SublimeCement says it is ready to begin scaling up "the world's cleanest cement" that meets industry performance standards, relies on room-temperature electrolysers instead of fossil fuel furnaces, and uses a variety of zero-carbon input materials. The traditional concrete production process produces one ton of carbon dioxide for every ton of cement produced. Global carbon dioxide emissions in 2022 will be approximately 4.1 billion tons, with China alone accounting for more than half.

So this is also a huge opportunity; as decarbonization mandates and carbon taxes come into effect, whoever can be the first to produce a cost-competitive, completely zero-carbon alternative to cement on a global scale could be sitting on a huge money printing press.

There is no shortage of low-carbon and carbon-capturing concrete alternatives, but MIT spinout Sublime Systems is of particular interest because it uses a novel electrochemical method to produce cement that is "true zero carbon," rather than "net zero carbon" or "carbon sequestration" -- because it can completely eliminate carbon emissions at two key points in the production process where they are typically produced.

The resulting zero-carbon lime is identical to existing products and emits one ton of CO2 per ton of concrete. It is a drop-in replacement in the ready-mix concrete process.

In conventional cement production, limestone is heated to approximately 1400°C (2552°F), the first major source of emissions, by burning fossil fuels in order to produce pure lime. However, limestone is about 50% carbon dioxide by weight, and when the furnace heats and releases the carbon dioxide, it is difficult to capture because the carbon dioxide mixes with the flue gases.

Sublime's approach is completely different, using electrolysis. Company co-founders Dr. Leah Ellis and Dr. Jiang Yiming published an article on Medium explaining this method in detail, but the basic process is as follows.

Water splits at a near-neutral pH, producing hydrogen and oxygen while creating a pH gradient between the two electrodes. Calcium-containing minerals react with the acid formed at the anode to form dissolved calcium ions, which then migrate toward the cathode. When their pH reaches at least 12.5, they react with the alkali produced by the cathode, precipitating out as solid calcium hydroxide, which is also known as building lime or hydrated lime.

Sublime's process uses room-temperature electrolysis to separate the necessary calcium from a range of different input materials

This lime can directly replace traditional cement production. The founders say Sublime has developed this method, which is now more energy efficient, does not require the production of hydrogen, can separate the oxygen and carbon dioxide gas streams, can continuously extract lime from the reactor, and can be produced using off-the-shelf electrolysis equipment.

The process is very flexible and can use a variety of different raw materials. It can also be used if low grade limestone is only available. This process cannot avoid the production of carbon dioxide, but at least it is produced in a pure, low-temperature state and is prepressed to 10 bar, making it easy to capture and store. Compared with ordinary cement production, energy consumption and emissions are greatly reduced.

For truly zero-carbon cement, the process can extract pure calcium from impurities such as silica, magnesium, iron or aluminum and refine them into higher-grade materials during production. Best of all, it all happens at room temperature, with no need for a burner or electric heating.

Sublime claims it is "the only concrete that uses no fossil fuels, is scalable and can be a drop-in replacement for traditional cement."

A few days ago, Sublime announced that its products have received ASTMC1157 certification. This is an industry standard based purely on performance requirements, with test parameters including strength development, durability, shrinkage, water retention, air content, setting time, density, early hardening and cracking, meaning Sublime cement can now be used in compliance with major US and international building codes.

Zero carbon lime produced using the Sublime process

Carbon capture in cement production costs between $19 and $205 per ton of carbon dioxide, according to a 2022 policy brief from Harvard University's Belfer Center. So without knowing which end of the scale Sublime is talking about, the company's first clean cement product could cost anywhere from 15-158% more than the shelf price of its dirty competitors.

Concrete production accounts for around 8% of total global CO2 emissions - it's a huge industry and decarbonised alternatives will make a huge contribution in the race to reach zero carbon by 2050.