Zero Emissions Cement?

The alchemy bringing steel and concrete together in a 'virtuous loop'

Let’s start with the problem. Fundamentally, to create cement you need to ‘cook’ calcium carbonate (generally limestone) to extract the calcium and oxygen, creating a product called clinker. However there is a major bi-product, carbon dioxide (CO2). Over half of the CO2 emissions from cement production are from this chemical reaction, not from heating of kilns to actually enable the ‘cooking’.

The solution?

It begins with the deconstruction of existing buildings, specifically the reinforced concrete elements. Cambridge Electric Cement (CEC) is eyeing an almost closed loop of steel and concrete.

The existing concrete it crushed, separating the aggregates from the mixture of cement binding them together. The old cement is ground down to a powder and conveniently can can be used instead of lime-flux in steel recycling.

A bi-product of the steel making process is normally that the lime-flux turns becomes slag, but in this instance the ground cement forms something that is virtually identical to clinker, or a new ‘novel slag’. Small pilot trials have proven the near identical chemical similarity between traditional clinker and the novel slag.

I’ve drawn it out below, in case (like me) you prefer diagrams to words.

This is a diagram I produced to explain the ‘virtuous loop’ of CEC

As I write, I’ve got a question. How is this different to GGBS (Ground Granulated Blast-Furnace Slag)? When you say the full name, it does sound rather similar to me! I assume CEC’s innovation is not the bit where they grind the blast furnace slag, but rather the input of ground cement as the replacement of lime-flux as an input into the furnace, and effectively ‘closing the loop’.

Making the comparison to GGBS does make me question how perfect this solution is. For example, GGBS is widely used in concrete production today, and can replace cement up to around 85% in extreme mixes. But it does have limitations such as longer curing time, some durability issues in certain environments and inferior workability. This is why it is not used as a 100% replacement. So does CEC have similar qualities?

I’m also slightly sceptical when it comes to the scale of different materials within the process. Are we crushing enough concrete to feed the steel furnaces around the world, and is steel production sufficient to provide the novel slag for the ever increasing cement demand? Yes, it is a virtuous loop as drawn, but only if the quantities add up.

I look forward to seeing how CEC develops, and how it fits into a wider selection of already available cement replacement products such as Cemfree (which also uses GGBS as a replacement). And watch out for Concrete Zero, a new initiative by Climate Group looking to send demand signals to the industry that a shift is needed.