Highly Energy Intensive & Massively CO2 Intensive!

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It's called cement. And for over 200 years since the 18th century—by and large—this has meant a product called "Portland cement". We seek to change that!

Yet, no matter how amusing the Twitter video above, many people might struggle to even fathom the difference between cement and concrete. This general ignorance plays to the cement industry's advantage...allowing either for wild claims to be made, or for a real intransigence towards tackling the deep issues in solving Portland cement's truly vast climate-change implications.

Billions of tonnes of CO2. Although there have been process innovations, in fundamental terms the production of Portland cement has not changed in over 200 years — by which many billions of tonnes of CO2 have been released in about 100 years!

Why's that? Because Portland cement is made by burning limestone at enormous and truly fearsome temperatures for many hours. Below, we take a look at that... 

Many might not know the difference between a cement and concrete. Even fewer will know what "typical" cement is, let alone how it's manufactured. Even fewer understand the process, and very few get to operate the equipment needed to produce 4 billion tonnes of Portland cement (OPC) every year.  

A geological cocktail. The #1 ingredient in OPC is "clinker". Clinker is made principally (but not exclusively) from limestone. Limestone comprises calcite (CaCO3) bound by what was once sand (SiO2), in a geological process called diagenesis. The SiO2 content in limestone varies widely (as high as 30%) with calcite typically in the 50 to 55% range by mass. Other trace oxides are present, together with the remainder made-up from organic material (e.g. from soils). To start the process, other raw materials can be added if the limestone is not of the correct profile. For example, if the limestone does not have sufficient SiO2, then more sand is added too. 

Then add-in heat. Lots of heat... The temperatures at the firing-end of the kiln can approach 1,870°C (3,400°F) and are so high that "NOx is produced in this environment due to the reaction of nitrogen in air with excess oxygen" (PDF here). NOx is poisonous and also an indirect greenhouse gas as it produces ozone. Further, nitrous oxide (N2O) is about 298 times more potent as a greenhouse gas than CO2 (here and PDF here) and can be a sizeable by-product of dealing with NOx (i.e., to keep the NOx emissions within legal limits).

Some may think that the purpose of burning the limestone is to only make calcium oxide (CaO). That's a misconception. Instead, the purpose is to make two products using CaO. So, each can only be made from having first made CaO. It's multi-stage process — all of which requires enormous heat in the form of ever-increasing temperatures. The multi-colored diagram below charts the process...

First, there's the scale. The photo of the long kiln is taken from an OPC facility. Enormous? Some can be even longer than 300 meters (1000 ft) in length! So now juxtapose that image with the obvious heat shown in the video of a small kiln. Now add-in that this process can take up to about 18 hours!

Second, there's the energy. The video makes it obvious why OPC uses somewhere in the region of 1.5 MWh energy per tonne produced. By comparison, the average household in the UK uses 8kWh electricity per day (so to produce one tonne of OPC uses the electrical equivalent of 160 days of the average UK home). Why? Because there's two heat phases performed in the kiln (sometimes called a "calciner"), the chemical dynamics of which are captured in the coloured diagram above. The purpose is to cause actual chemical change (i.e., to form new chemicals), and also to cause a physical change in the structure of those newly-formed chemicals:

• Phase 1: Calcination at ~850°C/1560°F. If Phase 2 below is to form new products from calcium oxide (CaO), then this Phase has the purpose of first making the CaO that's needed. To do this is simple "school book" chemistry: it's only about the application of heat. This causes the calcium carbonate in the limestone to convert into CaO, which is caustic and can cause chemical burns. Also, it conveys enough thermodynamic energy (i.e., heat) to cause some of the newly-formed CaO to bind with the SiO2, to form a new compound sometimes called "belite". Fearsome? Yes, but actually the majority of the removal of the CO2 is performed in equipment called "pre-heaters" which are bolted on before the long kins. So it's maybe a bit confusing because "pre-heaters" typically account for 95% of the CO2 removal process (PDF here) and are themselves running at very high temperatures (but are not "kilns" for the sake of the cement-maker's lexicon).

• Phase 2: Sintering at ~1450°C/2650°F. This is what the video shows. So, here the purpose is two-fold:  (1) to "melt" the Phase 1 material, causing it to form small "glassy" beads (amorphisation); and (2) to cause some of the calcium oxide, formed from Phase 1, to chemically bind with  "belite" from Phase 1 (dicalcium silicate) to form "alite" (tricalcium silicate) and other end products. Because each respective energy requirement in this Phase is so large, both of these processes require a very high heat — at temperatures way above that needed for Phase 1. The end result is a chemical mix of four end-products: alite (C3S), belite (C2S), tricalcium aluminate (C3A) and tetra-calcium aluminoferrite (C4AF). Calcium oxide, formed from burning the limestone in the first place, is by now only a trace compound. Hence, 98% of a typical modern good-quality general purpose grey clinker might contain 72% C3S, 9% C2S, 7% C3A and 10% C4AF by mass — and further, "C3A and C4AF contribute no useful properties, and can be to some degree deleterious. They are present purely for the convenience of the manufacturer, because, without the liquid from which they form, the clinker can't be formed at an economically viable temperature" (here).

Third, there's the CO2. The red chunk in the coloured diagram above shows the output of CO2, which is totally driven out by about ~850°C. Let's put it this way: typically, over 1.6 tonnes of limestone is burned to make a tonne of clinker (PDFs here and here). Another way of putting it? The associated loss of dry mass is lost predominantly as CO2 — and that's over 500kg (PDF here)! But that's not all. Because then, there's also the CO2 produced during the actual fuel combustion used to get to those terrific temperatures...

All in all? The typical figure of the total CO2 as cited by the Global Cement and Concrete Association ("GCCA"), is that produced in 2016 by the WBCSD (PDF here). For its membership, that's 842 kg CO2/tonne clinker. But that leaves out an awful lot, so the figure is likely a low estimate (see section below, Notes on Energy intensity / CO2 outputs). Further,  since the 2016 version the GCCA has now taken over the GNR and no longer publishes a figure for clinker. 

Here's the thing:

• There's nothing that can be done to prevent CO2 being produced in Phase 1. This is because this aspect has the express purpose of driving-off the CO2 held as calcium carbonate in the limestone. Based on a "typical" composition of limestone, the figure for Phase 1 is cited at 502 kg CO2/tonne clinker. The only mitigation feature to reduce the CO2 produced is to replace the combustion fuel with hydrogen. But, unless it is "green" hydrogen then there's little point, because the energy required to produce hydrogen in the first place would be so enormous that it's inevitable the energy-need would have to be met by fossil fuels...

• For Phase 2, the temperature is so high that hydrogen is not a technical option. This is because at temperatures greater than ~1350°C/2460°F, hydrogen reacts with atmospheric nitrogen to promote the formation of toxic NOx (here). Further, studies have shown that burning hydrogen-enriched gas can cause NOx emissions to increase up to 360% that of methane—the most common element in natural gas (PDF here), with others claiming "burning hydrogen-enriched natural gas in an industrial setting can lead to NOx emissions up to six times that of methane" (here). Indeed, there are numerous other studies confirming the difficulties of controlling NOx emissions from hydrogen combustion in industry —including a U.K. Government review in 2019 (PDF here). NOx can produce ozone which is poisonous. Yet, the issue is not just dealing with the NOx because of its toxicity, but that the conversion technology to render it non-toxic can easily produce N2O—which although non-toxic—carries a whopping 298x GHG factor (here and here). Thus for at least the foreseeable future, methane or coal will still have to be used for Phase 2. Hence, for this aspect of OPC production there's no option to reduce the CO2 produced in the first place. 

• Even hydrogen carries a significant Global Warming Potential (GWP). Switching to hydrogen combustion even for Phase 1 carries a Global Warming risk, because of the volumes of hydrogen required to meet the bulk temperature demands even at Phase 1's lower need of ~850°C/1560°F. Given its lowest of all molecular masses, hydrogen has the highest nascent kinetics in the universe. Hence it has an unmatched ability to escape. The logistics required to supply a clinker plant with sufficient hydrogen even just for Phase 1 will be considerable. A UK Government study of April 2022 stated the expected leakage from hydrogen's production, transport, storage and end use. With 99% confidence, it stated electrolysis hydrogen would result in 9.2% produced making its way into the atmosphere as an average — the worst offender being tanker transport with 13.2% of its hydrogen cargo leaking into the air, followed by above-ground compressed-gas storage (here). So why does this even matter? A second April 2022 UK Government study has stated, “For a 20-year time horizon, we obtain a GWP for H2 of 33” and hence, "Any leakage of H2 will result in an indirect global warming, offsetting greenhouse gas emission reductions made as a result of a switch from fossil fuel to H2" (here). That report does not take into account the GWP of producing hydrogen, only the impact of its release into the atmosphere...

For further insight see section below, *Notes on GNR / CO2 outputs... 

EMC Volcanics reduce the overall environmental impact of making concrete. Nothing is going to change the fact that all mineral production requires mining. And there's no doubt that a lot of care and attention regarding land management can be given to ensure the land that has given-up its exploited reserves, can be returned to some sense of use (e.g. re-wilding).

But here's the thing: The multi-colored diagram above shows a large area colored in red. That's the CO2 expelled. So, let's examine what this means:

• It takes an average of 1.66 kg of limestone—depending on the nascent chemical composition of the raw material—to make 1kg of cement (PDF here). 

• Whereas to replace a kilo of clinker with EMCs requires one kilo of EMC Volcanics. The difference? A kilo of raw material volcanic materials makes one kilo of EMC Volcanics. 

• Therefore, to replace a tonne of clinker with a tonne of EMC Volcanics means 666kg less mining is required. 

• This means using EMC volcanics reduces the overall environmental impact of concrete.

Making a reasonable estimate of Portland cement's energy intensity and CO2 is highly nuanced:

• We want to make it clear that when we mean "Portland cement" we mean clinker cement designated as "CEM I" in the EU and as "Type 1" in the United States. It is 90-95% clinker. 

• According the United States' PCA (here), one tonne of Portland Cement comprises 92.2% clinker on average, releases about 1.1 tonnes CO2 by virtue of production and has an energy intensity of about 5.9 GJ (~1.65 MWh). According to ECRA in its 2022 review (here), the "thermal energy demand for cement clinker manufacturing was 3,460 MJ" (excluding drying of fuels), a figure that "covers 22% of the cement production worldwide and all technologies", to give a total of about 1.1 MWh per tonne clinker for the best-in-class (10% of those reporting, therefore 2.2% of the global cement production). 

• While it is true that steps have been taken to move Portland cement away from CEM I into blends comprising less relative quantities of clinker, when we talk about CO2-outputs in the macro context, we rely on a generally-agreed statistic (see for example, here) that 2018's ~4.1bn tonne cement production yielded ~2.5bn tonnes CO2 — for both the CO2 driven-off from limestone (sometimes called "process emissions") and for the CO2 from the fuels burned in order to meet the high temperature requirements of production.

• Returning to clinker, the true CO2-output rate for clinker is difficult to establish with any degree of precision or certainty. Many monographs on the subject ultimately concede that the derived CO2-output rates are estimates based on the WBCSD number called the GNR ("Getting the Numbers Right"). 

• The GNR seeks to derive a Worldwide CO2-output rate for clinker even if the EU-ETS figure is lower.  The GNR number is reported from the "Cement Sustainability Initiative" (CSI) membership first formed in 2003. According to the GCCA, China is still not a member.

• The GNR we rely on (2016) states, "in 2014, participants reported that 42% of data was verified to a reasonable standard, with 35% verified to a moderate standard." 

• In contrast, the UK's Institution of Civil Engineering's data tables first rated 1 tonne of clinker cement (i.e., CEM I or OPC) at 950 kg CO2 (PDF here). This has been revised downwards to ~920 kg CO2 in a later table. However, it must also be stated that Australia's EPiC database v.9 rates one tonne of OPC at 1,300 kg CO2 (here).

• As a further example, China's 2015's NOx emissions associated with clinker's high heat requirements are estimated at 1.6 million tonnes — against 1.1 billion tonnes CO2 (2021 study here). NOx is not a direct GHG even though NOx can produce ozone (although short-lived, has the highest high greenhouse effect factor). Nonetheless, NOx is toxic. Therefore, its output is either strictly limited, or where the limit is to be exceeded, converted.  But conversion technologies typically convert NOx also to non-toxic N20 (here).

• Maybe that doesn't sound so bad? However, according to the UN's IPCC (PDF here), N2O has a GHG factor at a whopping 298 as compared to CO2—even on a 500-year measure its GHG factor is still at 153—with a lifetime of 109 years (PDF here)!

• So that's up to ~476 million tonnes CO2∙e for N2O. Just for China. That's nearly 43% of the CO2 count on top of the CO2 count itself. Worse? No carbonation of that is possible (i.e., to act as an inherent mitigation feature).

• According to the GNR, 1 tonne of OPC puts out about 800 kg CO2. However, given the data-limitations that may impair accuracy, this is likely a very low figure as compared to the reality. 

Having reviewed a number of articles and research papers, together with a range of EPDs, for the purpose of our analysis on this website:

• Carbon score: For OPC, we assume the low figure at 800kgs/tonne implied by the GNR. This is likely a low figure, with the global average probably at about 1000 kg upwards, for CO2 outputs.

• Energy intensity: For OPC, we assume the figure of 1,350 kWh/tonne. This is a mid-range figure between the 10% BAT declared by ECRA (i.e. 2.2% of the global cement industry) and the figure declared by the U.S. PCA in its EPD.