Thanks Martin - I think it is especially important to recognize that CSHubMIT is funded by the cement industry. The issue of carbon uptake in cement via carbonation requires a thorough rebuttal, but I’ll jot down some thoughts to get the conversation started.
When considering the lifetime carbon emissions of a pavement / building / structure, the carbonation of the concrete generally requires replacement, usually by more concrete, leading to further emissions. To assume that there is no replacement and thus only account for the uptake of carbon dioxide via carbonation is quite disingenuous.
In service the uptake of CO2 is typically ~3% of the production emissions, and although at end of life, crushing and exposing more surface area can greatly increase this number, assuming that 40% of your emissions will be absorbed in 100 years and thus can be discounted today is a massive problem when we know we need to reduce emissions to 0 as fast as possible. This also does not account for the time value of carbon / social cost of carbon, which is estimated to inflate by 3% p.a., due to compounding damages, thus the benefits of absorbing carbon at end of life are greatly diminished.
Concrete should be made to be durable and resist carbonation to get the most out of the ‘carbon investment’. What the concrete industry should be looking at in my opinion are the following options:
- recognizing that zero emissions cements are technically challenging and some years away, focus on how concrete elements will function in a circular economy, not just downcycling / recycling at end of life, but reusing concrete elements.
- rapidly shifting to electrification of kilns, perhaps via microwave calcination.
- paving the way for alternative cementing technologies, including geopolymers / AAMs, and biocementation.
Finally, I really like Blue Planet aggregates, and this could be one avenue to make true net zero concrete (without offsetting), provided that the energy source is renewable.