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This is the Discovery Files podcast from the U.S.

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National Science Foundation.

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Concrete is the most widely used construction material in the world.

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It is the backbone of what is called the built environment.

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A crucial ingredient for housing and infrastructure development.

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Advances in materials science and processing can enhance the long term

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durability of many building materials, including concrete,

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enabling significant economic and societal benefits.

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We're joined by Sabbie Miller, an associate professor in the Department

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of Civil and Environmental Engineering at the University of California, Davis,

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whose research is dedicated to advancing the built environment

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through the development and optimization of infrastructure materials.

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Professor Miller, thank you for joining us today. Thank you so much for having me.

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So I want to start with kind of defining one of the terms

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that’s kind of key to what we're going to be talking about today.

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What is the built environment?

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So folks do use the built environment in a couple of different ways.

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But typically what they're referring to is basically all of our buildings.

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So houses apartment buildings, hospitals, school buildings, offices,

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as well as all of our infrastructure systems and roadways.

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So sewers and highways and all of those things together

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are the environment that we as humans are building for ourselves.

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And so it's often referred to as a built environment.

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The most common material is concrete.

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So we're going to be talking about a lot of concrete today.

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I want to start with the manufacturing process.

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What are kind of the problems with manufacturing currently?

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So there's a couple of things to think about with concrete.

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Yes, it is our most consumed building material worldwide.

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It's actually a composite material. So it's made out of cement, water

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and crushed rocks, which we refer to as aggregates.

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So sometimes cement and concrete are used as synonyms.

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But in reality, cement is this powder that reacts with water and holds together

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rocks to make the synthetic rock that we refer to as concrete.

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And some of the manufacturing challenges associated with the production of concrete are actually tied to that cement.

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So when we're worried about things like the environmental burdens

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for concrete, it's a function of a couple of things.

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One, we use a heck of a lot of this material.

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So whenever you use a lot of something, it's impact scale accordingly.

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The other side of that being that the production of cement

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requires the utilization of limestone, that's our main ingredient

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in the production of cement. There are other ingredients as well.

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But that limestone, to create that reactive compound that could interact with water,

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we actually have to carbonate it. We actually have to break off effectively carbon dioxide from that limestone.

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And that leads to a direct emissions from a chemical conversion.

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And then on top of that, to get the reactions to take place,

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we require thermal energy. So then we also have energy derived emissions tied to the production

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of cement. So when we're talk about the impacts of concrete,

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oftentimes the impacts are things tied to cement that we're worried about.

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We hear about a lot of buildings when they get torn down

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going straight to the landfill. Can we crush them and reuse them?

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As an aggregate, let's say in any new concrete?

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Or is there issues with that carbon cycle and the bits of the cement that you just

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explained chemically that make it so we can't really do that?

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It's a yes and no response to that. We can crush our concrete and get some benefits out of that product.

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One of the benefits, actually, is that we really increase the surface area to volume ratio.

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And if we have proper exposure to atmospheric CO2,

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the hydrated cement in that concrete actually has a chance to interact

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with the CO2 that's in our atmosphere and pull a wee bit back out.

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The other thing, though, that we end up seeing from our crushed concrete is exactly as you mentioned.

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It's made out of hydrated cement and aggregates,

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which means it's not the same performance as the aggregates alone.

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It's now got this like caked on hydrated cement stuck on the material so it doesn't have the same performance

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as our normal aggregates that we would normally crush and use in our concrete.

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As a result of that, it doesn't always necessarily perform the way we want it to in a new concrete,

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but we are able to use it for sure in certain applications.

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So for example, in California, where I am, I know that our Department

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of Transportation actually uses the concrete from the roadways that we have,

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crushes it up, and uses a road base for the new roadway

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that they're about to put in, because it has great performance characteristics for that.

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So we can use it in some ways, but not all ways.

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So thinking about potentially reusing it for structural purposes,

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it might have a weaker fracture value or something.

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Yeah. The paste itself can have certain performance characteristics

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associated with it. The cement that's interacted with the water and created this kind of binder material,

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the aggregate is going to have certain performance. And then there's what's referred to as the interfacial transition zone.

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This region where the cement is actually bound onto the aggregates

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that has its own micro structure associated with it.

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So yes, we end up with potentially different fracture performance associated with that.

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Other types of durability issues can also be tied to having

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this effectively, like a three phase material as opposed to just the aggregate.

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So yeah, there's a couple of different issues that could happen that might hinder use in certain applications.

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So the paper that kind of brought your work to my attention

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is looking at using building materials for carbon storage.

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So how might that be possible?

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There's many different ways. So many of our building materials are already carbon based right.

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They're already using carbon. It's not necessarily carbon that we've pulled out of the atmosphere,

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but it is carbon. So part of what could happen is if we're able to re-engineer

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those materials such that the carbon that's in the material is coming from a direct air capture system, or we're basically pulling

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atmospheric CO2, concentrating it and then using it in those materials.

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Then we could potentially store them for a really long time in the built environment.

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The other way we could do that is capturing flue gas.

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So carbon capture, actually trying to capture the CO2

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as it's coming out of industrial processes, energy generation processes,

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and then again, concentrating it, using it in our built environment

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as the carbon source and potentially even having the materials

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themselves directly capture carbon dioxide from the atmosphere.

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This happens a little bit more with our biogenic materials, materials

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that are living organisms, like our photosynthetic materials.

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So if they're able to already interact with the atmosphere

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through photosynthesis, they can pull in carbon.

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And then we could use that material as something in the built environment.

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So there's a couple of different ways to get that carbon stored in our materials.

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But it depends on the material that you're talking about.

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I know part of the study you were looking at the effectiveness

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of different kinds of blocks like concrete, brick, asphalt, plastics, woods.

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Can you talk a little bit about the effectiveness of these different forms?

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Yes. So depending on the amount of carbon that you could put in any material,

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you're going to have a certain degree of potential effectiveness

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based on the carbon content. Right. So this is a very high level of carbon for this material.

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So in theory we could store more carbon in this material on a weight basis.

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The other thing though that we found was a much larger driver than that

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component was how much of this material do you plan on using?

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So even if you're getting less carbon stored in the material,

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but you could scale it to an enormous quantity, like concrete,

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then potentially you have this huge body that could store carbon dioxide

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as long as you're getting desired performance out of the material.

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So if the material has any loss in strength or the ability to place it

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during construction, or if it fails earlier than our conventional material,

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then you could end up not seeing these benefits.

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So the performance of the material is always priority.

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The idea, though, is that if we're able to get equivalent

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or better performance out of these materials and store

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atmospheric CO2 in them, we could have this really net benefit

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of leveraging this huge mass of materials that is available to us.

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One of those materials that I was curious to ask you about is bioplastics,

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because we hear about plastic and think about it being like more of a manufactured toxic thing.

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What is a bioplastic? So bioplastics are plastics where the carbon and the long

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chain molecule that makes up our plastics is coming from a bio resource

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as opposed to a petroleum based resource.

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So the vast majority of our plastics that we interact with are coming from

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petroleum based resources. That's where their carbon is coming from.

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But instead, if we're able to use things like food waste, not priority food,

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not food that we would otherwise eat, but waste that we would have otherwise

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disposed of, or residues from, different agricultural or forestry practices.

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So a biomass that we otherwise need to get rid of isn't

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going to be a priority for something else in terms of its utilization.

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If we could use those as the source of carbon,

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then we could potentially reduce our dependency on petroleum

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and leverage a bio resource that has pulled CO2 out of the atmosphere.

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We might have access to it a little bit more

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locally because of the ability to use different types of bio resources.

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So there's a couple of different strategies that one could use, basically leveraging

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that source of carbon to replace

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our more petroleum based carbons and those materials.

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Would this be like, say in the corn industry, like the husks or something that probably

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you don't have a lot of use otherwise but could potentially be used in this way?

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Exactly. So it's not the kernels that we would otherwise want to eat.

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But yes, the husks, straw leaves, all of those sorts of things

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that would otherwise be cultivated but not necessarily have as much value.

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Now, I will note that we don't want to take all of that off of the farmland,

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because farmers actually do need some of those nutrients to go back into the soil.

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But if it is already being removed, then that type of biomass can be really valuable for products like bioplastics.

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Interesting. One of the other things I wanted to ask

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you about was geo polymers and their use in kind of alternate cements.

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Can you talk a little bit about what these things are?

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Yeah. So there's a class of alternate cements that are referred to as alkali

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activated materials. Basically it's leveraging two main components a aluminum silicate

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solid precursor. So something that has a lot of aluminum and silica in an amorphous

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kind of like a chaotic, crazy state, not very well

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aligned in a crystalline state, along with alkali activators.

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When those are combined appropriately,

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then we can actually create something that acts like a binding material,

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just like our regular conventional cement with water.

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So we're basically able to replace our normally highest CO2

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component of concrete with something that doesn't require the same decarbonization of limestone, nor

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does it require that same energy demand that our conventional cement requires.

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So if we're able to replace that, there's this idea that potentially we could reduce a lot of the impacts

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that we would normally associate with our cement and our concrete.

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Geo polymers are a subclass of alkali activated materials.

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So they happen to be one of the ones that's really well studied.

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But it's in that class of materials that have these two key components

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associated with them. And there's a lot of work going on right now trying to understand

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where can we get those aluminum silicate solid precursors,

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where can we get those alkaline resources such that they are globally available

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and they themselves don't have high impact?

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Because if we have to process them a lot, then we could kind of counter our own benefits.

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That was going to be my next question, like trying to think about that for people

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that aren't too specific about the chemistry there,

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can you talk about something that might be an example of what that ingredient would be?

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So our most commonly used alkali

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activators would be things like sodium silicates and sodium hydroxide.

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We use those kind of alkali resources in a bunch of other applications as well.

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In terms of the solid precursors, a lot of work is looking at things

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like the utilization of coal fly ash and, ground granulated

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blast furnace slag, which we already use in the cement and concrete industry.

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Coal fly ash is actually it's exactly what it sounds like.

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So we use coal for the generation of electricity in many parts

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of the world, and the vast majority is carbon.

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That's what we're trying to oxidize to get our energy resources.

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But there's a bit of mineral in that coal, and the minerals are going to contribute

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to the formation of ashes, and some of the ashes

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will settle down to the bottom, others will fly upwards.

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And the ones that fly upwards are a bit fly ash.

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And those ashes actually have a really desirable characteristics

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associated with them. For this kind of perspective of utilization of aluminum silicates,

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they have great characteristics for reactivity, a nice disordered

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structure, etc.. So that's one class of these types of materials.

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The blast furnace slag that I mentioned is actually a byproduct

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of the treatment of iron oxide to form iron.

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So iron is our main precursor to the formation of steel.

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But we use iron and other things as well. Steel obviously has like a wee bit of carbon,

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so we get better performance out of that material

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when we're trying to make things like iron.

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We usually start off with something like iron oxide,

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and then in order to make that iron product,

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we actually have to send it through a furnace. The utilization of, certain types of compounds

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within the furnace, particularly a lime to purify that overall material,

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leads to the formation of a slag byproduct.

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So we're still getting our iron, but we've also got this

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byproduct associated with the general process.

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It also has fantastic characteristics the slag does for use in things

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like the production of our alkali activator materials

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or the use in concrete, because it can interact with the hydration process.

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So there's a couple of uses of these industrial byproducts

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that can get leveraged. There's also a bunch of other resources though as well.

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So a lot of our agriculture products have a wee

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bit of mineral in the biomass itself.

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So again, not the food, but those residues like the corn husks, like rice straw, etc..

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When we use those biomass products, it's predominantly carbon again.

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But if we tried to recover energy from the biomass

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through something like oxidation, we could then take the mineral compound

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that's left over as an ash form and use it as a solid precursor

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for the formation of alkali activated materials.

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So there's a couple of different sources that we have worldwide.

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And our current estimates suggest that if we wanted to replace Portland

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cement, Portland cement as our conventional cement

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with something like an alkali activator material,

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about two thirds of our current demand for cement could, in theory, be replaced.

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If we're able to leverage all of these different types of residues

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the industrial byproducts, the agricultural residues,

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forestry residues, if we're able to really utilize those properly,

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we can actually start to make a pretty notable dent in the material.

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Again, assuming that we get the right performance, we do need to engineer these things

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so that we get what we need out of the materials.

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So as you develop these kind of different materials,

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is there kind of trouble getting industry to buy in with using byproducts

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and using different kind of techniques to get at these things?

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They've traditionally used with limestone and Portland cement?

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Yeah. So engineers are very focused on performance for good reason.

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We want to create products that work.

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That is our main goal and are a little bit of tweaking to make them work

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even better is also one of our big goals.

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That said, civil engineers tend to be on the even more risk

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averse side than conventional engineers.

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We really try to make sure that things are working properly.

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The reason for that being things like life safety issues,

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you don't want to hop on a bridge and have it collapse or have a building collapse.

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We really want to make sure our systems are working incredibly well,

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and we use a few things to make sure that we are reducing risk.

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A lot of probabilistic modeling, trying to understand exposure conditions,

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trying to understand how we could best design these systems to reduce

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likelihood of any type of failure associated with them,

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and also our historic knowledge.

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I put this material in here 20 years ago.

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It's still doing great. I'm comfortable using it again.

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That ends up being a really strong driver. Also, validation from other parties that other person use this material.

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It was really successful. That means I have a higher likelihood of it being successful.

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As you can imagine, incredibly valuable.

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We don't want to remove this idea of minimizing risk.

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We want to make sure that safety and performance are number one always.

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But if you come up with a brand new material

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that there's a little bit of a version of using it, because this kind of knowledge

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of how it's going to perform and comfort associated with its use

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isn't necessarily there. So we end up seeing that civil engineers are a little bit less likely to rapidly

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adopt novel materials, because we need those materials to perform well.

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So that we make safe structures, and we need them to perform

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well for a long time. They have to keep going for decades

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because there's a lot of work going on right now trying to understand how we improve adoption

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of alternative materials, how do we make sure

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that we have the proper validation so that we're removing that risk

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from the practicing engineer? They should not be the one who has to take that on.

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Do we change our overall insurance structure

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so that there's more comfort trying to use some of these materials?

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Should we change how we're structuring kind of later

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stage testing so we're better understanding durability.

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Is there something that we can do in order to understand any type of barrier for actually placing the material?

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All of that is a really active area of research.

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Everyone's really excited about AI right now.

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Are you guys using AI in your lab as part of any of that kind of analysis process?

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It's a tool. Engineers love tools.

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So yes, we do use AI machine

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learning algorithms in order to kind of help predict certain things.

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So rather than conducting test over test over tests

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so that we get a battery of information,

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there are researchers around the world who are collecting fantastic information.

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So yes, we are leveraging things like AI in order to use many different

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data sets that might not all have been performed in the exact same way as such,

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that we can still predict robustly the likelihood of material performance.

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And we're also leveraging it to inform things where we have otherwise

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data poor environments. So quantifying environmental impacts, for example, is a very and data

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intensive field. And sometimes we have gaps in some of the inputs that are necessary.

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But we can leverage AI to fill in some of those gaps.

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Still with a bit of uncertainty, but better than our just guessing.

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And then also we have utilization of AI for some, overcoming barriers

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for adoption, trying to understand, okay, we have very limited data

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for these particular types of performance metrics.

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How can we predict what we would expect behavior to be, or predict what tests

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we should be doing in order to fill in some knowledge gaps in that realm as well?

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I also wanted to ask you about NSF support.

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What difference has say your career award made for you?

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NSF support has been the best support that I've had in my career.

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I realize that that kind of sounds like I'm pandering.

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At the same time though, NSF support actually allows us to do this

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kind of more foundational, understanding type research.

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NSF has facilitated my group actually looking at new areas

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where we don't have a solid understanding of how this could pan out,

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or where we should be focusing our attention.

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So it has facilitated work, like the work that we've been discussing about what would potentially store carbon

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and where could we get the biggest bang for our buck in terms

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of trying to utilize carbon in the built environment?

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And we end up seeing that NSF has actually been incredibly valuable

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in trying to support that type of really groundbreaking work.

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Put on a speculative hat for a second and think about

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where would you like to see cements or concretes in, say, 75 years?

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I'm a bit of an optimist, so I would like to see them having

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higher performance characteristics so that we don't need quite

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as much of the material. Obviously, we can't get rid of all of it because there's only so much

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you can cut back and still get the right amount of roadways, buildings, etc.,

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but higher performance characteristics out of the material.

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I would love to see the material, obviously, at least at net

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zero emissions associated with it, but also ideally something

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that could be a net uptake, a net storage system for our environment.

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And I would love to see more on circularity of resources tied to our use of concrete.

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So just like you mentioned, this kind of use of concrete, crushing it

19:47

and getting to use it again, we haven't been big on resource circularity in general.

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Not calling out concrete, but in general, humans

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have been a little bit more linear and the life cycle of our materials.

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We’ll extract, we’ll process, we’ll use, we’ll dispose.

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I'm really excited about the kind of concept of how we can start

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to re-engineer things so that when we're taking it out of use,

20:07

let's use the resources again such that they're circling through our economy.

20:11

Perhaps not straight into the same class of materials

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or the same class of products if we are losing performance.

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But how can we start to reuse those resources so that we don't have to keep extracting resources,

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which would facilitate our potentially being able to continue building in a very robust way.

20:27

So for the very last question today, I want to ask you about what's next in your work.

20:31

We actually are continuing doing work in circularity

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because I find it so exciting, also because there's some issues tied to resource consumption

20:38

and localized scarcity. So we might end up having areas around the planet where we have plenty of the material globally.

20:44

But in this particular area, we don't have enough access to it.

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Now, we've got to import it from other areas, which can cause a variety

20:50

of different stressors on the environment as well as other types of stressors.

20:54

So trying to better leverage the resources that we're taking out of use,

20:57

that circularity is something that we're incredibly excited about in the group.

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We're also looking at paired material and energy systems.

21:04

So humans are fans of energy, electricity in particular, and trying to understand,

21:09

okay, if we are going to need to continue to produce electricity,

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how can we make sure that we have access to that electricity while also creating

21:17

co products that benefit other types of systems, such as materials production?

21:21

How can we pair those together such that we're able to generate energy

21:25

and create a net storage mechanism tied to that energy generation?

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So we're getting the benefit of our energy resource,

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but also removing CO2 from the atmosphere or undoing air pollutant issues

21:35

that we’ve sent into the atmosphere, trying to integrate those together.

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And then of course, continuing work on decarbonizing materials production

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and creating net uptake, net storage systems,

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those are areas that I'm incredibly excited about.

21:48

Special thanks to Sabbie Miller. For the Discovery Files, I'm Nate Pottker.

21:52

You can watch video versions of these conversations on our YouTube channel by searching @NSFScience.

21:57

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