But the clinker has to be cooled down before it can be used.This cooling process is what releases the other 60% of emissions.
Let’s imagine a modest-sized skyscraper, about 100 meters and 30 stories tall. Such a building would likely have been constructed with concrete, which contains cement. In this case, it would have taken around 6,000 tons of cement to build, and making that cement probably emitted about 4,600 metric tons of carbon into the atmosphere - the equivalent of driving a car for 12 million miles. Now, when considering all the buildings in the world, this doesn’t even begin to capture cement’s carbon footprint, as it is also used for sidewalks, roads, and energy infrastructure, like dams and power plants. In fact, cement production accounts for 8% of global carbon emissions - more than the aviation and shipping industries combined.
Currently, much of these emissions are due to China’s rapid development. In just two years, China produced more cement than the US did in the entire 20th century, and other developing countries will use a lot of cement as they build tall cities and infrastructure. If we want to reach net zero in a few decades, we’ll have to figure out how to build a building like this without emissions.
At a typical cement plant, the emissions come largely from two places: the kiln and the preheater. The kiln is where fuel is burned to heat the cement to around 1450 degrees Celsius, while the preheater is where limestone, clays and other additives are dropped, and the 850 degree temperature here causes limestone to release its stored carbon dioxide.
The first step to decarbonizing cement is to use less concrete altogether. Architects and structural engineers often use more concrete than necessary, as it is valuable for its compressive strength and makes the building more reliable. Carbon conscious design can help reduce emissions, but replacing concrete altogether won’t be possible in the near term. Cutting down on excess concrete can cut emissions on our high rise by roughly 26%, according to one analysis.
We’ll have to do more, and the next place to start is how cement is made. The 40% of emissions that go into firing the kiln can be reduced by switching fuels, such as industrial waste, trash, or used tires, which can cut emissions by roughly 7%. Additionally, the other 60% of emissions that come from the heated limestone can be reduced by cooling the clinker before it is used. Cement accounts for a majority of concrete’s emissions, so one strategy is to find a substitute for the clinker. Startups have been in a race to develop a green cement that avoids clinker altogether, but so far, nothing is as abundant and commercially viable as the limestone-based stuff. Adjusting concrete formulas takes a lot of time and safety testing, and in North America clinker accounts for about 90% of cement due to safety standards. Globally, the average clinker-to-cement ratio was about 72% in 2020, made possible by clinker-like substitutes. Experts have highlighted a new cement mixture that gets the clinker ratio safely down to 50% by supplementing it with more clay and unprocessed limestone, which meets existing building codes and can cut emissions from the cement and concrete industry in half.
However, until a zero-emission cement is found, any clinker production will inevitably have process emissions. Carbon capture and storage will be necessary to decarbonize the cement industry, and a cement company in Norway is piloting one of the first cement plants in the world to capture carbon and store it under the North Sea in their oil and gas deposits. Some new companies are also finding a way to inject stored carbon back into cement and concrete during production, taking advantage of rock’s natural ability to reabsorb carbon.
Changes to design in the concrete and cement processes will be the easiest to implement, but carbon capture and storage is at very early stages and all this will be expensive. The US Inflation Reduction Act is helping create a market for carbon capture and storage by increasing tax credits, and China, the world’s biggest producer, has many energy efficient plants and at least one carbon capture project in the works. The goal is to get to net zero emissions by 2050, so we must start now and aggressively. He was able to film the building under construction and he gives a great explanation about how replacing traditional materials like steel and concrete with wood changes the entire building process. Though wood may not be able to replace concrete in all of the world’s buildings, an expert I interviewed said that in order to scale up the use of wood, we would have to triple the amount of wood harvested, which could come with its own environmental problems. Nevertheless, our big concrete problem requires a variety of solutions and using wood when possible is definitely one of them. Plus, as you can see from Phil’s video, it looks really cool, too.