Carbonation in the Built Environment

Recent studies have shown that exposed concrete in the built environment continues to absorb carbon dioxide from the atmosphere throughout its lifespan and even more so at the end of its life during deconstruction and reuse. The extent of the reabsorption depends on a number of factors, but it is significant enough to warrant more intensive research. The built environment is emerging as a massive carbon sink that is not counted in official carbon statistics but could alter the standings of many countries if it were.

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@frankcame Do they numerical estimates for this?

The research is continuing and we might have a new study ti share oresently. This article summarizes the results from the original Swesish research paper “Carbonation - The New face of Concrete” http://bit.ly/2CKN7o5

An excellent paper by CalPortland is available and has useful insights on the metrics of CO2 absorption. " Incorporating the Effect of Carbonation in Concrete Life Cycle Assessment"

Hi folks,

My research group has published on this as well. We derived a more generalized model that will help folks plug in the amount of fly ash, slag, etc. and type of cement.

Please see the following:

We showed implementation of the model in LCA in regular concrete and pervious concrete (and recently extended it to hempcrete – that paper is in review):


The moral of the story – we can only get back <20% (oftentimes way less if using SCMs) of the initial carbon through carbonation. In addition, it is still very much worth using high-volume SCMs and lower compressive strength concretes where possible to minimize total life cycle carbon.

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Thanks for posting this Wil - and great research. The two issues I see are:

  1. Carbonation causes corrosion of the steel reinforcing, often necessitating patch repair or demolition, that will consume yet more cement with associated emissions. This is touched on, but not explored in depth - perhaps the model needs to limit the volume to the cover zone for the proportion of cement used in reinforced concrete.
  2. The model doesn’t account for the time value of eCO2 emissions, due to feedback loops of global warming some inflation rate needs to be applied to emissions made today that will be absorbed in the future. I’m not sure what the appropriate inflation rate is, but this could also be a way to account for timber and forestry practices impact on the carbon cycle.

Forgive me if I’ve missed anything in the literature.

Hi Will,

Great points!

For (1), you are correct — what we wanted to simulate, is maximum possible carbonation (assuming the building is demolished and the crushed concrete is used as a CCS technology) to compare to upfront emissions. We wanted to show that carbonation would
never “reabsorb” CO2 to recover the initial emissions. For your own analyses, you could certainly find the time it would take to reach the steel rebar depth and limit your analysis to that time frame. The beauty of the model is that it is flexible and can
be used for a myriad of analyses. We just chose one to illustrate implementation.

For (2), you’re totally right; I do think that could be layered on top of the quantification of CO2 emissions vs. sequestration. Since we didn’t know if users would be using this model today or 10-20 years from now, we opted to leave the time-value-of-carbon
calculation to the specific user.

Hope this helps!

w

Wil V. Srubar III, Ph.D.

Assistant Professor

University of Colorado BoulderCivil, Environmental, and Architectural Engineering

Materials Science and Engineering Program
ECOT 441 **• **UCB 428 **• **Boulder, CO 80309

Living Materials Laboratory /
Embodied Carbon Network

With regards to the time value of carbon I took a crack at modelling the emissions balance for Type I OPC concrete vs. 20% GGBFS SCM, using the 30 MPa mix data from the cradle to gate paper, XC1 exposure at 300 ppm atmospheric CO2, square geometry [SA 3.79 m2, V 0.3 m3]. With no interest / discount rate applied the sequestration over 100 years looks like this:

Applying an inflation on emitted carbon of 1.4% (in alignment with the Stern review), the emissions balance looks like this:

I applied the inflation rate as follows:
E_y = (E_y-1 - deltaC_s) * (1 + r)
where: E_y is the CO2 emissions balance for year y,
E_y-1 is the CO2 emissions balance for year y-1,
deltaC_s is the change in carbon sequestered for year y,
r is the inflation rate.

The idea being that the CO2 is acting on the climate from the day it’s emitted, therefore emissions today compound into the future. I don’t know if this approach is appropriate, and what the inflation rate should be, and am happy to discuss. However, the underlying point is that, even with a modest inflation rate, the contribution of carbonation to sequestering CO2 is swamped by the emissions intensity of OPC.

One last point I’d make is that if carbonation of cement is to be included in LCAs, how can excluding sequestered carbon from wood products be justified?

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To the point of including carbonation of cementitious materials, but still ignoring biogenic carbon. I like to differentiate between the two as “carbon sequestration” (carbonation) and “carbon storage” (biogenic). Carbon sequestration is long-term carbon storage (~1000+ years), whereas carbon storage is less permanent (i.e., timber). The carbon sequestered by concretes and mortars will stay in the form of calcium carbonate until it is heated up again to high temperatures (either by humans, or by a geological process) which is unlikely to happen in the near future. Carbon storage on the other-hand describes the temporary storage aspect of biogenic carbon, since trees/grasses decay if left to the elements, and the carbon (or methane) is released back. I recognize that I’m omitting a lot of nuance that comes with forests and natural carbon cycles, but this is my general take on the topic. So within this paradigm, biogenic carbon uptake, and cementitious carbon uptake should be considered separate, yet in their accounting, they can be treated the similarly (idealy with dynamic LCA). For example, I like this study which accounts for both biogenic carbon and cementitious uptake with dynamic LCA (https://doi.org/10.1016/j.buildenv.2017.12.006).

There is not good consensus on how to treat the dynamic aspects of LCIs and LCAs that you raise Will. I find that Charles Breton has done a great job summarizing the discussion around accounting for the temporal aspect in this open-source review: https://www.mdpi.com/2071-1050/10/6/2020 (see Table 5 for a good summary of all approaches people have taken).

While discounting can be applied (see references in Breton et al.), a GWP100 metric (the common metric used in LCIs/LCAs), already accounts for the cumulative impacts of a pulse of a greenhouse gas emissions at t_0 for 100 years, including both direct and indirect impacts. So, the future effects of a GHG emitted today is included in the metric (albeit with many caveats that I’ll leave to the climate scientists to discuss - this is how the IPCC AR5 report handles the issue though - see Ch. 8). Is the carbon discounting that you are applying accounting for this effect already included in GWP100? Or is it something else? Is the 1.4% value you reference for the economic cost of climate change mitigation, or for greenhouse gas emissions? If you could point me to resources on this, I’d appreciate it as I don’t have much familiarity of discounting in the context of GHG accounting.

Now, how this now applies to quantifying the carbon uptake of concretes and mortars in a static LCA. In our simple screening LCA, we have treated the GWP of future carbon the same as the GWP of present day carbon. In reality, the two values do not have the same time horizons and need to be differentiated. With timber, others have treated this difference with a GWP_bio metric (for use in a static LCA). For carbonation GWP, a similar metric, say, GWP_carbonation could be applied. This is a metric that I plan to develop in the next year or so that builds upon the carbonation work Wil linked previously (more to come :slight_smile: ).

Hi, there is a cPCR supplementary to EN15804 called “EN16757:2017 Product category rules for concrete elements” it includes an annex with detailed calculations for carbonation. So an EPD following the standards should be including carbonation in the calculations.

Hi Jay, I’m still wrapping my head around how this should be dealt with. The inflation of emissions is related to the Social Cost of Carbon and I used the 1.4% from the Stern review. It is kind of like asking, what would you have paid to not emit that carbon last year, or 5, or 10 years ago. The required reductions in emissions is increasing every year, Carbon Brief provides this illustrative chart:

My whole career we have been trying to prevent carbonation of concrete - because once it carbonates, the steel corrodes, and then we are patching or rebuilding using even more concrete and producing more emissions. The argument that it will absorb CO2 seems absurd from this point of view, essentially forming quicklime so that it can reabsorb a portion of the emitted CO2 as useless lumps of rocks. All emitted CO2 influences the carbon cycle equilibrium point, whether we can survive at that equilibrium or it ends up as a Venus-like atmosphere is the worry. The climate models don’t include feedback loops as far as I’m aware, and if we started to include thawing of the permafrost, loss of albedo, ocean acidification and loss of blue carbon sequestation, etc. etc. then there would be a real inflation rate on the carbon emitted - likely much greater than 1.4%.

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In looking a bit more at the discount rate approach, I’ve begun to understand it more fully. For others following along, I found this paper helpful for using the methodology alongside a simple LCA for context. I think this discounting approach is valid, and underscores the importance of avoiding emissions today - which is also a key takeaway from our work. That the concretes with the lowest lifecycle emissions, also had the lowest cradle-to-gate emissions and lowest carbon uptake through carbonation. So maximizing carbon uptake potential is NOT the appropriate decision. Again, I’ll point to Table 5 in Breton et al. 2018 that points at all the different dynamic approaches that have been taken to account for the issue of time (your approach Will, is described as the Discounted Global Warming Potential - note though that this approach does not use a time horizon of 100 years for GWP, but rather an infinite one).

I agree that designing to prevent carbonation induced concrete corrosion is the primary thing to do. Build something once that is designed to last as long as possible while avoiding emissions today. Yet, carbonation is going to occur when we hydrate cement to form CH and C-S-H, so we can account for it. And there is (and will be) so much mortar and concrete out there, carbonating, that we are nearly at the Gt per annum uptake of carbon just through the carbonation of cementitious materials. It’s also useful to see that concrete is only 68% of cement usage.

LCA is a useful methodology for comparing design decisions, rather than arriving at exact emissions due to the high uncertainty surrounding LCIs and the GWP values they present. We are also typically using GWP100 which is a midpoint indicator, rather than an end-point one (such as Global Temperature change Potential, GTP), so accounting for potential climate feedbacks are outside the scope of nearly all building-scale LCAs. Using an end-point indicator such as GTP has much higher uncertainty (+/- 90% or so), while GWPs have uncertainties of +/- 26% or so at 100 years (I believe these numbers originated from the IPPC AR5 report, Ch. 8).

Re: Pat’s comment, the methodology presented in the EN 16757 PCR is a great starting point for understanding how to include carbonation in an LCA (for those not familiar with cement chemistry). It is a simpler model that doesn’t account for some new understandings in cement hydration/carbonation mechanisms and how to deal with SCMs - which the model presented in Souto-Martinez et al. 2017 (and others) includes.

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Thanks Jay - I have a lot of reading to do! Perhaps with the urgency of emissions reductions a timescale limit of 10 years is appropriate?

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