The construction industry is (slowly) beginning to see increased focus on the environmental impacts of buildings and construction. One area that seems to be getting a lot of attention recently is the carbon emissions due to cement and concrete. This seems at least partly due to Bill Gates’ book “How to Avoid a Climate Disaster”, which talks about emissions from cement (and I’ve been told everyone in Silicon Valley read.) Cement production is a huge industry, and is responsible for somewhere around 5-10% of global CO2 emissions [0]. Because concrete is such a fundamental part of the built environment, it’s worth taking a look at this problem in the context of how, as a civilization, we use concrete.

First, a bit of background – concrete is formed by mixing together cement, aggregates (such as sand and gravel), and water. The cement (which mostly consists of calcium silicates), reacts with the water in a process known as hydration, and binds the entire mixture together into a single solid mass. Beyond its strength and its low-cost, it’s the property of starting out liquid that makes concrete so useful – it can easily be formed into any shape, and can be leveled with the help of gravity to form a flat walking or bearing surface. Essentially all modern concrete is reinforced with steel bars, which provide tensile strength and arrest cracks.

The CO2 emissions from concrete are mostly due to the production of cement [1]. This is partly due to the energy intensive production process, where limestone and clay are heated in a kiln to ~2500 degrees F. And it’s partly due to the chemical reaction that produces cement (calcinization), which releases CO2 as a byproduct. Roughly 50-60% of emissions of cement’s carbon emissions are due to calcinization [2], and thus wouldn’t be addressed by moving to less carbon intensive electricity sources.

The scale of concrete production

The most important thing to understand about concrete is the scale of its production. The world produces somewhere around 4.25 billion metric tons of cement annually [3], which works out to about 30 billion tons of concrete produced each year [4].

How much is 30 billion tons?

It’s not easy to get a sense of scale once you start to deal with numbers this big (billion doesn’t feel 1000 times bigger than million). One way of looking at it is we produce around 4 metric tons, or just under 60 cubic feet, of concrete for each person on the planet every year (this is a cube just under 4 feet on a side).

Another way of looking at it is to consider the total amount mass that gets input to civilization each year. Estimates here vary quite a bit, but it seems to be in the neighborhood of 100 billion tons [5]. So of the total volume of material that gets extracted and used each year (including all oil, all agriculture and tree harvesting, and all mining), around 30% of it by mass goes toward making concrete. The amount of concrete produced each year exceeds the weight of all the biomass we use annually, and all the fossil fuels we use annually.

Another way of looking at it is that the total mass of all plants on earth is around 900 billion metric tons. So at current rates of production, it would take about 30 years to produce enough concrete to exceed all the Earth’s plant (dry) biomass.

And because a) we’ve been producing concrete for a while, and b) concrete tends to last a long time, we seem to be on the cusp of this happening. Elhacham et al 2020 estimates that total human-created mass (roughly half of which is concrete) reached the total weight of all earth’s biomass sometime in 2020. Eyeballing their graph, concrete alone will exceed total biomass weight sometime around 2040.

In a pure mass-flow sense, human civilization is basically a machine for producing concrete and gravel (and to a lesser extent bricks and asphalt), with everything else as just a byproduct.

Where concrete goes

So civilization uses a lot of concrete. Where is it all going? 

Geographically, everyone probably knows the punchline here – most of this concrete is being used in China. In recent history, China has been responsible for roughly half the world’s cement production and consumption (and by implication, concrete use) [6].

Here’s another view from around 2010, showing what this has looked like over time (data after 2010 is a projection).

This gets summarized in the oft-repeated statistic that China used more cement in 3 years than the US did in the entire 20th century.

But since China has such a large population, we can get a more intuitive understanding of this by looking at cement consumption per-capita. Here’s per capita consumption sometime around 2015 (via Globbulk):

We see that the official numbers from China make it a huge outlier in cement consumption, using around 8x the amount per capita as the US (though some Middle Eastern countries exceed it. Saudi Arabia is higher, and Qatar, which is somewhere over 2000 kg/capita, doesn’t even show up on the graph).

The official Chinese numbers are so huge, in fact, that some analysts suspect that they’re inflated, either by manipulating the data, or by producing construction projects that don’t have actual demand (or both). The graph above also includes a more “realistic” estimate (which is still 3x as high as US per-capita use).

What does all this concrete construction mean in practical terms? Well, China has somewhere around 50%-60% of the floorspace per capita as the US does, or roughly as much living space per capita as most European countries [7]. This is the result of a massive trend towards urbanization over the last quarter century. In under 20 years China quintupled its urban residential floor space, and urbanization rates went from around 25% in 1990 to 60% in 2017, during which it also increased its population by 250 million (roughly the size of the 1990 US). China is what you get if you double your total floorspace in 20 years in a country of over a billion people, and use concrete construction as the way to do it.

Beyond China, we see high rates of cement use in the rest of Southeast Asia, as well as the Middle East [8].

One reason you see this volume of concrete use in lower income, urbanizing countries is that concrete construction is comparatively labor intensive. The materials for concrete are extremely cheap, and much of the cost (especially in high-cost labor countries, such as the US) is due to the labor to produce it – building and setting up the formwork, laying out the reinforcing, placing the embeds, etc. If you’re a country with a lot of low-cost labor, this is a pretty good tradeoff.

In addition to the current largest users, one trend to keep an eye on long-term is India’s concrete use. If it ever proceeds on a path of mass-urbanization similar to China, (as some folks speculate it will), we could see a massive uptick in global concrete output – India’s current urbanization rate of 34% is around where China was in the late 90s. A shift in India towards a per-capita cement consumption more consistent with the rest of southeast Asia (say, around 600 kg/capita) would increase worldwide cement consumption by about 13%, and it does seem as if India’s cement use is trending upward.

By contrast, one thing clear from this data is that the US actually uses an unusually low amount of concrete – per capita, it uses