Brimstone’s Cement Process Tries To Dodge Carbon Dioxide Through Costlier Rocks & Higher Heat

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Several years ago I concluded that Cement’s CO2 Emissions Are Solved Technically, But Not Economically. Since then I’ve had numerous conversations with deep experts on the subject that made me realize that it wasn’t just cost, but regulations that held alternatives back.

I’m digging through some of the current crop of alternatives in this mini-series on the subject. Recently, I assessed Sublime Systems’ electrolysis approach, which electrolyses water into oxygen and hydrogen, plays with the ionic chemistry and pH balances, and feeds any lime bearing substance in crushed form through it, including limestone, concrete and electric arc furnace slag.

Another thread that was handed to me was California’s Brimstone, which takes a different, less technically avante garde approach.

Let’s start with why cement is a problem in terms of climate change. Its primary ingredient is calcium oxide, known as quicklime or lime, with former being more specific and the latter good enough in context. That’s made today by heating limestone to several hundred degrees Celsius in kilns. Limestone contains calcium and oxygen, which are good for calcium oxide, but it also contains carbon. When limestone is baked, it turns into lime and carbon dioxide. Then the lime is turned into cement into a clinker kiln, which the article on Sublime gets into more for those interesting.

Every ton of cement results in about a ton of carbon dioxide between the heat for the limestone process, the carbon dioxide from the limestone itself and the heat for the clinker kiln. Electrifying the kilns gets rid of that source of carbon dioxide, but not the carbon dioxide from the limestone itself.

We produce about 4.1 billion tons of cement annually to make 30 to 40 billion tons of concrete. That’s about 4 billion tons of carbon dioxide, or about 10% of global emissions. It’s a very big climate deal.

Brimstone’s solution is to pick a different rock, a group called calcium silicates. There’s a clue there. When we talk about limestone, that’s it. Limestone is very chemically homogenous. Calcium silicates have all sorts of stuff in them by comparison. Calcium oxide is typically around 55-65% of the rocks. Silicon dioxide is 25-35%. But it usually has 2-5% aluminum oxide, 2-3% iron oxide and smaller amounts of magnesium oxide and other trace elements.

The convenient thing about the unneeded parts of limestone is that when they it baked, they go up the flue and disappear. The bad part is that it is carbon dioxide and that’s the big greenhouse gas.

The inconvenient thing about doing the same thing with calcium silicate rocks is that the waste, up to 35% or so of the mass, doesn’t go up the flue and have to be separated out from the lime. One process step becomes two to five. A waste stream that disposes of itself as long as the atmosphere is used as an open sewer is replaced with a waste stream that has to be disposed of.

The good news is that the mass of carbon dioxide is a greater percentage of the whole for limestone, about 44% than the waste elements in calcium silicate rocks, about 35%. So that’s two advantages for calcium silicate rocks and a couple of other disadvantages. Given how obvious this is, literally 19th Century science and technology, are there any other reasons why limestone is used everywhere for cement and calcium silicate rocks aren’t?

Yes. Cost, cost and cost.

Brimstone’s website accurately says that calcium silicate rocks are abundant and voluminous at the surface of every continent in the world. What it doesn’t say is that limestone is all that with added limestone on top. It’s the most common sedimentary rock in the world, it’s more evenly distributed than calcium silicates, it’s typically very shallow and comes in very big, very homogenous deposits that are cheaply quarried. Deposits can cover hundreds of square kilometers and be a couple of hundred meters deep. As a factoid, 67% of all crushed rocked mined in the USA in 2007 was limestone. Delivered, limestone only costs US$25 to $35 per ton.

Meanwhile calcium silicates, while still prevalent, cluster in certain geological formation, meaning that if you need cement where calcium silicates aren’t, transportation costs will be higher. Those geological formations are mostly where there was a lot of subterranean heat. Calcium silicates like wollastonite and larnite are metamorphic rocks, one form of rock, including limestone, that’s been exposed to a lot of heat and pressure, into another form of rock. Another set of calcium silicates are igneous, meaning cooled and solidified magma.

What they have in common is that they are both much harder than limestone, 2 to three times harder on the Mohs scale, and typically deeper underground, where there are deposits close to the surface at all. While a tremendous amount of calcium silicates exist in the earth’s crust, it’s usually one to 30 kilometers below the surface. The number of surface and shallow deposits of calcium silicates is much smaller than for limestone.

The combination of not quarrying but mining, whether open pit or subsurface, the harder rocks and the greater distribution distances means that calcium silicates are more expensive, from $50 per ton in the best case, 30% more expensive perhaps, to $150, up to 600% more expensive than limestone.

That few more percent of calcium dioxide per ton of rock makes up for this a bit, as does limestone costs not being the majority of cement costs. About 10% to 20% of Portland cement costs are from limestone, so taking all into account, that’s going to increase. $150 per ton Portland cement might see $15 to $30 from limestone, and that’s going to increase on average by a factor of three. $200 to $240 per ton based on this factor.

Then there’s the next cost factor, which is higher process heat. Limestone is advantageous because it decomposes into calcium oxide and carbon dioxide at a lower temperature. Limestone kilns run at 900-1,000° Celsius while calcium oxide kilns run at around 1,450° Celsius. 50% more heat means 50% higher energy costs.

The process heat is 30% to 40% of Portland cement’s total costs, and limestone kilns are 40% of that. Of the $150 for Portland cement, $18 to $24 is from the limestone heat. Increasing that by 50% brings the price of up as well, making the $150 cement with the additional cost of the calcium oxide and the heat, up to $210 to $250.

And then there’s the waste material, about 35% of the mass. It has to be separated from the useful calcium oxide, so there are probably additional mechanical or chemical separators that probably add another 10% to the cost, at a guess. The ton of cement is now at $241 to $275.

And then there’s the disposal of that 35% of waste material. That’s mostly silicon dioxide, which currently costs around $1,800 per ton. That sounds amazing, except that the total global market is only around 6 million tons, and cement manufacturing would produce around 330 million tons of the stuff if calcium silicates replaced limestone. Swamping a market that completely and globally would turn a reasonably high value commodity into a waste mineral that is expensive to get rid of. Industries that use silicon dioxides would rejoice however. For context, all aluminum produced globally is only around 60 million tons.

That much waste would probably add 5% disposal fees, to be generous, bringing the total cost of a ton of Brimstone’s cement up to $250 to $290.

Is this necessarily a problem? No. Carbon pricing under the EU’s budgetary guidance for 2030 suggests that standard Portland cement would have a €200 (US$215) per ton carbon price added, bringing it up to $375, well above the cost of the Brimstone cement process. While the USA does not have a carbon price, it almost had a national one under the guise of a China-focused carbon border adjustment that included a domestic tax, it does have a methane tax indicating reality is setting in, 13 US states have carbon prices, and the US EPA’s social cost of carbon is very close every year to the EU’s budgetary guidance.

But wait. There’s a kicker. And it’s not kicking in favor of Brimstone. The cost of the heat. The costs for cement in Europe and the USA are based mostly on natural gas costs per MWh or gigajoule or Btu. This doesn’t matter for wind turbines or solar panels vs natural gas electricity generation plants because the wind and the sun are free. This doesn’t matter for electric vehicles vs internal combustion vehicles because while natural gas generation has turned out to be not that much better regarding greenhouse gas emissions than coal, electric vehicles are absurdly more efficient well to wheel than internal combustion vehicles. This doesn’t matter for heat under 200° Celsius because heat pumps are four times more efficient than natural gas on a well to basking in warmth on a cold winter night basis.

But it does matter for 1,450° Celsius heat. Any process that requires that much heat loves cheap gigajoules, MWhs or Btu, regardless of how you count the units. And Brimstone requires heat. While overall energy requirements will plummet with electrification and there are no real barriers to it except operational cost, high temperature heat is one of bits where energy costs are going to go up. Costs per MWh of high-temperature heat, which actually is roughly equivalent in natural gas, coal gas or electricity, are going to be higher. When 40% of the heat requirements. 10% to 20% of total costs, suddenly become three or five times higher, that makes economic competitiveness hard. The $250 to $290 per ton jumps up again.

Right now, as long as the atmosphere is treated as an open sewer and no one cares about climate change, that $20 to $30 per MWH for natural gas isn’t much higher delivered. For electricity, it’s tough to get that low except in places that haven’t privatized everything and have a lot of big, old hydro electricity such as Quebec in Canada. Delivered cost of heat is in the $10 to $40 per MWh range. That’s going to strongly favor solutions that don’t need a lot of heat, like Sublime Systems, if its unstated energy requirements are lower. That’s probably true.

I’m strongly bullish on electrify everything, including heat. But I make no pretense that renewable electricity used for high temperature heat is going to be cheaper than dirt cheap fossil fuels when the atmosphere is a gutter. The Sublime solution doesn’t require high temperature heat. The Brimstone solution does. The Brimstone solution is easier to cost with external data. How will this play out economically? Market question. I don’t pick winning companies, I pick winning and losing solutions sets based on climate change and carbon pricing, and with that metric, Brimstone, Sublime and standard cement processes are competitors for low carbon cement. I’m skeptical that carbon capture and sequestration will win, but if it does, good enough. As long as there are many competitive solutions that are much lower carbon, I’m comfortable that one or three will become the primary winners.

Anything else? Yes. the tonnages of calcium silicate ores today, due to the greater depth, harder rock and uneconomic value of the materials is very low. The total tonnage of wollastonite, the primary rock in this category is only 0.9 to 1.0 million tons annually. The requirement is billions of tons. That’s three orders of magnitude. Anyone who thinks that calcium silicates are a slam dunk for decarbonization of cement has never bothered to look at the denominators.

Pricing the carbon dioxide in cement is coming, and it’s going to add costs to the production of it. Brimstone’s technology, after this cost work up with available data, is higher cost than any cement with carbon price adder in the world today, but below the EU’s budgetary guidance and the US (and Canada’s) social cost of carbon. It is, however, competing with carbon capture and sequestration around cheaper limestone, Sublime’s approach and others.

At the beginning of this piece I mentioned regulations, and this is a sticking point. Building codes may be suggested nationally, but every smaller jurisdiction down to even small towns can do whatever they want in many countries. And as one of my discussions with a cement expert illuminated, that means that every small town can specify a lot more about irrelevant characteristics of cement than is remotely rational. Both Sublime and Brimstone assert clearly and constantly that their processes produce cement that adheres to ASTM C150. But there are three other international standards. And then a myriad of locally rational — in the sense that local people benefit — but more broadly irrational variants. Getting ASTM C150 right is the baseline, but doesn’t guarantee salability. A bit of national nudging on standards would be a good thing.

The market will decide which technology is best, as long as the fossil fuel industry doesn’t convince governments that only carbon capture can possibly work and governments decide to pay for it. In that case, carbon capture will probably ‘win’, likely with significant downsides for everything else. May carbon pricing be the big thumb on the lever instead of more subsidies for the fossil fuel industry.