Industry is responsible for 23% of carbon emissions, close to the amount of electricity production (25%) and transportation (28%). We talk a lot about transportation and energy, but industrial carbon is a harder nut to crack. Also, the 23% is direct carbon release from industrial processes, if you include the energy used by industry the contribution is 30%. Industry and manufacturing is increasing at a steady rate worldwide, and by some estimates we could be seeing a 90% increase in CO2 release from industry by 2050. This could wipe out any gains we make in the energy and transportation sectors.

The two largest contributors to direct industrial carbon release are steel and cement. Cement production is responsible for about 8% of CO2 emissions, about a third of industrial release. Another third is from steel. So these two industries are a ripe target for reducing carbon emissions. To put the carbon footprint into perspective, if cement production were a country, it would be the third largest emitter after the USA and China.

The world produces 4.4 billion tons of concrete each year (cement is a main ingredient in concrete). This is excepted to increase to 5.5 billion tons by 2050. Some type of cement has been used by people for about 12,000 years, with the first concrete dating to about 800 BC. The Romans perfected concrete, and built an empire out of it. Today cement and concrete are essential building materials for our modern world.

About half of the CO2 from cement production comes from the following chemical reaction – CaCO3 + heat → CaO + CO2. CaCO3 is limestone, and the CaO is what is known as clinker, a main ingredient in cement. The other half of CO2 emissions comes from the energy necessary to heat the limestone to drive the reaction. Some of that CO2 is then absorbed back by concrete, but that takes decades. There are lots of proposals for how to reduce the carbon footprint of cement and concrete, or even eliminate it. The main barrier is that any such process needs to be done on a massive scale – able to make 4.4 billion tons per year and growing.

One proposal is to recycle old concrete. In this process the old concrete is crushed into a powder. That powder is then used as flux in the recycling of steel. When the ash is then collected from that steel production, it can be quickly cooled and hardened into clinker, which can be used to make fresh cement. At least this allows for a completely circular process for the recycling of old concrete. It also uses an existing industrial process of recycling steel. In order for this process to be truly green, however, the steel mills would have to use electric furnaces powered by green energy. There was about 360 million tons of concrete debris created in 2015, so that can take a chunk out of new concrete production – but this is obviously not a solution by itself (about 9% of concrete production).

Some proposals involve reducing the energy demands of cement production, mostly by using electricity in replace of heat. This can produce so-called room temperature cement. And again – if the power for the electricity comes from low CO2 sources, this can be a huge advantage. This same approach, by the way, has also been proposed for steel making – using electricity instead of heat generated by fossil fuel.

There are also many alternative cement recipes proposed. From what I can see these come in two basic flavors. The first adds ingredients to concrete to make the resulting concrete stronger and lighter. Perhaps the best option here, in terms of resulting physical properties, is carbon nanofibers, or graphene. This makes for strong and durable concrete, which means we can use less of it. In one study, “The resulting concrete was 25% lighter than concrete made with a normal aggregate, and showed a 32% increase in toughness, 33% in peak strain, and 21% in compressive strength.” So if we can use 30% or so less concrete, depending on the application, that right there reduces the carbon footprint of cement by 30%. This also has the added benefit of reducing the use of sand in concrete, which is increasingly becoming a limited resource.

The other type of concrete alternative replaces the clinker in the original cement. One proposal is to replace clinker from limestone with other materials, such as coal ash. This takes an existing waste stream and uses it to replace cement, up to 80% of it. This could be a good option for the next decade or two, but I hope that we will phase out coal power as quickly as possible, so this should not be a long term solution. But in the meantime, it can be one of many solutions.

Another approach is to inject CO2 into the concrete mixture when it is being made, instead of waiting decades for it to slowly absorb CO2. This potentially could create carbon neutral concrete.

The problem with all of these approaches, which all seem to work, is getting them up to industrial scale at a competitive cost. Of course, if any of these methods produced cheaper concrete at scale, then problem solved. But that is not the case. We can keep looking for a cheaper alternative, but it may be difficult to get cheaper than a process we have been doing for hundreds of years with an existing infrastructure. One approach is to consider the externalized cost of the carbon footprint of the cement industry. We could charge them for that cost by pricing carbon, making low carbon alternatives price competitive. Economists generally agree this is the best solution, but pricing carbon is a political heavy lift. Another approach is to subsidize the lower carbon but more expensive alternatives in some way. If this ultimately reduced climate change this is a good investment for public funds.