Unlocking Possibilities with Cheap Renewables in Cement Production

The cement industry, responsible for roughly 8 percent of total global carbon emissions, is notoriously difficult to decarbonize. But a new startup, Sublime Systems, aims to manufacture zero-carbon cement that can easily be substituted for the traditional version. In this episode, Sublime CEO Leah Ellis talks through the company’s vision and process.

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David Roberts:

Of all the so-called “difficult to decarbonize” sectors, cement is among the most vexing. Making cement produces CO2 not merely through fuel combustion (in kilns that reach temperatures of up to 1400 C), but also through chemical processes that split CO2 off from other molecules. It is responsible for roughly 8 percent of total global carbon emissions.

Most gestures at decarbonizing cement to date are fairly desultory — things like adding special additives or injecting a little CO2 when the cement is mixed into concrete. The only widely available method that could theoretically produce no- or low-carbon cement is post-combustion carbon capture and sequestration. And there are plenty of people who would question whether that's actually viable at all, much less widely available, given that it would roughly double operational costs for a cement plant.

There are lots of startups out there attempting to solve this problem (as reported by Canary last month). Perhaps the most intriguing is Sublime Systems, a team that has developed something truly new and exciting: a system for manufacturing cement that requires no high heat (thus no combustion emissions) and uses inputs that contain no carbon (thus no chemical emissions). That makes the cement, at least potentially, not just low-carbon but zero-carbon. What’s more, the company says that, in form and performance, its product is a perfect drop-in substitute for traditional Portland cement, so it wouldn't even require any changes in the construction industry.

A carbon-free drop-in cement substitute — at scale and at competitive cost — would be genuinely transformative. I contacted Sublime CEO Leah Ellis to talk about cement chemistry, the company’s process, and the plan for reaching megaton scale. This one was truly fascinating and educational for me; I think you will really like it.

All right then. Leah Ellis, CEO of Sublime Systems, welcome to Volts. Thank you so much for coming.

Leah Ellis

Thank you so much for having me.

David Roberts

I'm excited today to talk about concrete, everybody's favorite subject. But first I wanted to ask you, I know you and your partner originally were trained as and educated as battery scientists. I'm just curious how you ended up here. What drew you into this area, this problem?

Leah Ellis

Yeah, my co-founder is a professor at MIT, Yet-Ming Chiang, and he's in the material science department, and I'm a chemist by training. I like to think of chemistry as the central science that combines everything from physics to biology. All of the good stuff you can sort of spread into anything from a foundation in chemistry. So I did my PhD in lithium-ion batteries. I worked with a prolific inventor, Jeff Dahn, and after that I wanted to continue working with an inventor. As you may know, in academia, there are so many different styles of research.

I mean, some people like microscopy and mechanisms, but I really like the creative aspect, like discovering something that could be useful or to solve problems. And not many academics and professors think through that lens. So I've always been very lucky to work with prolific inventors, both in my master's and my PhD. So for my postdoc, I sought to work with people who thought like that. So my co-founder, Yet-Ming Chiang at MIT, is a prolific inventor and also a serial entrepreneur. So Sublime is his 7th startup, and five of the previous six have been very successful.

So I didn't join him with the aspiration of becoming a founder. I really knew nothing about entrepreneurship or anything like that, but I did want to invent, and I did love the way he approaches his work from a problem-solving standpoint. So that's what brought us together.

David Roberts

And he's the one who sort of flagged the problem of concrete to you.

Leah Ellis

That's right. So I was always aware that cement was one of the biggest levers for decarbonization. But I suppose after my PhD, where I'd worked with one of the most illustrious battery scientists, I sort of always had thought that my career would be in batteries. Like, I thought I'd painted myself into a corner. And so when I first met Yet-Ming Chiang, he asked a question that at first I thought was a trick question. He was, "Hey, Leah, like, you've got this Canadian grant to come work with me, and I know you're a battery scientist, but aren't you a little bit bored of batteries?"

And I thought that was a trick question because he's the battery guru and I didn't want to insult him, but honestly, I sort of shared his opinion that, well, maybe this isn't his opinion, maybe it's just my opinion. But I think batteries are exciting, but I think there's like sigmoidal growth in any technology where it starts out slow to build momentum, and then it goes through a period where it's super hyped, and then you sort of squeeze all of the innovations out of things. And I think with lithium-ion batteries, which was my expertise, it comes to making the cans a bit bigger and the separators a little bit thinner and tweaks. And you know — I don't know, I just want to do something totally outside the box.

And I think that's what Yet offered me the chance to do when he said, "If you're bored with batteries, why don't we think of a way to apply our electrochemical toolbox to cement?" And so the way he came up with that electrochemical cement tagline was he spends a lot of time thinking about reducing the emissions in the utility sector. And I think we have all of the technology needed to do that. I mean, not saying it will be easy to deploy solar, wind, long duration storage, but at least we have the technology. So that's not necessarily where most of the early stage R&D is needed.

And so he was thinking at the time, like, how do we use low cost renewables, assuming that we'll figure out all of the utility stuff and really get to low cost intermittent renewables? And how do we take intermittent renewables and apply that to decarbonizing the next biggest tranche of emissions, which is cement? So cement, if it were a country, would be the third largest emitter after China and the US. So it's 8% of global CO2 emissions back in 2018, it fluctuates, but it's like 7% or 8%. So it's big game hunting when it comes to decarbonization. So we've always worked backwards from that electrochemical cement tagline.

David Roberts

And it's 8% of current emissions. But also, you make the point that's just going to go up, right? There's just going to be more and more cement as far as the eye can see.

Leah Ellis

Right. And as everything else, I hope, goes to zero. I think these so-called hard to abate sectors like cement and steel, they may be both seven or 8% now, but in coming years, as the grid gets decarbonized, these numbers are just going to get larger because neither cement nor steel are going to go away. And in fact, we're going to use more cement, especially in places like India and Africa, that will undergo a phase of dirty growth unless we develop and deploy these clean technologies in time.

David Roberts

Right. So, let's talk about cement then. I mean, if you're looking for the big problem with no solution yet, it seems like you really nailed this one. So, let's talk about cement. I've been reading sort of the background materials of your company and reading about cement in preparation for this. I have tripled my knowledge about cement over the last 72 hours, starting from an extremely low baseline. What is interesting to me is that the process whereby cement is made, the emissions mostly come from trying to get lime. So, let's talk a little bit about how cement is made.

So, cement, the recipe for cement, as you say on the website and in all these briefing materials, is calcium silicate hydrate (CSH). There are, I'm sure, details that matter, but that's the basic recipe for the basic kind of cement that we've been using for hundreds of years. CSH, they call it calcium silicate hydrate, and that is made of lime and silica and water. We can talk about silica and water later, but they're pretty easy to come by. So, mostly it's about getting lime, it's about how do you get lime? And the way we get lime now is starting with limestone.

So, maybe you can just briefly describe the process whereby conventional cement is made. How do we get from those raw materials to the cement that we are familiar with in bags, mixing with water, et cetera?

Leah Ellis

Yeah, as you say, cement, the principal ingredient is lime, calcium oxide. And fun fact is, that's where Sublime Systems got their name. I was looking in a rhyming dictionary for something that rhymed. No shortage of cement puns. I can keep them coming if you like. Or maybe you don't like.

David Roberts

That's excellent.

Leah Ellis

So, lime is calcium oxide or hydroxide. It's a reactive calcium that reacts with silica and water to make cement. Or when cement reacts with water, then it becomes concrete, that calcium silicate hydrate phase you mentioned. So, today, and historically, the path to making cement has been by thermally decomposing limestone, which is calcium carbonate, which is 50% by weight CO2.

David Roberts

Yeah, that kind of blew my mind. I don't know anything about chemistry, so all chemistry blows my mind.

Leah Ellis

Yeah, it is mind-blowing. I mean, you think of cement, which is the world's largest industry by mass. So we use more cement than any other material besides water. And the prime ingredient in cement, it's about 65% weight calcium oxide. And then that comes from limestone, which is 50% by weight, CO2. So you just do the math. It's very easy to visualize. Just the massive CO2 emissions is staggering.

David Roberts

So the emissions from cement are almost all around this process of getting lime. They're 50/50 — you break it down 50/50. So 50% is getting the kiln hot enough to break limestone down, which requires extremely high temperatures. What is it, 1400?

Leah Ellis

Yeah. So the way we make Portland cement, which is the modern cement that we've been using for the past almost 200 years, is you take limestone and then heat it first to 900 degrees Celsius. And that is the temperature at which limestone decomposes into reactive lime and CO2, and then to make Portland cement, which is a specific chemistry of cement, you heat it further to 1400 degrees Celsius, at which point the lime and the silica fuse together to make a phase called alite tricalcium silicate. And then that is quenched. So it's dropped very quickly from that hot kiln and sort of freezes that phase.

That when you grind the cement into a powder that now reacts very vigorously with water, it releases a lot of that extra heat and that turns into the hardened concrete.

David Roberts

So just to give people a visual here, a conception, this process of heating lime up and getting the lime out of the limestone, that's where all the emissions come from. They're about 50/50 the emissions.

Leah Ellis

Yeah, about 50/50. And of course, it depends on the design of the kiln. Some of them are more efficient than others, but it's about 50% limestone emissions, 50% fossil emissions. And to get to that very high temperature, you often require a very high caloric, luminous flame, which requires bituminous coals — is often the primary fuel for this type of kiln.

David Roberts

Right. So it's 50% this special kind of coal heating up this special kind of kiln to the extraordinarily high temperature of 1400 C. And then the other 50% is the CO2 in the limestone being broken out and then vented, I guess emitted. Which is just the reason I'm harping on this point, is just that I think a lot of people think of decarbonization as primarily an energy thing. But it's worth noting here that even if you find a decarbonized way to get that heat, that 1400 C heat, you're still left with 50% of the emissions. Right, because you still have CO2 coming out of the limestone as it's heated.

Because we've talked a lot on this pod about alternative sources of heat and about heat batteries, thermal storage batteries that conceivably could get up to 1400. They have that in their sights. But I just want to emphasize that even if that problem is solved, you still got 50% of the problem left, which is all the CO2 that's embedded in limestone attached to the lime. So that's our current cement. 50/50, heating up lime, breaking the lime out of the limestone. Let's talk about ways that people have tried to approach, before we get to Sublime's approach, let's talk about other ways people have thought about this.

It's a big problem. And if you think about decarbonization long enough, you end up here. What else have people tried to do with cement? I know there are a couple of things floating around, but maybe you just tell us sort of like, what are the other alternatives here?

Leah Ellis

Yeah, there are, as you say, not many technologies, if any, besides Sublime's, that can address both halves of the cement CO2 problem, the limestone and the fossil fuel, simultaneously. So you could electrify the heat in some way with a thermal battery or find some sort of electric kiln. And electric kilns have material problems. It's difficult to get things that withstand that temperature — the heating elements — if you're using resistive heating, for example.

David Roberts

Is anyone doing that yet? Is there an electrified heat source specifically making cement yet, or is that just an idea?

Leah Ellis

Not that I know of. I mean, I think that would be the obvious solution, I think is to electrify the kiln. And I know there's some efforts with plasmas and solar concentrators and stuff like that, but I think that one would be, if it was easy, someone would already be doing it.

David Roberts

Getting extremely high heat with electricity is vexing.

Leah Ellis

Right. And you can do it. The challenge is, like, can you do it efficiently? And then even if you do it, have you really solved the whole problem?

David Roberts

Right.

Leah Ellis

So, there's that. And then I'd say, like, what the industry is doing right now. And to their credit, they are doing everything they can to decarbonize, especially in the past three to five years. So there's a focus on alternative fuel. So, cement actually plays an interesting role in the garbage ecosystem because it's such a high temperature. It's just a really great place to dispose of things because everything at that temperature vaporizes into CO2 within a matter of seconds. So it's a great place to burn tires. It's a very clean way to burn tires.

David Roberts

What, in cement kilns?

Leah Ellis

Oh, yeah.

David Roberts

They're throwing tires in there?

Leah Ellis

Yeah, I've actually visited a kiln that had this tire injection port, and I was able to stand on top of it and watch them drop tires into this kiln. About every 20 seconds, a hatch in the kiln, rotary kiln, would open, and it would drop in. I actually have a video I can send you a link to. It's one of the coolest things I've ever seen, but, yeah. The plant manager told me that the tire turns entirely to CO2 within 20 seconds. I think many of us have seen a tire fire which creates, like, black, choking smoke.

But if you get to 1400 degrees C, it just undergoes complete combustion almost instantaneously. So tires, like tar shingles, unrecyclable plastic, medical waste, solvent waste. The list goes on. In fact, I have an employee that used to work at a cement kiln close to the US-Mexican border, and they would bring all kinds of contraband that was seized at the border and they would just unload pallets of contraband into the kiln. So, almost anything goes. But that can't be your entire fuel source. So that's a supplementary fuel source.

David Roberts

Does anything except coal work here? Like, is there a substitute for this specialized coal here that can get to that kind of heat?

Leah Ellis

Yeah, I mean, some of them use like a blend of natural gas and other things. And often it takes a blend of fuels to really get up to that temperature. So first you have to start with propane and get it to a certain temperature, and then you add more oxygen. More coal is the typical fuel that's used. And so besides all of these giant cement plants, if you look at them on Google maps or something, there's usually you can see a little mountain of coal besides the kiln.

David Roberts

Right, but for decarbonizing that, you basically just have to capture the CO2 after it's emitted, right? From the kiln.

Leah Ellis

Yeah. So the ways the industry is decarbonizing now include alternative fuels and then what's called supplementary cementitious materials. And then of course, post-combustion carbon capture, which hasn't yet been proven at scale. But there are plenty of pilot plants because that's really the only other way, in my opinion, besides Sublime Systems that can get you to a fully decarbonized cement.

David Roberts

I mean, theoretically, right, that there's no carbon capture working even close to 100%, as far as I know, on any industrial facility. Theoretically, we could get to mostly decarbonized.

Leah Ellis

Theoretically. And I'm afraid to share your opinion that post-combustion carbon capture has never been successfully demonstrated at scale for cement and for coal either. I mean, I sure hope it works because I think that is, if it worked, it would be the most expedient way to decarbonize existing assets. But I think there have been many challenges in employing post-combustion carbon capture at scale.

David Roberts

Among other things, it basically doubles the operating cost, right? I mean, it's just like adding another industrial facility onto your industrial facility. And I just come back just on first principles. I was like, however we solve this problem, it's not going to be through some Rube Goldberg system where we double the cost of every single concrete plant and then bury kajillion tons of — and then build CO2 pipelines to all of them and then carry all that CO2 and bury it. It's just like, there's no way that's going to be it.

Leah Ellis

Yeah.

David Roberts

Although that's kind of where people are going. That's sort of the only option.

Leah Ellis

I like to use the leaky tap analogy when talking about climate change. So imagine you have a tap that's leaking or gushing water. There's really three ways to fix it. And one is to stop this flow of emissions and through carbon avoidance technologies like Sublime Systems like renewable energy. And then the other thing you could do is put a bucket right underneath the tap and collect it point source. And of course, that's just a patch solution if you don't have the means or the ability to fix the tap right away. And that, of course, is point source carbon capture.

And then the third way you could go about fixing this problem is mopping the water off the floor. And in this case, the analogy is director carbon capture, which is collecting the water in a much less efficient, more energy-intensive, more dilute way. And I don't think it makes sense to mop the water off the floor unless you've done steps one and two to mitigate the stem of the flow of CO2.

David Roberts

I just want to mention these materials you're talking about adding to the process.

Leah Ellis

Yes, that's important.

David Roberts

Just to explain that. So you want the lime to react with the silicates. That's the point of all this. But through this process, you end up with a lot of lime sort of left over that has not reacted with a silicate. And so you add these supplemental materials and that kind of boosts more of the lime to react. Is that basically right?

Leah Ellis

Yeah, that's right. So that phase that I mentioned that is made in the kiln is called tricalcium silicate, or alite. And that is three calciums, tricalcium to one silica. And then that hardened phase of concrete that cement turns into as it hardens into concrete is calcium silicate hydrate. One calcium to one silica to one water, approximately. So Portland cement has three times more calcium than it really needs. Those two extra molecules of calcium are just on for the ride. They don't significantly contribute to the strength or to the durability — in fact, they make today's cement maybe less durable than, let's say, Roman concrete was.

So this actually started decades ago. Let's say in the '60s, '70s and '80s, people started using coal fly ash, which is a silicate, and blending that into cement. And at first they did it to save cost, because at that time, coal, fly ash, people would pay you to take it off their hands and it dilutes the cement, so — they did it for cost saving reasons. But really it makes better cement when you balance out that calcium to silica ratio and you also can lower your CO2 emissions. But that being said, you can only lower the CO2 emissions of your concrete by about 30% before you start having too much silica to calcium, and then that starts deteriorating your performance.

So, there's fly ash, and that is going away. So the cement industry is now moving, trying to find other sources of supplementary cementitious materials. These include natural sources of silicates like the Romans used, volcanic ash, so pumice, certain types of calcined clays. All of these are being developed as ways to dilute the Portland cement with no or low emitting minerals, which do in fact make better concrete at the end of the day. But again, these supplementary cementitious materials can't, like, they can only solve 30%. Maybe you could get to 50% if you accept some differences in performance, but they can't get you to zero because these are bringing silicates, they're not bringing that lime, and there's no way to decarbonize lime, unless I think you're going about our process, which uses renewable energy and non-carbonate sources of calcium.

David Roberts

Right. These materials you add, they're going to come back into our story later. A little Chekhov's gun there. All right, so I've made you and the audience wait for 20 minutes now. So let's talk about how you solve this dilemma. So we've got limestone that has to be broken down into lime. And right now, the only way we know how to do that is heating it up extremely high with a fossil fuel kiln, which emits things, and then you break it out of limestone and it gets emitted. So you need basically what you and your partner came to, where all of this sort of leads you as you sort of analyze this process, is we need some other way to get lime.

Basically, that's the nub of the concrete problem. Basically, we need a way to get lime that does not involve emitting a bunch of CO2 as we break it out of limestone and does not involve super, super, super high temperatures. We need a room temperature way to create decarbonized lime. So I think anybody who sort of reviews the process of how cement is made from a carbon perspective, is going to end up there, right? Lime is the thing. So then you say in your Medium post, once the goal was clear, it took us less than a year of focused effort to invent a way to make carbon neutral lime at ambient temperature, which kind of like, blew my mind a little bit.

I guess it's just that no one had looked before. Or it's just crazy to me that once you started looking, there it is. Which to me indicates that they must not have been looking very hard before.

Leah Ellis

I think that's true. I don't think people had been looking very hard before, maybe for two reasons: So, one reason is that never before have we had a pathway to low cost renewable energy. Which is why something like this may have been invented decades ago, but it wouldn't have made sense. So I like to call Sublime the electric vehicle of cement making, because we're replacing these high temperature fossil fueled kilns with something at ambient temperature that avoids combustion but —

David Roberts

I love this. Can I just pause and clap here? Because I swear, like, almost every one of my pods, no matter what technology you're talking about in terms of modern technology you're talking about, it's super, super, super cheap renewables that are unlocking these possibilities, like one after the other. Stuff that it would have been too energy-intensive and expensive to do up until very recently. But renewables getting so, so, so cheap is just like one after the other, unlocking all sorts of possibilities in all sorts of areas. You wouldn't even necessarily think, right, like cement. The first thing you would think is like, renewable energy can do that, right?

But it turns out there it is again. There's cheap renewables coming to our rescue yet again. Sorry, go ahead.

Leah Ellis

Yeah, well, totally. I mean, it expands your toolbox and just like an electric vehicle, if you were to power your Tesla with a coal fired power plant, I mean, there's really no point. Same with us. We use the same or similar number of kilowatt hours in terms of energy for our process. But if you're going to power that with a coal fired grid or just fuel it with coal directly, there's really no point. And so I don't think I'm really that smart. I think someone 30 or 40 years ago would have arrived at this conclusion. I think it's obvious in retrospect.

David Roberts

So many discoveries seem that way, right?

Leah Ellis

Yeah. And then the other reason why I think my co-founder and I were special in being the ones to discover this obvious in retrospect pathway is that we blurred the lines. I think many people, especially in academia, they're sort of pigeonholed into their area of expertise. And I don't even credit myself for this move. I think it was my co-founder, who's a tenured professor at MIT, very celebrated. He's already made his career, so he's quite comfortable putting it on the line, being exploratory and cross-pollinating two disciplines that don't see each other. Like civil engineering, is where cement lives.

Electrochemistry is in material science or in the chemistry or physics departments, and those often don't touch. And the grad students may not even meet each other on campus. And so I do think it required a certain degree of boldness and courage on the part of my co-founder. And I was just trying to make him happy and be his conspirator and co-inventor, but that original concept definitely came from him.

David Roberts

So let's talk about how you do it, then, how you make lime with an ambient room temperature process that does not produce CO2. It turns out to involve another theme that comes up a lot on Volts, turns out to involve an electrolyzer. So, walk us through a little bit how you end up with lime.

Leah Ellis

Yeah, and I have to say, it may be quite simple in retrospect, although there's a lot of high tech technology and patent applications will be filed. But at a high level, it is so simple. I can explain this to someone with high school chemistry. So, when you do water splitting, you make hydrogen at one electrode, oxygen at the other electrode. But if you start with neutral pH, water or a brine, you make acidic solution at one electrode and an alkaline solution at the other electrode as a function of that water splitting reaction. Now, once you have that pH gradient, you can use it to drive a chemical reaction.

In this case, the dissolution of calcium from a mineral. If you're using limestone as a source of calcium, which we don't, you can think of this as like a Mentos and Coke reaction, where you have a carbonate reacting with acid to dissolve those minerals and release the CO2. So we could use limestone, and we could capture all that CO2, which, just like the Mentos and Coke, the CO2 would be cold, clean, and compressed. If we do this in a pressurized unit, and we could likely collect a lot of money for 45Q tax credits. But we chose to abandon that path and go for that true zero approach of using a non-carbonate source of calcium.

So, there are many calcium silicates, both natural minerals and also waste materials that are calcium silicates. We can dissolve the calcium from those minerals, leaving behind a reactive silicate. Those calcium silicates are inert. You put it through a reactor, the acidic side of the electrode dissolves the calcium, leaving behind a — so we've broken those inert bonds — leaving behind a reactive silicate. Then the calcium swings over to the negative electrode, precipitates as a hydroxide. Now you have reactive calcium, aka lime, and you have a reactive silicate and you dry those off into free flowing powders, blend them together, and now you have a cement that is very much like Roman cement where Romans, they didn't make Portland cement at 1400 degrees Celsius.

They made what's now called a pozzolanic cement, where they took volcanic ash, the silicate, and they took lime, which they made at 900 degrees Celsius, that first step. And they blended those together with water to make a cement that has evidently stood the test of time. So we're following that old recipe.

David Roberts

It's all lime, silica and water.

Leah Ellis

At the end of the day, yeah.

David Roberts

At the end of the day you're getting your lime. So you're precipitating lime out of lime containing calcium silicates.

Leah Ellis

That's right.

David Roberts

And so that would be your raw material instead of limestone. So just on the materials side, you're going to need a lot of whatever that is. What is that going to be? What is that material that you're going to break the calcium out of that you're going to be able to find in bulk that does not contain CO2?

Leah Ellis

Yeah, broadly, it's basaltic minerals. So anything with 10% calcium, more or less could be a suitable input material. So basaltic minerals, this is the type of rocks that are typically used as aggregate in concrete. So we can almost use the same supply chain that people use for aggregate can be used as input for our material as well.

David Roberts

So there's no choke point there. That's an abundant material. There's no worries about finding enough stuff, basically.

Leah Ellis

Yeah, that's right. There are no worries about finding stuff. Although of course, even for cement plants today, making sure that your quarries are in close proximity to your market, it is a big exercise to site a plant and to minimize transportation costs because cement is so massively produced, which makes it very cheap and it's also very heavy. So there is a game here that we are also playing alongside any other cement company about siting your plants in proximity to market or in a way that you could float it by boat, since that way it travels very inexpensively. So you find cement plants on the Mississippi River, on the Hudson, near ports where the cement can float.

David Roberts

Got it. You know, you're talking about using just sort of aggregate, just sort of like junk rock as an input. Is there any input you could use where getting rid of it would actually be an extra value stream? Like some sort of waste that you could dispose of by doing this.

Leah Ellis

We have looked at loads of waste. So we have an ARPA-E grant that looks at ponded bottom ash. So there's billions of tons of ponded bottom ash. Like historical. So this isn't the fly ash, the stuff that comes out of the top of the smokestack. This is like the bottom ash, like the stuff with heavy metals that sort of is at the bottom. It's the icky stuff that they've ponded for hundreds of years. So there's billions of tons of this that needs remediation. This is the stuff they'll pay you to take it away. And with our process of dissolving the metals, extracting it, leaving behind a silicate, and then precipitating the metals and isolating them, could be a way to clean up bottom ash.

We've also looked at other things, like you could even use our system to recycle demolition debris. So dissolving the cement and turning it into fresh cement. We've looked at eggshells, we've looked at municipal incinerated waste, we've looked at clamshells, I don't know. For s***s and giggles we've tried out a laundry list of things to make sure they work. But really what we're going for is to optimize our process around materials that are widely abundant, and especially in places like India and Africa. Because Sublime's stated mission is to have a swift and massive impact on global CO2 emissions.

And we know that both India and Africa are going to undergo a period of dirty growth if their population is going to grow and enjoy the same quality of life that we do, and if they build carbon intensive cement and steel plants. Those plants are designed to last 50 to 100 years. So if we work quickly to scale up this technology, we want to make sure that all greenfield cement plants in these developing regions are made with technologies that will avoid CO2 emissions for decades to come. So we really have a small window of opportunity to really get ahead of generations worth of CO2.

David Roberts

Yeah. So you need a cheap and abundant feedstock, which makes sense. But getting back to this, like, say you're using bottom ash, the way you're getting lime out of this stuff is that lime precipitates out at a particular pH level, right? Sort of like a threshold. And presumably other chemicals will precipitate out of that mess that you throw in there at other pH levels.

Leah Ellis

Different pH. Yeah.

David Roberts

Right. So could you, if you're just dumping a bunch of sort of bottom ash slurry into your machine, you can pull out the lime. Can you also pull out some of the other, like those heavy metals you're discussing that we don't want in our water or soil? Could you precipitate those out too? Like, can you isolate other things other than lime if you wanted to repurpose this in the future for some reason?

Leah Ellis

Yeah, you could. And in fact, if we're using basaltic minerals, those aren't only containing calcium and silica. Those are also containing iron, magnesium, aluminum, which we are isolating as a function of our process, like you say. And what my director of R&D likes to say is, "We use the whole rock just like hunters use the whole deer." So all of these things are coproducts. And in fact, things like magnesium are actually worth more than cement.

David Roberts

Oh, really?

Leah Ellis

We're actually not — yeah, I mean, it's a fine chemical.

David Roberts

Are you pulling out enough magnesium out of these materials that that could be a —

Leah Ellis

So it could offset the cost?

David Roberts

Yeah, like a meaningful value stream.

Leah Ellis

I mean, we think so. Although our focus is cement, we know that people invest in us to have that swift and massive thing. So we are focused on doing this cement work. And everything else is just gravy. We actually haven't factored those costs in and done that business development work to see how much it sweetens our cost, because we're just focused on making the cement. And I have to say one of the reasons why Sublime has a cost advantage over other ways of decarbonizing cement is that, as you said before, we're not adding on to a Portland cement kiln.

Adding carbon capture is going to at least double the cost of making cement. And because our technology is carbon avoidance, it's green cement, not blue cement. We are replacing the kiln with a new technology. Now, we may not be able to compete on cost with our first plant because cement has undergone almost 200 years of improvements and redesigns of the kiln where they had wet kilns, semi-wet kilns, and dry kilns. And finally, these five-stage preheater, precalcinator kilns, which I won't get into, but they've undergone tremendous efficiency improvements over the past 200 years. And so we think we can get really, really close.

And I don't think any other technology can do that because they're always adding on to a Portland cement kiln.

David Roberts

I mean, it seems like if you could sell the magnesium, you bring down the cost of your cement and make it more competitive.

Leah Ellis

Totally.

David Roberts

So instead of a kiln, you have an electrolyzer, which does not require high heat. All this process you're talking about with the metals precipitating out is an ambient room temperature process. Yes?

Leah Ellis

Yup.

David Roberts

And you end up with lime. And then you can mix lime with the silicates that you also got out of your raw material. Or do you have to bring in extra silicates to mix with the lime? Once you've got the lime. And what are they?

Leah Ellis

No, we're using our own silicates. And of course, if we want to get the desired calcium to silica ratio, it may mean blending in certain inputs to get to the right ratio. But yeah, it's just a tweak to get the input ratios to match the output ratios of what we want.

David Roberts

And so this cement that you are producing with electricity and non-CO2 containing rocks, I assume you have produced it. And it has been through whatever tests, whatever governing bodies put cement through?

Leah Ellis

It has. So our cement meets what's called ASTM C1157, the performance-based specification for hydraulic cement. This was a standard that was created 30 years ago. And really this standard is so important because previously cement was defined by the crystal structure of what was made in the kiln. So that alight tricalcium silicate phase that you can only make, it's only thermodynamically stable at those volcanic temperatures. So you cannot solve for alite, you know, unless you go through that thermal process. So, actually, when we were at MIT, my co-founder and I, we believed that if you weren't making Portland cement, nobody would use it.

That was the prevailing myth at the time. I got out and networked with the industry and spoke to the customers and realized that people who work at a readymix concrete producer, those folks running the spinning trucks, they don't know what alite is. They've never heard of tricalcium silicate. They talk about slump, they talk about alkali silica reactivity and set time and finishing properties. And so we realized that it was the performance of this cement that matters. It's not the chemistry and the standard bodies have prepared for this moment. And all of this work with supplementary cementitious materials, which are gaining traction, especially in Europe, is just winds of change that are blowing in the right direction.

Because as you start blending cement with other things, it becomes much less about the chemistry that you're making in the kiln and more about the ultimate performance.

David Roberts

So the standard is about performance.

Leah Ellis

There are multiple standards. So there are three standards baked into the international building code. One is for pure Portland cement. The second one is 50% Portland cement, 50% SEM. So a blended cement. And then the third one is pure performance-based. So we don't care what you do, as long as whatever you have performs this way. And there's an exhaustive list of tests that you have to do. You want durability and strength and freeze-thaw and chloride ingress and all these different things that you test to make sure that your cement is performant. So we've done those tests.

We do them in-house. We have a world-class cement testing lab, but that's not good enough. We send it out to an accredited third-party lab, we test it again, and then — just so that we don't waste our customers' time — and then the third test is done by our customers. And that's really the test that matters. And so this quarter, we finished our pilot plant early January this year, and then spent a few months optimizing our production, ramping it up, testing it in-house, testing it with the third party, and now we're testing it with the customer, and the results have come back very strong, positive, and we're working with some of the best people at Boston Sand and Gravel.

David Roberts

Well, presumably, what you would want to hear from a customer is just nothing. They just used the cement and it worked fine and —

Leah Ellis

It works exactly.

David Roberts

There's nothing to report. I said those supplementary —

Leah Ellis

Cementitious materials.

David Roberts

Yes, thank you. I said they would come back into the story. Those supplementary materials that are being added into traditional cement to incrementally lower its carbon intensity, those are the silicates that you're using to mix with your lime. So you're actually just using the lime and the supplementary materials and just skipping, basically, the Portland cement kiln part of it.

Leah Ellis

Yeah.

David Roberts

And so you've produced this cement that is now passing performance tests being used in the field. It's working well. One point I did want to make before we leave behind the process part is if you use a feedstock that doesn't have CO2 in it and you use renewable energy to power your electrolyzer, then you've got bona fide zero carbon cement, the holy grail. But you make the point. And I think this is maybe just worth pointing out. Even if your process uses limestone as its feedstock and thus has CO2 in it, and even if it uses average US grid electricity.

Right, which is not clean. Even if you do both those things, you still end up with cement that's lower CO2 than conventional cement.

Leah Ellis

That's right.

David Roberts

So basically, switching to this process immediately improves the carbon performance of cement, even if you don't have your electricity and your feedstock dialed in perfectly yet.

Leah Ellis

That's true, although we will accept nothing less than perfection.

David Roberts

Well, you say in the medium post that you're currently 70% decarbonized. Does that just mean there's still some limestone in the process?

Leah Ellis

Yeah, exactly. But it is our goal to get to 90%. And so when we model our megaton plant with optimized feedstocks, optimized electricity, we just did a lifecycle assessment with Climate Earth, which is the cement industry's preferred lifecycle assessment partner. So we did show that we can get to 90% lower emissions than today's Portland cement using our process at scale.

David Roberts

Why 90 though? What's the 10% left over?

Leah Ellis

Oh, I believe that was from the mining and transportation. And those are sort of outside of our gates. So again, if you're traveling by truck or going back and forth between the quarry and the kiln and that type of thing.

David Roberts

Ah, interesting, interesting.

Leah Ellis

But I'm sure those will go to zero too, as all those heavy trucks get electrified.

David Roberts

So, your big plant that you are envisioning here, your mega-scale plant, your full-sized plant to be certified as basically super low carbon, let's say 10%, 90% carbon-free, you're going to have to find the right quantity of feedstock, a non CO2 containing feedstock, a steady, large supply of that, and you're going to have to somehow certify that your electricity is completely clean, which is going to get you in the same sort of tricky situation that the hydrogen people are in right now, which is how do you certify that electricity is totally clean? You can't just use grid electricity, right? Because then you're taking the clean electricity from someone else. You basically are going to have to like — it's additionality.

You end up with the additionality problem. I don't know, that's probably way down the road for you. Maybe you haven't even thought about that.

Leah Ellis

Well, we do think about this. So we have a kick-ass head of project development and strategic sourcing. So shout out to Becky and she's doing the work to cite our megaton plant. And we're looking at places that have renewables. And I know the hydrogen folks, as you say, struggle with this too. So we require — anything six cents a kilowatt hour or less is fine with us. And we're looking to site first in places with a high degree of renewables. And again, it doesn't have to be zero, although zero would be great, but it's just as low as reasonably possible.

David Roberts

Would you ever build your own renewables attached?

Leah Ellis

Yeah, we might. We're looking at that too. And then to your question of additionality, we're actually gearing up to publish something recently that shows that because Sublime Systems is a great way to use a renewable electron because it has similar embodied energy as today's kiln. But you're not just using that renewable electron to replace a fossil electron, you're also getting avoided carbon through avoiding the limestone as well. And so when you calculate the amount of CO2 avoided per kilowatt hour, you compare that over many other technologies using that the highest and best use of a renewable kilowatt hour could be for Sublime cement.

David Roberts

Interesting.

Leah Ellis

According to our calculations.

David Roberts

Interesting. So let's talk a little bit about scale and about cost. Right now you've got basically a little lab set up. You're producing this stuff, but on a very small scale.

Leah Ellis

Well, I wouldn't call it little. It is a fully functioning, continuous pilot plant that can do up to 250 tons a year. But it is small. And we affectionately call it our cement plant for ants, if you know the Zoolander reference.

David Roberts

Which is just to say that scale will bring lower costs.

Leah Ellis

But totally, it's all about scale.

David Roberts

Right. Insofar as you know, the costs now, currently, because as you say, traditional cement is very cheap. So are you currently in the ballpark or what's the sort of timeline? How do you see getting into the ballpark? Where are you now and where are you kind of targeting?

Leah Ellis

Right. So where we are now is definitely less than cement plus post-combustion carbon capture.

David Roberts

That's a low bar, but yes.

Leah Ellis

Well, is it a low bar? So one of the things is that cement is very inexpensive, and that is a double-edged sword for us. So that makes it very difficult for us to compete on cost with a new technology that hasn't yet achieved scale. But on the other hand, that makes it — the so-called green premium — really that small. Especially when you compare it to other things like direct air carbon capture or sustainable aviation fuels. And also, if you think about the total installed cost of concrete. So 80% of the total installed cost of concrete is labor, and it's often unionized labor.

The cement is 10% of that, and the aggregate is the remaining 10% so even if we were to charge a two to four x green premium, really, it doesn't really affect the overall cost of a building that much.

David Roberts

Right.

Leah Ellis

But your carbon savings are huge. So, again, if you're thinking all across clean tech, that green premium or the carbon credits, if you think of dollars per ton of CO2 captured or avoided, I think Sublime cement is really attractive. When you do that analysis, especially as we come to scale.

David Roberts

How big does the green premium need to be currently to bring you down to Portland cement costs?

Leah Ellis

Yeah. So presently we're scaling up. I mean, I don't even dare calculate what it costs at pilot scale because it's so handmade. But at this next level of scale that we're doing. So between where we are now, which is pilot scale, and where we need to be at megaton scale, we do have to build what we're calling the kiloton plant.

David Roberts

An in-between plant.

Leah Ellis

Exactly. An in-between plant. And that is to validate the product, the process, the economics, the performance, to enable us to attract low-cost project finance so that we can build the megaton plant, ideally many megaton plants, and work with partners to deploy this all over. So at that megaton plant, you don't have the economies of scale that we would have at the megaton.

David Roberts

Kiloton

Leah Ellis

Yeah, at the kiloton. You know, we like to say the megaton plant is like achieving Costco level status. And at the megaton plant, I do think we'll be able to compete on costs.

At the very least, we'll be able to undercut cement decarbonized by post-combustion carbon capture by a long shot. So I'm very confident saying we'll be the cheapest low carbon cement, especially after we crunch the numbers with all the co-products that you mentioned. But at the kiloton scale, I think this is at the farmers market pricing. So the people who will be taking offtake from our cement, from that plant, they will realize that they're paying a higher price for something that's special, for something that comes with the added value of a lower profile of harm against the planet.

And the premium that they're paying is reinvested into our technology, into the people that are making this happen, and that it is very much catalyzing this technology that I believe will be obvious in retrospect in a post-carbon world.

David Roberts

Right. Well, they'll have warm feelings, but I think there are also — we'll talk about this in a second — but there are also some entities who are under sometimes self-imposed, sometimes legally imposed requirements to find low carbon cement. Some people are doing it because they need it, not just because they'll feel good about it. So, scale wise, the kiloton plant is roughly when?

Leah Ellis

So that depends on a lot of things, but we're ready to have that. We are on track to have that built and commissioned and operational by early 2026. But of course, that timeline requires everything to go perfectly. But we're all geared up for that.

David Roberts

And that will be a working, like, you'll be selling concrete. It'll be a full-size selling concrete plant. No longer just a test or a demonstration.

Leah Ellis

That's right. We'll be selling cement to concrete producers.

David Roberts

And then, assuming everything goes well. Of course, everything never goes well. Assuming everything goes well, you have your kiloton plant in early 2026, you're selling cement, it's working, your customers are happy, your financiers get more confident. They extend to you some low-cost capital for you to scale up. Then you build your megaton plant, everything goes well. When does that happen?

Leah Ellis

That could be as soon as 2028. Again, these are very aggressive timelines, and this assumes everything goes well. But once that kiloton plant is built and validated, assuming we have sites ready to go, those megaton plants could be built as quickly as within two years.

David Roberts

And then a megaton plant produces how much?

Leah Ellis

A million tons a year.

David Roberts

A million tons a year. And so to replace any meaningful amount of cement supply, you would need a bunch —

Leah Ellis

Hundreds of them.

David Roberts

Hundreds of those eventually. Have you thought about, I mean, you're selling cement. Have you thought about licensing the process and just having other people work on joining you and trying to scale up?

Leah Ellis

We do all the time. So, like I said before, we're capitalists around here, as you know. So I hope we'll make a healthy profit for the investments. But the goal is to have a swift and massive impact on global CO2 emissions as quickly and as bigly as possible. And I hope we'll make money along the way, and I expect we will, because that's how value is measured. But really, it's all about speed and scale, because we could do this ourselves. But really, I'll just be honest here, we're nerds. What gets us fired up is this technology and the product that we're making.

There's a lot that we don't know. One of the reasons why the cement industry has taken so long to change is because innovation is not part of their DNA, per se. They run operations like nobody's business. They keep these megaton plants up and running around the clock with high-quality, reliable material that's distributed in the most efficient way to get everything built with minimal disruptions. It's really, truly remarkable.

David Roberts

Yeah, you pretty quickly get beyond the science part to just logistics and stuff like that. There's like a whole set of those considerations for large-scale businesses that are unique to scale, I think.

Leah Ellis

Exactly. So the operational know-how is there. And I think we would be foolish to try and reinvent the wheel entirely by ourselves. And I mean, would we be able to build? So let's just say we address half a gigaton of CO2 emissions per year. So cement is 8%. Let's say we set our expectations low, aim for 1% of CO2 emissions or half a gigaton. That's like 500 megaton plants. So how will we get to 500 megaton plants by 2050, if that's our goal, working backwards, we would need at least ten up and running by 2030, let's say.

So I really think if we're going to get this s**t done, we have to take what we do well and merge it with what the industry evidently does well, which is the operational logistics and know-how. And so, we speak to cement majors almost every week, and they're coming to us, and they have a legitimate desire to not be the world's largest CO2 emitters. And this is also an interesting fact. So cement companies, they have the highest scope one emissions of any company. You would think it's Aramco or Exxon, but no, they're making fuel that other people are burning. It's cement companies.

David Roberts

Yeah. Scope three, that gets the oil companies.

Leah Ellis

Yeah, totally. But cement companies, it's their scope one, so they have a great big X on their back, especially in Europe.

David Roberts

Well, it also seems like, I know there are improvements. It's not fair to say that you're sort of stuck with what you're stuck with when you're using fossil fuels. I know there have been improvements in the kiln process and in the input and the kind of kiln, but there's a certain sort of baseline, like when you're using limestone and you're using Portland cement process, there's a certain floor, right? Whereas when you're using electrolyzers technology and you have a bunch of different feedstocks to choose from and a bunch of different metals that you can pull out of those feedstocks, it just seems like you're opening up a lot of new areas for innovation.

Basically, the runway for cost declines seems huge here.

Leah Ellis

Yeah, I believe so too. And I think what's really exciting is that if Sublime succeeds, and I believe we will, that technology that we're starting to build now, and that will continue to improve over time, will be the way we make cement for the post-carbon world, like, for the next millennia. So I feel like this is a really cool place to be where cement innovation happened with the Romans. And we still go back and visit those monuments that they made. And then cement was innovated a second time by the British with Portland Cement. And now we're creating new cement in America that's for this post-carbon world.

And so often when you think of buildings and the architecture, the way you make a building is often a representation of the values of the people who've built it. And so I think it's a very exciting time for us to be making this new material and to be incorporating it into the most cutting-edge buildings that we're building today. For some of the best and most forward-thinking companies.

David Roberts

These electrolyzers, are they a special kind of electrolyzer that you need custom built, or are these just normal, off-the-shelf electrolyzers? What is there to say about the electrolyzers?

Leah Ellis

They leverage a lot of existing technology. So electrolyzers broadly are very similar. You've got electrodes, often with a carbon backing, often with a rare earth catalyst, you've got separators. There's kind of like assembling a sandwich of electrode, separator, electrode. And so many components of our electrolyzer exist and have been scaled up to megaton scale, and we're simply taking that and leveraging that for our own purposes.

David Roberts

So like your own kind of sandwich, but made with standard ingredients.

Leah Ellis

Exactly. We're taking the bread and the mayo and the lettuce, and we're just slapping them together in our own special way.

David Roberts

Right, but in terms of electrolyzers for your process, you're the one making them. So that'll scale up too, and presumably get cheaper too.

Leah Ellis

Yeah, exactly. And I think many of the trends we see with hydrogen and all these other electrochemical things taking off, I mean, we do anticipate that as well — we've got the dropping price of renewable electricity, especially intermittent renewable electricity — and I believe we'll also see electrolyzers come down in price as well.

David Roberts

Yeah, yeah. I talked to the woman in Iceland who's making ammonia directly with electrolyzers. I clearly need to wrap my head around electrolyzers better and do a whole episode on electrolyzers, because I had thought they were just for hydrogen, but now people are doing all sorts of other things with them. Anyway, so final, I promise, question: What sort of policies would help you? I mean, I think some of them are obvious, just like lowering, requiring lower CO2 emissions, maybe requiring government procurement. Like government could say, we're going to buy lower cost cement. What are the other sort of big buckets of policy that you think would help accelerate this process?

Leah Ellis

Government procurement would go a long way because the US government, in its various forms, army, GSA, federal highway administration, DoT, procures 60% of the cement produced in the US.

David Roberts

Oh, wow.

Leah Ellis

So if we don't have a commitment from them that they want this cement, it makes it more than twice as hard. But the policy change that I would really like to see, and what really grinds my gears is that when you look at tax incentives and production tax credits and also the voluntary carbon market, they reward disproportionately direct air carbon capture. For example, I'm forgetting the acronym. Is it 42? Oh my gosh. Can you remind me what?

David Roberts

45Q? That's the carbon capture one.

Leah Ellis

C three. Anyway, the carbon capture one, it's $180 a ton for direct air carbon capture, $80 a ton for point source carbon capture, and zero for carbon avoidance. And we applied for a production tax credit for our low carbon production of cement and we were discouraged from submitting a full application. And I think it's unfair, using that leaky tap mop bucket analogy, that you should be incentivized more for mopping up CO2 than for fixing the tap. And I believe there should be a technology agnostic implementation of the tax credit to put all of these things on a level playing field and then let the invisible hand of capitalism choose the most efficient way to decarbonize.

I don't think the government should be picking winners.

David Roberts

Well, this is the danger of industrial policy, right, is that once you get down in the gears, you're going to inevitably sort of get some things and skip other things and get out of balance. Presumably there are other policies besides carbon capture, policies that would help you, right?

Leah Ellis

Yeah, I actually think direct carbon capture is great. I'm definitely not anti-direct carbon capture. I'm just advocating for carbon avoidance to be valued the same as a ton of carbon removed from the air, especially when the carbon avoidance is technology enabled, like Sublime Systems. I think there's a role for this premium, this farmers market stage that we're at, to get that added boost so that we can get to a stage where we don't need that incentive anymore. But you will never get there with DAC. It'll always need to be propped up somehow.

David Roberts

Yes, I know. Have you gotten help from the LPO? From the loans office?

Leah Ellis

No, they haven't helped us yet. Although had some interesting conversations with Jigar Shah and others from the LPO. And I do hope that once our kiloton plant is built, we will definitely work with LPO.

David Roberts

It seems so obvious this is such a huge opportunity here and exactly at the stage of business that they're designed to help. Okay, so some sort of production tax credit would be great. Some sort of government procurement strategy. Is there anything like that in place? Is there demand pull? I know that a bunch of companies are saying they want to reduce their own emissions. A bunch of concrete companies are saying, where are we at on demand pull? Like, is there a big developing market for low carbon concrete that's going to help pull you through this?

Leah Ellis

I am seeing huge demand for low carbon cement and I would say that our customers are leaning into us just as much as we're leaning into them and they want more of it sooner. It's one of those things, though, that's difficult to manage. And I think maybe like setting up a carbon market, people are not used to seeing new products in the cement world and so getting these advanced market commitments and these offtakes. Right now, when people build a new cement plant, cement is vertically integrated with concrete, oftentimes. So you never need an offtake for building a new cement plant and even the supplementary cementitious materials.

The industry just knows that if the product meets a certain quality, that there will be, at a certain price, it will be bought.

David Roberts

Right, right.

Leah Ellis

So these offtake agreements have sort of never been done before until now. But, yeah. so we're still in the stage of figuring that out and standardizing those agreements. And one day I hope there will be a sort of book and claim model for selling our material as well as other low carbon cements.

David Roberts

Yeah, it does seem like, especially once it's wider known that this is available and that it performs as well, it just seems like there's just going to be huge demand from every which way. Leah, this has been absolutely fascinating. I've been meaning to dive into concrete forever and this was a good excuse to learn. It's funny. I'm going to finally learn how it works right on the cusp of it changing to a new process. But thank you so much for — I feel like you've really found the right target here. This is such a leverage point for so much change.

So thank you for coming on and walking us through it.

Leah Ellis

Thank you so much for having me, and for being an enthusiastic listener as I talk about my favorite subject for a full hour.

David Roberts

Awesome. That's what we're here for. All right, thanks so much.

Leah Ellis

Thank you.

David Roberts

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