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...
China's production-growth says it all (link)
The video below is taken from inside a small kiln making "clinker", the majority ingredient in Portland cement.
The temperatures involved are so high and the volumes so huge, that much is now automated.
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...
Lots of heat: the process can last 18 hours...
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 1.35 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:
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)! 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, is that produced 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 GNR / CO2 outputs)*
Here's the thing:
For further insight see section below, *Notes on GNR / CO2 outputs...
Production is highly energy-intensive with huge prolonged temperatures (PDF of graphic)
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:
Oasi La Madonnina in Italy shows re-wilding after the mining for limestone has ceased (here).
>> Click here or image to read the PDF (free)
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