Fly Ash, Silica Fume, & Natural Pozzolans Are Terms To Know When Talking Green Cement

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Every domain brings with it a wealth of new language. Being the domain junky that I am, my head is creaking at the seams with jargon and technical terms from a multitude of places. Cement and concrete are no different. Today’s exploration is of the interesting world of supplementary cementitious materials, or SCMs as cement-heads are prone to throw into conversations without explanation.

SCMs are a key component in modern concrete technology, especially in the context of sustainable construction and cement production. SCMs are materials that, when used in conjunction with Portland cement, enhance the properties of the concrete. They are industrial by-products or natural materials that can contribute to the performance and sustainability of concrete.

Emphasis is on the by-products bit, in case you were wondering. Pretty much any concrete in anything around you contains industrial waste of one type or another, repurposed because its characteristics make the cement a bit better, because they reduce the carbon dioxide emissions a bit, and because they are cheaper than Portland cement.

Table 1: SCM cost and emissions by author

This is actually a pretty good news story. Lower emissions from concrete at a lower price for buildings and bridges. Naturally it’s the lower price that has meant it’s already being heavily used, wherever the industrial wastes are handily available. It’s safe to say that no one would have been using them for concrete unless they were cheaper.

The first three are pure industrial waste. Fly ash is the solid waste from coal plants, as opposed to the waste that goes up the chimney, causes acid rain and global warming, causes people living nearby to have serious cardiorespiratory health issues, and puts a bunch of bioaccumulating heavy metals like mercury into our ecosystem where they poison other people. I did the research a few years ago to find that the average US coal plant killed about 80 people a year, mostly from lung diseases of various sorts.

Of course, coal plants are going the way of the dodo bird, being shut down rapidly in the developed world as ‘cleaner’ natural gas and actually cleaner renewables have been replacing their electrical generation. Before anyone mentions China, the coal story there is much more nuanced than the black and white picture usually painted. While the sheer tonnage of waste of all kinds from coal plants is horrendous, it pales beside the tons of cement we manufacture and use.

Table 2: US coal plants and fly ash usage by author

I pulled together this retrospective look at coal plants in the USA over the past four decades and a couple of things surprised me, although they shouldn’t have. The first was the rapid increase in plants through 2000. The rapid decline in the 2010s was due to replacement with natural gas plants for the most part, which have fewer health impacts, but far from none.

More importantly for climate change, the gas plants in the USA come with the highest methane emissions full lifecycle of any country in the world. That’s in part due to fracking and shale oil inherently creating a lot of emissions, but also because unlike Europe, regulations regarding venting, flaring, and other sources of emissions were very lax, and of course the emissions were unpriced.

That’s changed as of January of 2024, as an actual greenhouse gas pricing scheme made it through Congress, with methane leaks priced at $900 per ton, which equates to a $36 carbon dioxide or equivalent cost. I’m somewhat surprised it managed to get through, as Manchin’s West Virginia is the fourth largest producer of natural gas. I’m not that surprised, as Manchin gets 70% of his investment income, in the range of $600,000 a year, from a coal brokerage company he started in the late 1980s, so while the gas industry lobbies him, the coal industry owns him.

I worked out not long ago that the USA’s increased methane emissions since 2000 have completely eradicated carbon dioxide reductions from electrical generation, meaning that the USA’s CO2e per kWh has not declined at all as Americans like to claim.

Back to cement, however. You’ll note that the percentage of fly ash used for SCM has climbed steadily over four decades, reaching almost 60% in 2020. That increase is excellent news.

And sharp eyes will notice that while the number of coal plants has plummeted, the amount of fly ash used actually went up. That was another surprise, but one with an obvious explanation. The low utilization of fly ash over the past 140 years in the country means that there is a very large amount of fly ash sitting in landfills and slurry ponds. That’s an environmental disaster all of its own, much like the Superfund site that is Solvay, New York.

In December 2008, a dike ruptured at the Kingston Fossil Plant in Tennessee, releasing 1.1 billion gallons of coal fly ash slurry into the Emory and Clinch Rivers. The spill covered about 300 acres and caused extensive environmental damage. In February 2014, a stormwater pipe under a coal ash pond at the Dan River Steam Station in North Carolina collapsed, releasing 39,000 tons of coal ash into the Dan River. The spill affected 70 miles of the river, harming aquatic life and posing risks to human health.

Fly ash is pretty nasty stuff, so the fact that it’s starting to be mined to create SCMs, where the nasty stuff is sealed away to reduce cement emissions, is good news.

And these volumes are significant for cement. In 2023, the United States produced approximately 93 million metric tons of Portland and masonry cement. The volumes indicated to me that fly ash is being used for more than SCM, so I went and had a look. It is used in soil stabilization to enhance the engineering properties of soil for construction projects, and in the production of geopolymers for various construction applications. Fly ash is also used as filler material in road base and embankment construction, and in waste stabilization to reduce the leachability of hazardous wastes. Additionally, fly ash serves as a soil amendment in agriculture to improve soil properties and as a component in fertilizers.

That last one was a bit worrisome, as fly ash can contain trace amounts of various elements such as arsenic, boron, and heavy metals, but it seems that they are typically bound in the glassy matrix and are not leachable in significant quantities. That’s still a concern, as biological processes in soil, such as those mediated by microbes or root exudates, can alter the chemical environment, potentially releasing bound heavy metals. Factors such as soil pH, organic matter content, and microbial activity can influence the stability of these metals. Lots of testing and care is required with that one.

Fly ash is by far the most commonly used SCM in North America simply because there’s so much of it and it’s available wherever there are or have been coal plants. Just as with earlier discussions regarding limestone’s remarkable uniformity being a major factor in its use in cement, all things cement are heavy, bulky, and are cheapest when very, very close to the cement plant, which is cheapest when it’s very close to major consumers of cement.

Ground granulated blast furnace slag (GGBFS) is the stuff that comes out of coal-burning steel processes. Like EAF slag, which I dug through recently, it’s a grab-bag of things that cement and concrete love — silicon, iron oxide and the like. In the USA, blast furnaces just aren’t everywhere in the way that coal plants were. Most US blast furnaces were in the northeast, and concrete is used everywhere. As a result, GGBFS only saw 2.4 million tons of use as an SCM in 2020.

By contrast, in China where blast furnaces have been going full … errr… blast for decades, GGBFS is a big deal, with 169 million metric tons used in 2020 alone. Similarly, fly ash is a major cement supplement in China, with 540 million tons used that year. As noted in my recent piece assessing ways that we can avoid using cement at all, including mass timber and building reuse instead of demolition, China is currently producing and consuming more than half of the cement used globally, about 2.1 billion tons a year. 709 million tons means its cement has maximized SCM content already. The other point in that assessment was that China’s cement demand is going to decrease in the coming years as it’s mostly finished its massive infrastructure and city building spree, a good news story for climate change.

The natural in front of pozzolans in the title is necessary because fly ash and GGBFS are unnatural pozzolans. The term pozzolan originates from the ancient Roman use of volcanic ash from the region of Pozzuoli, near Naples, Italy. The Romans discovered that mixing this ash with lime produced a hydraulic cement that could set under water, leading to the development of Roman concrete, which was highly durable and used in many of their monumental structures. All natural pozzolans do is repeat that process, grinding soft volcanic ash and rock deposits into a fine powder.

That soft bit matters. Pumice is used for this as its texture and inner bubbles make it easier to crush than basalt, which has a more tightly interlocking crystaline structure that’s much more resistant to being broken apart by mechanical or thermal processing, hence the higher heat required to use the much smaller amount of calcium in it to make lime for cement, something that keeps being brought up as a solution by commenters.

In a vain attempt to prevent basalt from being mentioned again, it has a sixth of the lime that can be produced as limestone and costs four times as much per ton, so would be in the range of 24 times more expensive if basalt a quarry were right beside a cement plant, just for the rock. Then it’s harder to crush and requires higher temperatures, 50% higher, to decompose into limestone and mostly silicates, so more expensive again. Then it is 90% solid waste, and that solid waste dwarfs any possible use for it making it solely a landfill problem. The waste is actually a decent SCM, but as noted, we’re actually fully loaded with SCMs already, so it would have to be cheaper, and there’s nothing cheaper than massive tonnages of stuff already headed for landfills. Basalt is not a solution for cement.

Natural pozzalan deposits are igneous in nature, which means that they cluster around historical or current tectonically active zones. As a result, they are used when the cement plant happens to be sitting on them instead of near a blast furnace or coal plant.

I’ll ignore silica fume, at least for now, simply because it’s more expensive than Portland cement, so the only places where it would possibly be used as an SCM are in places with lots of electric arc furnaces and strict regulations on its emissions and use. It comes from the flue gases of EAFs, so tonnages will invariably be a rounding error compared to the rest of these.

And so we get to calcined clays. When used as an SCM, they aren’t necessarily limestone calcined clay cement (LC3), which I dug into in depth recently, but LC3 uses calcined clays as an SCM. It’s just a specific formula for using calcined clays, one that’s gaining a lot of attention.

As I noted, calcined clays are basically weathered feldspars, and feldspars are igneous rocks. That makes calcined clays another natural pozzolan. It also means that its distribution is more like basalt, pumice, and other igneous rocks, in that it’s patchier than limestone, being where there was or is a lot of tectonic activity. Limestone being just the beds of long vanished seas where lots of shellfish died and calcium precipitated out of the water over millions of years, it’s much more evenly spread.

People have been following along on my most recent cement week might have noticed a discrepancy in the price of the clays between my earlier findings and the table near the top of this article.

Additional cost of kaolin clay compared to limestone in three geographies by author
Table 3: Additional cost of kaolin clay compared to limestone in three geographies by author

The data from the table at the top says that they are 20% to 30% cheaper than cement, but according to global prices per ton, the kaolin clay is more expensive, quite a bit, and as a result would lead to LC3 and other cements which use calcined clay being more expensive than Portland cement, if all else were equal. Given that limestone is everywhere and calcined clays aren’t, this might be why no one has used calcined clays in any great amounts in cement despite its properties being understood for centuries.

It’s not like we don’t mine kaolin today. We do. In the paper industry, kaolin is essential for improving gloss, smoothness, and printability, serving as both a coating agent and filler. The ceramics sector relies on kaolin for producing porcelain, fine china, and sanitaryware due to its high purity, whiteness, and plasticity, which contribute to the strength and translucency of these products. In paints and coatings, kaolin enhances opacity, texture, and durability as an extender and filler. The rubber and plastics industries use kaolin as a reinforcing agent to improve product durability. Additionally, kaolin finds applications in cosmetics and pharmaceuticals, offering absorbent and gentle properties for facial masks, powders, and antidiarrheal medicines. In agriculture, it acts as a natural pesticide and carrier for pesticides and insecticides. Kaolin is also crucial in fiberglass production and the manufacture of thermal and electrical insulation materials. Moreover, in petroleum refining, kaolin serves as a catalyst in the cracking process, breaking down large hydrocarbon molecules into more valuable products like gasoline.

The variance from the LC3 association data points about prices and my bottom-up workup made me a bit suspicious of LC3 claims. The reality that no one uses LC3 or calcined clays as a big SCM confirms it for me. The LC3 price points don’t stand up to scrutiny. When I looked at their scenarios, they were looking at 10 km distances from kaolin deposit to cement plant. My assessment of major accessible kaolin deposits makes it clear that they are much more likely to be hundreds of kilometers to the average plant, not tens.

Kaolin clay is also more expensive to mine than limestone because the walls of clay pit mines will collapse without a lot of shoring up, while limestone quarries just sit there. Both limestone and kaolin require drying, as limestone is a porous rock that tends to have lots of water in its crevices, part of the reason why southern Florida, which sits entirely on limestone, is completely unable to prevent sea level rise from wiping out much of Miami. Between drying and firing, kaolin clay requires 0.2 to one ton less energy.

That’s a price difference per ton of material of $2 to $7 using coal or natural gas, which isn’t sufficient in my assessment to overcome the higher cost of the material and shipping charges. It does equate to lower carbon dioxide emissions from process heat, but only to roughly 0.005 to 0.01 tons per ton. When cement’s emissions are a ton, the process heat emissions difference just isn’t that big a deal. It’s the lack of carbon dioxide from decomposing kaolin clays that’s the really big hitter.

Of course, when the heat has to come from electricity because it’s possible to decarbonize it, then the price goes up. At average US and European rates, thats US$19 to $36 per ton, which does start to make an impact. As I noted about the electrochemical process that Sublime Systems uses, it should be cheaper to create the same results. Examples including hydrogen, modern ammonia processes, and chlor-alkali processes show that they are 50% to 60% of the energy cost of legacy pressure and temperature processes. There is no reason to believe that kaolin clays couldn’t be processed electrochemically by Sublime’s process or one similar to it, and so that might be a $10 to $18 difference per ton, but still the electrochemical process would be more expensive than burning dirt cheap fossil fuels and using the atmosphere as an open sewer.

Calcined clay SCMs, including LC3, are barely used today because they are more expensive in reality, not in the self-serving cost workups of the people who own the mineral rights to the kaolin deposits, or at least that’s how I read it. Further, they are just another SCM like all the other ones that are cheaper, existing in vast quantities from steel manufacturing and coal plants, sitting in dumps around the world, and typically closer to where cement is needed.

LC3 is not particularly outstanding in terms of carbon dioxide emissions compared to other SCMs either. That 169 million tons of blast furnace waste in China is probably doing as much good for cement as LC3 would, and at a lower cost. And from the data I have on tonnages of cement being manufactured and SCMs being used, it appears that the maximum usage is already happening, so LC3s would be directly competing with cheaper alternatives.

Where does this leave us? Calcined clays are undoubtedly going to be used in places where there aren’t cheaper substitutes, including the residue of volcanos, steel slag from electric arc or blast furnaces, or fly ash from coal plants lying around in massive heaps begging for a useful life. There is potential for kaolin dryers and grinders to be set up on kaolin deposits that are cheaply accessible, with the calcined clay being shipped shorter distances to relatively nearby cement mixing plants. That’s undoubtedly going to pencil out as cheapest in a bunch of places, but that’s for the local markets, just like GGSBF in Cleveland.

We are going to stop burning coal for electricity and we are going to stop using blast furnaces, so those sources of SCM are going to dry up, but there’s so much of the stuff lying around that we’re probably good for decades. At that point, calcined clays will be one of the only scaled, reasonably cheap SCMs available and carbon pricing will have probably kicked in globally to level the playing field with Portland cement a bit. But right now I’m seeing tough sledding for calcined clays vs fly ash and GGSBF.

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