Unfortunately, cement production is responsible for about 8% of global carbon emissions, making it a significant contributor to climate change.
In regular cement manufacturing, limestone is heated to around 1,400 degrees Celsius by burning fossil fuel to produce pure lime. There are two key points at which carbon is emitted during traditional cement manufacturing. Burning fossil fuel to heat the limestone is the first key point that releases CO2. Gases released from the limestone during the heating process introduces the second source of CO2. Traditional concrete manufacturing processes result in a ton of CO2 for every ton of cement produced. Since we’re talking about somewhere around 4.1 billion tons globally just in 2022—more than half of that in China alone—cement is a massive contributor to climate change.
One approach to reduce carbon emissions during cement production is based on electrolysis, which splits water at near neutral pH, producing hydrogen and oxygen gases, but also a pH gradient between the two electrodes. A calcium-bearing mineral reacts with the acid formed at the anode to generate dissolved calcium ions, which then migrate toward the cathode. When they reach a pH of at least 12.5, they react with the base generated at the cathode to precipitate out as solid calcium hydroxide, also known as builder’s lime or slaked lime. This lime is a drop-in replacement that feeds into traditional cement manufacturing from there.
Researchers have found that the process is quite flexible and works with a variety of different feedstocks. Where low-grade limestone is all that’s economically available, the electrolysis process can use that, although use of traditional or low-grade limestone as it’s calcium source will produce CO2 as a byproduct. If limestone is used as the calcium source, the CO2 that is released is pure, cold, and in a compressed format that enables easy recapture. For true zero-carbon cement, the process can draw pure calcium from impurities in silica, magnesium, iron, or aluminum, for example, refining them into higher-grade materials in the process. Crucially, it all happens at room temperature, with no need for a combustion furnace or electric heating.
These processes are moving from theory to practical solutions. A company called Sublime Systems has produced cement using electrolysis as outlined above, which has attained ASTM C1157, Standard Performance Specification for Hydraulic Cement, the industry standard required of cement that can be used in concrete.
Clients are increasingly seeking to reduce the carbon emissions for capital projects. If companies like Sublime can scale electrolysis-based cement production successfully, zero or low-carbon cement that meets US and international building codes will be more readily available in the marketplace.