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.

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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

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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: ).

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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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I wanted to pick this thread up based on the recent Dezeen article that is quoting the cement industry flag waving about a 50% sequestration that is quoted in the IPCC AR6 report.

The full report AR6 WG1 is here: https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Full_Report.pdf

The quotes from the Dezeen article are in chapter 5: “Direct CO2 emissions from carbonates in cement production are around 4% of total fossil CO2 emissions, and grew at 5.8% yr-1 in the 2000s but a slower 2.4% yr-1 in the 2010s. The uptake of CO2 in cement infrastructure (carbonation) offsets about one half of the carbonate emissions from current cement production (Friedlingstein et al., 2020)”

The Friedlingstein paper is available here: ESSD - Global Carbon Budget 2020

It references papers by Cao et al. (2020): The sponge effect and carbon emission mitigation potentials of the global cement cycle | Nature Communications, Guo et al. (2020): https://essd.copernicus.org/preprints/essd-2020-275/essd-2020-275.pdf, and Xi et al. (2016): Substantial global carbon uptake by cement carbonation | Nature Geoscience

It is the 2016 paper by Xi that contains the carbonation model. The model calculates carbon sequestration during three stages in the ‘life cycle’: service life, demolition and secondary use of concrete waste. They assume that building service life is 35 – 70 years, which is somewhat concerning. The bulk of the carbonation in their model occurs after demolition, and they state that because buildings have shorter lives in China (35 years) this is a benefit to the carbon cycle. Clearly they do not consider that the carbonated concrete is usually replaced with something, typically this is more carbon emitting concrete. Reductio ad absurdum the authors are advocating for digging limestone up, calcining it to release emissions and then crushing it to absorb back 50% of the emissions, clearly this is not a sustainable outcome unless we are going to live in rubble. (At least one of the authors of this paper is from the Swedish Cement and Concrete Research Institute.)

The real flaw seems to be in the carbonation rates used - these are taken from literature and field reviews of atmospherically exposed concrete. However, it is well established that carbonation rates vary with relative humidity, see this RILEM report for instance: https://link.springer.com/article/10.1617/s11527-020-01558-w I would contend that demolished concrete is normally going to be in a saturated condition, and thus the carbonation rates will be negligible. Therefore the effective sequestration of CO2 is likley to be <5% of process emissions. Does anybody have any sources that could shed further light on this?

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This rebuttal article puts it succinctly in that it’s inaccurate to call concrete a sink because on the whole it contributes more than it takes out. Additionally, the 50% number quoted doesn’t include fossil fuel emissions used to make the concrete (roughly half the total emissions).

Also interesting that the process requires certain conditions:

Shah also said that “cement carbonation requires very specific conditions” including humidity of between 40 and 80 per cent and open-air conditions.

“Submerged or buried concrete or concrete will not undergo carbonation,” he said, adding that “concrete carbonation happens at an extremely slow rate: an average of one to two millimetres per year.”

… implying that this might not occur in regions lacking this humidity and not at all if buried in a landfill.

Do go beyond the IPCC summary comments and look more closely at the key reference studies that were the basis of the observation. In particular look at Global Carbon Budget 2020, available here Earth Syst. Sci. Data, 12, 3269–3340, 2020, https://doi.org/10.5194/essd-12-3269-2020,
© Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License.

The rebuttal article in Dezeen is flawed. The logic of dismissing the carbon sequestered in concrete in the built environment as a carbon sink simply because the cement and concrete sector is still a net emitter applies equally to the use of wood in the built environment. Wood use has been sheltered by the concept of carbon neutrality for far too long and as any reasonable assessment will show the carbon emissions and permanent environmental damage from timer harvesting and wildfires seriously call into question the notion that our forests are true carbon sinks.

@FrankCame - Thanks for sharing. Could you give some of your takeaways from that research?
From what I can gather, the cement carbonization sink is generally included in the fossil fuel emission accounting or excluded. Yes, the cement sink is increasing because of how much the world has produced, but is not a net-sink like the land or ocean.

It’s fair to say there’s a balance of both source and sink, but the point is it’s important how this is presented. Giving your child $10 to buy ice cream and them returning the change (in this case, $2.50 in pennies over years and without interest) is indeed a cost to your financial budget. Labeling as a sink gives the perception that it’s we should do more of it and prolongs the perception that we don’t have to change (the opposite of IPCC report’s ‘red alert’ message).

Yes, timber has its own challenges, but calling the article flawed because it doesn’t address the whole life-cycle of another material is a distraction. Agree, we also shouldn’t fill our buildings with unnecessary timber either, but if people want to learn more about that topic, they should go here:

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Not only is this a critical distinction, but if you look into the papers cited in the studies the methodology appears flawed.

Within AR6 Chapter 5 the following statement is made:

The uptake of CO2 in cement infrastructure (carbonation) offsets about one half of the carbonate emissions from current cement production (Friedlingstein et al., 2020).

The first critique of this statement is that the timeline for carbonation is very similar to the decay of CO2 in the atmosphere, it is highly likely that this sink is already factored into GWP calculations, and thus any attempt by the cement industry to incorporate this into LCIs is probably double counting.

Secondly, a thorough review of the referenced literature finds that this figure is highly questionable, and at best is an upper estimate of the carbonation of cement. This error arises from a misinterpretation of the literature, and erroneous assumptions regarding the carbonation of concrete after service life. The studies quoted in (Friedlinstein et al. 2020): (Cao et al. 2020, Guo et al. 2020, and Xi et al. 2016), all rely on the carbonation model published by (Xi et al 2016), which makes assumptions about rubble stockpiling that are not borne out in practice, and the assumptions regarding the ongoing carbonation of demolished concrete and cement which is typically landfilled or used as roadbase are unrealistic.

Construction waste rubble is typically buried below grade and therefore is subject to much higher relative humidities than atmospherically exposed cement, in this condition the carbonation rates are controlled by leaching of calcium from the rubble, rather than CO2 diffusion. This process is strictly limited and unfortunately does not sequester CO2as efficiently as the authors contend. All studies to date rely primarily on the figure for buried concrete carbonation rates from (Lagerblad 2005), however the source explicitly states that this is an assumed carbonation rate, and no empirical evidence is provided to support the carbonation rates. Indeed (Lagerblad 2005) makes the following statement:

A normal concrete submerged in percolating water without erosion of the surface will leach less than 10 mm in 100 years and carbonates can only be observed in the outer 2-3 mm (Lagerblad 2001, 2003). Like carbonation, leaching approximately follows Fick ìs second law and the rate diminishes with the square root of time. One can, however, presume that all Ca-ions that leach will eventually form calcium carbonate either at the surface of the concrete or in the water. In soil the decay of organic matter may result in high CO2 concentration but on the other hand the speed of the diffusion of CO2 gas or carbonate ions in the soil may be slow.

This condition is the one most relevant to the discussion of carbonation rates after demolition, and would thus correlate to a carbonation rate of 0.2 - 0.3 mm/yr0.5. Furthermore, the authors don’t consider the kinetic limitations of diffusion of carbon dioxide, both in an atmospherically exposed stockpile, or below ground. Below grade, deterioration of concrete (e.g., by H2S and sulphates) is considered likely to greatly reduce the alkalinity available for reaction with CO2. For the brief period of time that the rubble is stockpiled they assume perfect conditions of CO2 exposure, humidity, and that the cement is perfectly spherical, perhaps these are reasonable assumption, but I’ve found no empirical evidence to support the carbonation rates assumed. (Pade and Guimaraes 2007) surveyed recycled concrete aggregate practices in Nordic countries and found that at best 5% of RCA ends up unbound and above ground. (Xi et al. 2016) go so far as to state that stockpiled rubble is very rarely stored under cover, because cement is hygroscopic it is my opinion that the particles in the stockpile will very quickly become saturated following rainfall or exposure to moisture (e.g., during hydrodemolition), restricting carbonation to very low rates.

(Xi.et al 2016) also provide carbonation rates based on 1,300 samples taken from across China, but provide no details on the source of these measurements, it is noted that their figures are approximately 30% lower than those of (Lagerblad 2005). They also assume that CO2 concentration is 3000 ppm below grade, without providing any empirical source for this assumption. Mattias Achternbosch and Peter Stemmermann provide a more detailed critique of the means and methods used to arrive at the 50% offset figure in the quoted studies, it is well worth a read.

Finally the point has to be made that the research used to underpin this carbon accounting fallacy has been funded by the cement industry, in particular the Swedish Cement and Concrete Research Institute.

Respectfully, the carbonation of cement stated in the report is highly dubious and based on simplifying assumptions that are unrealistic.

References

Friedlingstein, P. et al. Global Carbon Budget 2020. Earth Syst. Sci. Data 12 , 3269–3340 (2020).

Cao, Z. et al. The sponge effect and carbon emission mitigation potentials of the global cement cycle. Nat. Commun. 11 , 1–9 (2020).

Guo, R. et al. Global CO2 uptake of cement in 1930–2019. Earth Syst. Sci. Data Discuss. 2 , 1–28 (2020).

Xi, F. et al. Substantial global carbon uptake by cement carbonation. Nat. Geosci. 9 , 880–883 (2016).

Lagerblad, B. Carbon dioxide uptake during concrete life cycle - State of the art . CBI Rapporter (2005).

Pade, C. & Guimaraes, M. The CO2 uptake of concrete in a 100 year perspective. Cem. Concr. Res. 37 , 1348–1356 (2007).

Achternbosch, M. & Stemmermann, P. The carbon uptake by carbonation of concrete structures – some remarks by perspective of TA. 1–31 (2021). (web link)

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Thanks, Luke.

My principal takeaway points from the IPCC Report and the Carbon Budget 2020 study are summarized in this article I prepared for the Building Resilience Coalition websites.
See here https://bityl.co/8aSb

The key charts I took from the study are attached - they show the thin line of carbonization recovery. My cautionary points are that we have yet to fully develop the accounting metrics for the reabsorption process, but in time, we should be able to incorporate such figures based on full lifetime LCA models.

Frank

Frank Came

Per capita Emissions.png

essd-12-3269-2020-f03-high-res.pdf (126 KB)