Since the early eras of our history, humans have used concrete in many different forms. It was just as useful to the ancient Romans and Egyptians as it is to us today. Over the past couple of centuries, cement has literally paved the way for our industries, our roads, and even our homes. This comes as no surprise as concrete is the most widely used man-made material on the planet. It’s relatively simple to produce, it is incredibly versatile, it lasts for decades, and it’s weatherproof. Why should we care then? Well, to be able to create a lot of concrete, you also need to create a lot of carbon dioxide. According to some research presented at the Paris Climate Conference, humans produce roughly 4 billion tons of cement each year, which makes up 8% of global carbon dioxide emissions. In fact, for every ton of cement produced, a ton of carbon dioxide is released into the air. 

This is partly due to a chemical reaction that takes place when the raw materials are mixed together. Some main ingredients in the creation of concrete are raw limestone, iron, clay, and silicon. After they are ground up and mixed with other binding materials like ash, they are fed into a massive cylindrical kiln that gets heated up to 1450℃. Within these large kilns, a process called calcination occurs and separates the raw materials into calcium oxide and carbon dioxide, and the final product are small gray balls known as clinker. The central byproduct of this process is carbon dioxide. Again, according to the report at the Paris Climate Conference, 50% of emissions from this process comes from the chemical reactions inside the kiln. Another 40% comes from the fossil fuels used to heat up and manage the kilns, while 10% comes from mining and transporting raw materials. Long story short, 90% of CO2 emissions from cement comes from the kiln process. The cement industry is a continuously expanding and very profitable business, which if left unchecked could prove to be a huge obstacle in achieving net-zero global emissions by 2050. However, there is room for innovation and sustainable solutions, which gives cement the chance to reduce global emissions if done properly. 

Consequently, scientists and entrepreneurs alike are all scrambling to find the next alternative concrete. Some potential innovations include the following:  hemp bricks, growing bacteria bricks, or even mushroom bricks that can continue growing stably after being set, and  geopolymers that can harden at room temperature. Another exciting example is being produced by the manufacturing company Solidia, which has developed an alternative chemical process for cement, which reduces energy use and emissions by 30%-40%. However the most promising aspect of Solidia’s cement is its use of carbon dioxide. Typically the conventional way to harden cement into concrete is to mix it with water as the binding agent, but Solidia has found a way to do that with carbon dioxide pulled from the atmosphere. This means their cement not only conserves water, which, for an industry that consumes 9% of the total amount of freshwater on earth, is huge, but it sequesters carbon too. As a result of the decreased energy demands, production, and emissions, as well as the absorption of carbon during curing, Solidia claims it can reduce concrete’s carbon footprint by 70%. 

Meanwhile Blue Planet, a startup company in San Francisco, has developed the technology to create ‘synthetic limestone aggregates’ which are basically just rocks, from captured carbon. Simply put, these rocks can be used to replace traditional materials like sand or gravel in the final concrete mix, and in theory, the amount of carbon captured by Blue Planet could equal the amount of carbon emitted by the cement production process. They do this by taking high amounts of carbon out of the atmosphere, and use a chemical reaction to form a solid ‘limestone aggregate,’ similar to how a shellfish builds its shell.

These solutions, though innovative and clever, aren’t without barriers. Solidia’s cement curing process can only happen at a factory, limiting their application, and Blue Planet’s aggregate process only comes in after the cement has gone through the kiln, meaning that its carbon capture properties are really just trying to make up for the carbon already released into the atmosphere from the kiln, addressing the symptom but not the main issue. On top of this, none of these techniques and technologies have been proven at a large scale, and quite frankly, they may not get the chance to. These companies are entering an industry where safety and durability are the highest priority, and in the eyes of safety inspectors and builders, the tried and true conventional concrete almost always trumps new innovations. This means that government intervention and policy change is essential. Without it, the cement industry will remain a one-party system. Also, we can’t just rely on new technologies to save us. The way we build and how much we build matters too. If we can build our cities and towns in a more compact capillary way, so there isn’t such a dire need for a car or a bus to get around, we could reduce our car use by ⅔ and our concrete use by ⅓.

At the end of the day, developing new technologies and more sustainable materials for building is just one piece of the global concrete puzzle. Only combined with lowered consumption, smarter design, and government policy can we begin to truly lay the foundation of a net-zero carbon world.