ChatGPT & DALL-E generated map of the USA with piles of ash and slag and big limestone quarries

Concrete Is A Geography, Minerals, & Waste Game Played With Billions Of Tons Of Chips

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As cement day turns into concrete week turns into concrete month, it’s worth considering how the absurd tonnages of concrete manufactured changes, often radically, depending on where specifically in the world the concrete is needed. That’s all because those absurd tonnages means that we move the stuff that goes into concrete the shortest possible distances because it’s so expensive to move.

Let’s step through all the things that go or can go into concrete. First you need something to glue everything together. Mostly that’s Portland cement, but epoxy and geopolymers are used as well.

Portland cement is one of the big climate problems for concrete, with every ton of the stuff producing about a ton of carbon dioxide during manufacturing. That’s because we make all of it from limestone, which not only has the calcium and oxygen necessary to make the lime for cement, but also has extra oxygen and some carbon, enough to produce a waste stream of carbon dioxide along with the useful lime. Note that the waste stream took care of itself, dissipating invisibly into the atmosphere, something that we’ve found isn’t actually without a trace.

Why do we use limestone, especially know that we’ve found out the problem with its invisible waste? Well, it’s everywhere. Almost literally everywhere. All limestone is is dried up seabeds that were buried, compressed and lifted up to the surface. The limestone is the remains of millions or tens of millions of years of shellfish dying and their shells dropping slowly through the water, along with calcium carbonate that precipitated out of the ocean water. The processes for this started hundreds of millions of years ago and pretty much every bit of land that’s dry today was seabed in the past.

As a result, it’s easy to find shallowly buried, often hundreds of meters deep, deposits of limestone covering hundreds of square kilometers pretty much anywhere we want to make concrete. In addition to being relatively easy to quarry out of big rock holes that we don’t need to brace the sides of, it’s reasonably easy to decompose into lime and carbon dioxide in kilns running at 900° Celsius.

Everything else we might want to make Portland cement out of is nowhere near as convenient. Some startups want to replace limestone with rocks that contain calcium and oxygen, but not carbon. The only one of those rocks with high ratios of the calcium and oxygen is wollastonite. While limestone is about 56% lime and 44% carbon dioxide. Wollastonite, the lime champion of the rocks that aren’t limestone — emphasis on the lime — only has 48% lime and 52% silicon oxide. When it’s decomposed, something that occurs at 50% higher temperatures and hence energy requirements than limestone, we get less lime and a huge pile of silicon dioxide. We’ll get back to silicon dioxide in a minute, because some of the startups think it isn’t a massive waste problem, and they have some reason to believe that.

Anything we should know about wollastonite? Yes, it’s an igneous rock, meaning lava that cooled underground for the most part, although there are surface lavas in the mix as well, with the pressure making it hard and crystalline. That means a few things. The first is that it isn’t evenly spread around like limestone, but clustered around places that used to or still are tectonically active, especially continental plates and earthquake faults. It’s much less evenly distributed than dried up seabeds. The second is that heat and pressure and crystallization mean it’s much harder than limestone and needs more heat to decompose into the big piles of useful stuff and waste. The third is that most of it is 1 to 30 kilometers underground, not laying around in thick slabs on the surface. The combination means we just don’t mine much of it, less than a million tons a year, and it’s fairly expensive.

But that’s okay, say the startups, there are lots of more common rocks with calcium, oxygen, and silicates in them, like basalt, dacite, and diabase. We mine hundreds of millions of tons of basalt and diabase for aggregate annually, and millions of tons of diabase. How does that compare to limestone, ignoring distribution? We quarry about 4 or 5 billion tons of limestone annually, an order of magnitude more than basalt or diabase. We use it for most of the same things we use the other rocks for, and we use it to make cement. Basalt, dacite, and diabase clearly aren’t going to make up for the volume of limestone, but in theory they could at least make up for part of it.

Why so much more limestone than the others? Well, they are igneous rocks too, so they are much less broadly spread. They are harder as well. Basalt formed from old surface lava flows on or near the surface, so there are some places with formerly active tectonics where there are reasonable sized formations, though typically not nearly as wide or deep as limestone. Dacite quarries are smaller in breadth and depth than basalt quarries. Diabase forms in intrusions in the earth’s crust that don’t reach the surface. We quarry the ones that subsequent tectonic activity pushes to the surface. Smaller quarries over all, and not nearly as broadly spread as limestone.

Anything else about these rocks? Yes, the calcium and oxygen we want comes in much smaller percentages. Among the three, diabase is the calcium oxide king, with the lime representing 10% to 12% of its mass, and a bunch of other minerals comprising the 88% to 90% waste. Remember, diabase is the one that we only mine millions of tons a year of because it’s the least common near the surface. Then comes basalt, with lime comprising 7% to 12%, so 88% to 93% waste. Coming up last is dacite, with lime representing a meager 3% to 6% of the mass.

Remember, limestone — emphasis on the lime — is 56% lime, from 6 to 19 times more per ton than the also-rans that have calcium and oxygen.

Anything else? Oh yes, while limestone is pretty straightforward to decompose into lime and carbon dioxide, the other rocks aren’t. To decompose basalt, dacite, or diabase to produce lime, you typically need to crush and grind the basalt, use acid leaching to extract calcium ions, precipitate calcium carbonate from the solution, and then calcine the calcium carbonate to produce lime. During the precipitation step, a carbonate compound such as sodium carbonate or sodium bicarbonate is added to the solution containing the dissolved calcium ions. These carbonate compounds provide the necessary carbonate ions for the formation of calcium carbonate. Lots more steps, lots more expense, and it ends with the same temperature requirements as limestone for the remainder.

One hopes that they get the carbonate or bicarbonate from a carbon neutral source. Oh wait, there are none. The Solvay process emits about three tons of carbon dioxide per ton of carbonate or bicarbonate, and the carbon dioxide mostly comes from limestone. Mined carbonates and bicarbonates are digging up fossil carbon dioxide bound into the substance. Chemistry isn’t magic. But let’s pretend that they’ve invented a way to get the lime out of basalt, dacite or diabase without adding carbon from fossil sources. Perhaps they are using electrochemistry, microbial leach, supercritical carbon dioxide, high-temperature pyrolysis or innovative solvents. Is that easier or cheaper? No. Does it matter in any event? Not really.

Let’s do a little math. Let’s assume that the entire global consumption of basalt is 500 million tons a year. We divert that entirely to making lime for cement. That delivers us 35 to 60 million tons of lime. Lime is about a third of cement, so that’s 53 to 90 million tons of cement. We use 4.1 billion tons of cement annually, so that’s 1-2% of global cement demand.

What would that cost? Assuming basalt, it costs $40 to $200 per ton, compared to limestone which is $30 to $40 per ton. Assuming only doubling of raw material cost and the most lime rich basalt, so only requiring five times as much, and assuming that whatever process they use costs exactly what the Portland cement process costs, cement made with this process would cost ten times as much. This would be very, very expensive cement.

With it, we would get 410 to 447 million tons of various mineral compounds as waste. Are they good for anything? Sure, the firms that say that they are going to make cement this way claim that they are going to be very useful for supplemental cementitious materials. This means that what they are really doing at great expense is making supplemental cementitious materials and a little bit of cement on the side. Are we short of SCMs? Not now, and not in the future. But assuming that the basalt derived SCM was used because it was lying around in great massive piles of waste at the cement plant, then the cost of the cement and SCMs would be about seven times the cost today.

It still doesn’t pencil out. And that’s with every thumb on the scale for their cost case, including cheap basalt, high-lime content basalt, and no additional processing costs. Most of the time it would be double or even triple that. The best case scenario isn’t remotely economically viable.

Okay, so that’s cement. We use limestone — emphasis on the lime — because it’s everywhere, it’s cheap and easy to extract, and the majority of it is the stuff we want. Other rocks that contain the ingredients that some firms are counting on actually create massive piles of cement supplements at great expensive and very little cement, with no chance of scaling up to cement demand.

Let’s look at SCMs next. For chapter and verse, read this assessment, but here’s the summary. SCMs have various mixes of silicon dioxide, iron oxide, and aluminum oxide, with various other things depending on where we get them from. Where do we get them from? Virtually all SCMs are coal fly ash or ground granulated blast furnace slag. We get coal fly ash from running coal generation plants and from tips where lots of it was dumped over the past 140 years. Coal plants were spread wherever there were people and there was any even remotely local coal, or there was a more remote mine a rail line could be built to, or for that matter a port that coal could be shipped to with rail lines leading to generation plants. Coal fly ash is everywhere, so it’s used more than any other SCM.

Blast furnaces were typically concentrated in heavy industrial areas, not spread wherever anybody lived, so while there’s a lot of slag still being produced in parts of China, and there’s a lot of slag left over in the Rust Belt of the Northeast and Midwest, it’s pretty scarce everywhere else. Once again, geography and mass dictate the SCM that gets used, but we have a rather absurd amount of the stuff lying around, just as with fly ash.

Next up are naturally occurring pozzolans, mostly volcanic ash and pumice that happen to still be on or near the surface. Where there was a lot of recent volcanic activity — by geological standards — and it’s cheaper to dig up and crush the ash and pumice than to use fly ash or slag, we do that.

Then we get up into the barely or never used SCMs, electric arc furnace silica fume, called that because it is initially a gas, silicon monoxide, that rises out of the furnace and reacts with oxygen from the air to make silicon dioxide, a very fine substance that is captured because it’s a health risk to workers and nearby residents. That is, in places with good health and safety regulations. It is only produced when silicon or ferrosilicon metals are being manufactured, a tiny fraction of the output of electric arc furnaces. Where there are strong health and safety regulations and silicon or ferrosilicon metals are being produced in electric arc furnaces, the silicon fumes are used as an SCM, but it’s a rounding error.

Then there are calcined clays. These are weathered feldspar, and feldspar is another igneous rock, so they are clustered around places where there was a lot of igneous activity, just differently clustered and a bit more clustered than basalt. Once again, pretty big deposits but not nearly as evenly spread as limestone. Further, unlike fly ash and slag, which merely require grinding and crushing to produce SCMs, calcined clays can’t be used directly. They have to be fired in kilns themselves to remove the water content and to activate the cementitious properties at temperatures typically lower than limestone, 600° to 900° Celsius. They are also more expensive to mine than limestone because the walls of the quarries and mines tend to collapse because they are clay, not rock. The combination makes them more expensive than Portland cement in the vast majority of cases. Only in places with lots and lots of the kaolin clay and limestone that’s a fair ways away — a rare combination — will they be cheaper than Portland cement. Even then, they are competing with much cheaper fly ash and blast furnace slag.

For example, in India, the province of Gujarat has a lot of the right type of clay in large and accessible amounts and mines it for all the things we use it for, like paper and makeup, but it’s also the home of a very large number of big coal generation plants and a couple of primary steel plants. There’s lot of cheap waste lying around that’s much more convenient than clays.

Once again, whatever is cheapest and nearest that has silicon dioxide, aluminum oxide, and iron oxide gets used. We have enormous reserves of those things. Expensive SCMs manufactured from basalt would have to compete with calcined clays, which are cheaper. While basalt is more common, calcined clays are cheaper and rail shipping is pretty cheap, so in a great number of cases, where all of the fly ash, slag, and other natural pozzolans have been depleted, it will still be cheaper to ship in calcined clays than to create massive amounts of SCMs and tiny amounts of cement from basalt.

Basically, the firms that are claiming that they are making cement out of basalt are making uncompetitive SCMs that might have a small market decades from now. Bit of a head-scratcher that venture capitalists are putting money into them because none of the numbers add up to anything worth investing in.

The next category is cement replacement starting with geopolymers. These are used instead of cement, so run into challenges with regulations and building codes, but let’s set that aside. Invented in the 1970s by a French chemist, they basically turn industrial waste into an epoxy-like substance. Big sources of industrial waste that are used are coal fly ash and blast furnace slag, with red mud waste from bauxite refining as part of the aluminum value chain running a distant third.

The wastes are processed to greater or lesser extents depend on the particular waste into a geopolymer precursor. They are mixed with caustic and silicate activators, sand and gravel (which we’ll get to in a minute) and water. The precursor and the activator replace both the cement and the SCMs. Because the biggest sources of SCMs and the biggest sources of geopolymer precursors are exactly the same thing, fly ash and slag, they have the same geographic dispersal value propositions and challenges. Because cement isn’t used, more of the fly ash or slag are required for the same results.

The activators, sodium hydroxide and sodium silicate, are industrial chemicals we manufacture by the millions and tens of millions of tons, which is good because the geopolymer recipe is about four parts precursor to one part activator. The raw materials are sand, salt, water, and energy, so are reasonably well distributed. The smaller tonnages mean transportation is less of a concern, but they are expensive.

Another replacement for cement is actual epoxy, made from petroleum. The same approach applies, with the manufacturing of a precursor and the mixing with an activator. Less epoxy is required than cement, although more is required in very high strength solutions, but no SCMs are required, removing a geographical constraint. And while the burning of petroleum products for energy will stop, we already have a global supply chain that is scaled for billions of tons per year, a supply chain that will be scaled back but still exist for petrochemicals in the future. This isn’t a significant geographical constraint, and the tons that would be required is much less than the current tons we burn. It is, however, significantly more expensive, two to three times per my assessment recently, for the same mass of concrete. This is much cheaper than cement derived from basalt, so may be the best option, once we decarbonize crude oil extraction, processing, distribution, and refining, and bring epoxy’s carbon debt down.

What else goes into concrete? Sand and gravel. 65% to 85% of concrete is just rocks and grit. Whatever is cheapest and closest to where the concrete is being mixed is what gets put into concrete. A very large amount of the aggregate used in concrete is, unsurprisingly, limestone. Where there is far more limestone available than is required for cement, it’s used for everything else too, because, once again, there are huge amounts of it everywhere and it’s cheap to extract. Sand comes from rivers, beaches, or nearshore in seas and oceans, again, whatever is most cheaply available in the location in question. Where there’s a lot of basalt, it gets used for aggregate, but never for cement because it’s so expensive and there’s usually a lot of limestone around anyway.

Anything else? Water, about 10% of the total mass of concrete. Making concrete in the Sahara is a problem.

Anything else? Reinforcing steel mesh or rebar. Typically, in residential construction, steel reinforcement may account for about 1-2% of the total volume of the reinforced concrete structure, and 8% to 15% of the mass. In commercial and industrial construction, this percentage is generally higher, around 2-4%. In special cases, such as bridges, high-rise buildings, or heavily loaded industrial structures, the steel reinforcement can constitute 5-8% or more of the total volume. Steel is a small percentage of the mass, but 20% to 30% of the cost of reinforced concrete. Once again, whatever steel is closest and cheapest is used.

With the concrete game, what’s cheapest and closest wins, and unfortunately that means limestone for cement emitting carbon dioxide. There are solutions, but nothing is going to beat the price point of the gray glue that holds our economy together except avoiding using it entirely. As we price carbon, that’s going to be the biggest lever we have left to pull.

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Michael Barnard

is a climate futurist, strategist and author. He spends his time projecting scenarios for decarbonization 40-80 years into the future. He assists multi-billion dollar investment funds and firms, executives, Boards and startups to pick wisely today. He is founder and Chief Strategist of TFIE Strategy Inc and a member of the Advisory Board of electric aviation startup FLIMAX. He hosts the Redefining Energy - Tech podcast ( , a part of the award-winning Redefining Energy team.

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