairly straightforward. The second one you mentioned, may be a little bit less obvious, but alternative ways to heat the kiln basically, of which there are a number and could decarbonize that portion of the emissions, but don't do anything about the process emissions. That third one supplementary cementitious materials, I feel like that one actually is, at least for outsiders, a little more complicated to understand. So what are these SCMs and how much can you blend in? And how big a difference does that make on the total lifecycle emissions of cement if you are just using a little bit less Portland cement?

Leah Ellis: Yeah, that's a fantastic question. So what you're normally blending into Portland cement are silicates. So Portland cement, as I said before, is a tri-calcium silicate, which means there's three calciums for one silicate, but the hardened form of Portland cement, the phase that gives it its strength, is one calcium to one silicate to one water. So it's called CSH with a stoichiometry of approximately one to one to one. So when you hydrate Portland cement and add water to it, you have a lot more calcium floating around then you really need, and this doesn't do you any favors for a strength and durability standpoint. So folks add extra silicates to the cement and that soaks up the extra calcium and that improves the durability, improves the density, and also improves the performance in many ways.

So there are a number of different sources of silicates. Right now, fly ash is very popular. So fly ash is an amorphous silicate collected from coal-fired power plants, and of course those are going away and for a good reason, but it does mean that there's a shortage of fly ash, a near term shortage. And we're also increasingly using fly ash for sustainability reasons, although you could argue that perhaps it truly isn't a sustainable product since it has quite a bit of embodied carbon in it, depending on how you allocate that to the electrons or to the fly ash. So there's a number of different silicates, so natural silicates, clays, pumices, obsidians, and different materials coming to market to replace fly ash.

Shayle Kann: And how big of a difference can adding SCMs make on emissions and, I guess on cost?

Leah Ellis: Yeah, so you're right. Another reason why they're added is because of cost. They're oftentimes less expensive than Portland cement. And so these SCM supplementary cementitious materials have been used for decades before cement came into the spotlight as something that had a big CO2 footprint. And that's because they reduce cost. So you're blending in something that's much less expensive, but you can only blend in so much before you start... You're blending it in to remove some of that extra calcium, but if you add too much silicates, now you have too much silicates. And so it's that reaction between the calcium and the silica that make the hardened cement. So you can only blend about 30% in before you start diluting the performance.

Shayle Kann: Right, so in other words, you can have a meaningful but incremental impact. You can maybe lower cost, you can maybe increase strength. It may be the right thing to do in a lot of cases. From an emissions standpoint, there's no world in which that totally decarbonizes cement because you still need 70% Portland cement. So if you don't solve for the Portland cement production process, at best, you're getting a 30% emissions reduction.

Leah Ellis: Exactly. So going back to where the CO2 emissions from cement occur, a lot of it is in that decomposition of calcium. It's producing the reactive calcium, and so you really need to have both calcium and silicate in the cement. So you can't totally decarbonize just from that silicate angle.

Shayle Kann: Okay, so we've talked through three of the categories. The fourth, I think that's at least worth mentioning, is on the concrete side, more so than the cement side, which is curing, injecting CO2 into the curing process and using cement as basically a sink for captured CO2. How do you think about that fitting into the broader cement decarbonization picture or I guess concrete decarbonization picture?

Leah Ellis: If I may, before going into CO2 injection, I'd like to add in a fun fact about alternative fuels, and that is that cement kilns, because they run at such an absurdly hot temperature, they actually form a pretty important part of the garbage ecosystem. So you can burn a lot of waste that you couldn't end up getting rid of another way. So medical waste, tar paper, unrecyclable plastics, even cement kilns around the world use camel dung in Saudi Arabia. So I think it's actually quite cool that there's a use for these. There's a garbage ecosystem that cement can take any type of undesirable carbonaceous waste, and at that obscene temperature of 1,500 degrees, almost everything vaporizes into CO2 very quickly at that temperature. So I think we've all seen tire fires, and of course they produce a black choking smoke, but when you throw a tire into a cement kiln at that temperature, it just vaporizes entirely into CO2 within a matter of seconds. So it's actually a pretty clean way of getting rid of some pretty dirty materials. So I think there's a pretty cool role for cement kilns in the circular economy.

Shayle Kann: Good aside.

Leah Ellis: Going back to CO2 injection. So you have calcium oxide in the cement and you can add CO2 back and have it convert back into limestone, that calcium carbonate. So that goes back to one of the earliest forms of cement that I mentioned, just using burnt lime and then using it as a mortar, and then it reabsorbs CO2 from the atmosphere as it hardens. So there are a number of startups looking to use this type of technology for making precast blocks, so having the lime and then putting it in an autoclave, pressurizing it, and forming CO2 cured rocks, which is great. And then we also, we have carbon cure, which adds a small amount of CO2 into the ready mix concrete, so the cast in place concrete, and that, even though they're adding just a small amount of CO2, it is said to improve the strength so that you can end up using less cement overall.

So it's actually interesting, another small tangent about cement. So Portland cement does recarbonate over time. So if you have a piece of pavement, for example, it will continue to absorb CO2 from the atmosphere over timescales of decades to a 100 years or millennia will totally turn back into calcium carbonate. So it is a CO2 sink, which is a good thing. However, the fact that cement absorbs CO2 is also a degradation reaction. So it's a double-edged sword. So it's constantly absorbing CO2, but then at a certain point, it all falls down. So that's why structural engineers will often bury their rebar. They'll use even more cement than they have to for structural purposes to bury the cement so far into the building so that as the carbonation front increases and as the building starts absorbing CO2 from the surface, that rebar is so far into the structure that it's protected for a very long time.

Shayle Kann: That's fascinating. I did not know that. Is it enough of a CO2 sink to matter from a global CO2 flux perspective? I mean, we have a ton of cement out there.

Leah Ellis: So I think it does matter. However, it will only absorb the limestone emissions, so it'll only absorb about half of its original CO2 footprint.

Shayle Kann: Right, and over decades to centuries, which we don't necessarily have that much time. It's just interesting that we think of what are the major global CO2 natural sinks that are just out there sucking up CO2, things like trees and plants and all of that, and it's funny to think about cement being one of those.

Leah Ellis: Yeah, totally. And so a big factor in that is the surface area to volume ratio of the cement. So if you have an old building, crushing it up into finer particles will really accelerate the carbonation.

Shayle Kann: Fascinating. So now we come around to Sublime, which you're doing none of the things that we've described so far, all these different pathways, which are all interesting to partially decarbonize cement production. Sublime's taking a totally different approach. So what are you working on at Sublime and then we'll get into why.

Leah Ellis: So Sublime is pioneering a new way to make cement. So we use electricity instead of fossil fuel to drive the decomposition of calcium-bearing minerals into a cement. So my background is in electrochemistry, worked previously on batteries, as did my co-founder. So we were looking for a way to use our electrochemical toolbox, so working in ambient temperatures, working with intermittent electricity, and we were trying to find a way that we could use electrochemistry to displace the fossil fuel in the process.

Shayle Kann: Okay, so part one of what Sublime is doing that's novel is you're replacing the kiln, which is the thing that you have to normally heat up to a thousand to 1,000 to 1,500 degrees C with an electrochemical reaction that occurs at room temperature. How do you think about the benefits of that relative to the traditional kiln?

Leah Ellis: Yeah, so the benefit is that it's lower in CO2, so obviously we're getting rid of that half of cement CO2 emission that's from the fossil fuel. And also we can get rid of the other half of cement CO2 emission, which is from the minerals or the limestone that's used. And what's cool about Sublime's process is that it's not a thermal decomposition of the calcium carbonate. So remember calcium carbonate decomposes in a kiln and the CO2 escapes as a gas, and then you have a solid calcium oxide. So Sublime goes about this in a different way. So we take a source of calcium, it could be calcium carbonate, it could be something else, and then instead of thermally decomposing it, we put it through an electrochemical process that dissolves the calcium, so extracts the calcium as a liquid and then precipitates it in a second step.

So this means that we can use calcium carbonate, in which case when the calcium is extracted, we're left with pure cold compressed CO2, so inherently captured CO2 that's ready to go into a pipeline or to be used in food grade applications. Very low cost of capture, less than $10 a ton. Or we also have the option of using non-limestone minerals. So there's a long list of calcium bearing minerals. We could use, a number of natural minerals and waste materials, largely silicates, so we can pull the calcium out of the silicate and then use both the calcium and the silicate in cement to make that calcium silicate hydrate.

Shayle Kann: So this is one of the things that's cool about Sublime. So you have two different pathways to fully decarbonize cement production. So you've got what would've been the emissions from the kiln solved by electrifying it. Now obviously you got to use low carbon or zero carbon electricity, but let's assume we do. That part's solved. Then there's the other part which is the decomposition of the rock. And you're saying there's two different things you could do there. You could use the traditional material, in which case you are doing a form of carbon capture, but it's not really apples to apples with post combustion carbon capture. You're basically, just as a natural part of your process, you're getting a pure stream of CO2 to which you don't need to do a whole lot. You just got to figure out where the home is for it.

Or if you want to avoid that entirely, which is I think important to talk about because it is not the easiest thing in the world to do something with a huge stream of CO2. We're seeing this already in all these carbon capture projects. You have to have pipelines and you have to have permanent sequestration or a big sink or whatever. So in the event that you don't want to or don't have a home for all that CO2, you're saying you can use different inputs, still run your electrochemical reaction, and then you don't get any CO2 out at all. But in that case, you're not producing Portland cement. So how do you think about this back to our original conversation around how Portland cement ended up being so ubiquitous. How do you think about the future of cement types and is there room in the market for something besides Portland cement to take a big role and what would it take for that to happen?

Leah Ellis: Yeah, great question. So we don't use a thermal process in the kiln, so we don't have 1,500 degrees Celsius and therefore we don't make that tricalcium silicate phase that's only stable at that temperature. But we still make, in the end, that calcium silicate hydrate, so the final hardened phase of cement and all that durability and compressive strength associated with it are the same, but we go about it in a different way. That's all at very, very low temperatures. And I think the cement market being absolutely huge, and then also with so many performance requirements for these different applications, there's a lot of the future of cement, and it's already happening even long before Sublime, is all about the performance you need for a given application. This started way back in the '60s, '70s and '80s when slag and fly ash started getting blended into cement as a supplementary cementitious material that people started rethinking the way that cement was specified. So previously it was all chemistry based specifications, so your cement had to have a certain percentage of that tricalcium silicate phase.

But now architects are moving towards performance-based standards. So there's standards for blended cement where it's 50% Portland cement, 50% almost anything you want as long as it meets this performance based standard. And there's a bunch of tests to prove that. And there's also 100% performance based standard where structural engineers say, "We don't care what it's made out of, just give us something that works."

Shayle Kann: And so the idea being if those performance-based standards take hold more and more, then what dominates the market in the long term is not a particular formulation, it's not Portland cement or any other particular formulation. It is anything that has been blessed as meeting those standards. And that's the pathway to a market where you never needed to use limestone in the first place to make cement.

Leah Ellis: Exactly. And I think that's really exciting because there's so much room for innovation when you're trying to hit targets that the customers and the end users actually need, which is a strong, durable, fast-setting cement. And you're not limited to just trying to achieve a certain chemistry that is associated with these very high temperatures that are incredibly difficult to decarbonize.

Shayle Kann: So I guess the key question, or a key question anyway on Sublime's process then is the energy intensity of it. Obviously we're switching from what is currently relatively cheap fossil-fueled kiln heating to electricity, which is lower emissions, but currently more expensive. So it all comes down to how much energy it requires to run the process. How do you think about that?

Leah Ellis: Energy is super important when we think of all the different ways to decarbonize cement, be it electrifying parts of the kiln or moving to post-combustion carbon capture, all of those are energy adders onto the system and have the possibility of being cost adders as well, since electric heating is never as efficient as direct combustion heating. So Sublime's process produces a cement that has the same embodied energy as Portland cement today, and we also have a pathway to further reducing the embodied energy as well to making it a path to half as much embodied energy. And I think that's important because as we look over the millennia for how technology's evolved, it's usually the lowest energy, most efficient process ends up being the winner.

Shayle Kann: All right, so I guess final question. What are you seeing in terms of the market? How much demand pull is there for low carbon or zero carbon cement from major customers? And I guess who are those customers? And do you feel like it's, from the years that you've been working in this space, is it moving any faster? This is notoriously a conservative slow-moving though fast-setting market?

Leah Ellis: Yes, a very stoic industry and for good reason too. But I've seen in my three years at Sublime, a tremendous amount of interest in low carbon cement. So a lot of interest from corporations, even from individuals. And I think what's really interesting is that, like I said before, with 90% of the cost of cement being labor and only 5% being materials costs, even as these new technologies for making cement, until they get to scale, they're going to have some sort of green premium associated with them. But that green premium ends up being budget dust at the end of the day for the building owner. So this is even compared to HVAC or everything else you're putting into a building, it's so cheap and it's one of the biggest levers for a company's scope three emissions. And so we're seeing a lot of these companies with internal prices on carbon getting really excited about low carbon cement.

Shayle Kann: All right, well, I know we're out of time, but that's a good way to end it. Let's hope that all this demand materializes, let's hope that all these solutions scale up and that we end up in a place where we are still perhaps producing more cement than we are basically anything else in the world. But the emissions profile goes from 8% of global emissions to zero by mid-century. Leah, thank you so much for finally doing this with me.

Leah Ellis: Thanks Shayle.

Shayle Kann: Leah Ellis is the co-founder and CEO of Sublime Systems. What did you think? What did we miss? There's a lot going on in cement decarbonization world. We're always excited to hear feedback. You can find the show on Twitter at Catalyst Pod. You can find me there too. If you liked it, go over as always to Spotify or Apple Podcasts or wherever you get this show and leave us a rating and review. The show is a co-production of Postscript Media and Canary Media. You can head over to canarymedia.com for links to today's topics. And as always, Postscript is supported by Prelude Ventures, a venture capital firm that partners with entrepreneurs to address climate change across a range of sectors, including advanced energy, food and agriculture, transportation and logistics, advanced materials and manufacturing and advanced computing. This episode was produced by Daniel Waldorf, mixing by Greg Vilfrank and Sean Marquand, theme song by Sean Marquand. Our managing producer is Cecily Meza-Martinez. And special thanks to my dog, Primo for being quiet while I record this outro as he sits on my lap. I'm Shayle Kann and this is Catalyst.

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