Concrete Carbon Accounting Uses Isotope Fingerprints

By Dr. Noopur JainReviewed by Susha Cheriyedath, M.Sc.Jun 15 2026

Carbon accounting improves as isotope measurements distinguish fossil-derived CO2 from atmospheric uptake in cementitious materials, helping verify concrete sequestration claims for emissions reporting, carbon trading, and low-carbon construction systems today.

In a recent research article published in the journal Cement and Concrete Research, researchers developed a novel isotope ratio measurement technique to accurately distinguish and quantify carbon dioxide absorbed by concrete from natural versus industrial sources, improving carbon accounting methods.

Carbon Isotope Challenges

Concrete production is a major contributor to global carbon dioxide (CO₂) emissions, accounting for a significant portion of the greenhouse gases driving climate change. In response to pressures for sustainability in the construction industry, researchers have explored methods to enable concrete to absorb CO₂ rather than simply emit it.

One promising avenue involves special concrete production techniques designed to sequester carbon dioxide by incorporating it into the concrete material itself during manufacturing. However, a critical uncertainty that persisted was the precise source of the absorbed CO₂, whether it originated from atmospheric air or from industrial exhaust gases.

Differentiating these sources is essential for accurate carbon accounting and effective carbon trading, as emissions from industrial sources carry higher regulatory and economic implications than those from natural background emissions. Before this study, the difficulty lay in quantifying how much CO₂ absorbed by concrete was derived from industrial emissions versus atmospheric sources.

Isotope Ratio Analysis

The study employed Ordinary Portland cement pastes prepared with a water-to-cement mass ratio of 0.55. The cement was hydrated for over 2.5 years in saturated lime water to ensure full hydration, then crushed and ground under a nitrogen atmosphere to avoid atmospheric CO₂ contamination before experiments. The powder samples were sieved under 90 μm and freeze-dried to achieve stability before carbonation.

Carbonation was conducted under controlled relative humidity conditions, specifically 33%, 60%, and 95% RH, maintained using saturated salt solutions inside sealed desiccators. A CO₂-free atmosphere was maintained using air passed through a CO₂ adsorbent and electromagnetically pumped, ensuring that only designated fossil-based CO₂ (from a ¹⁴C-free cylinder) was introduced for the accelerated carbonation experiments.

Two types of carbonation experiments were performed: accelerated carbonation using fossil-derived CO₂ (¹⁴C-free) under controlled humidity and natural carbonation involving exposure to atmospheric CO₂ containing natural ¹⁴C. Sequential mass measurements ensured equilibrium of water content inside the powders before carbonation at each RH level.

Following carbonation, the total carbon content fixed in the samples was quantified by thermogravimetric analysis (TGA) and total inorganic carbon (TIC) measurements to ascertain CO₂ uptake. Carbon isotopes were analyzed using isotope ratio mass spectrometry (IRMS) for δ¹³C and accelerator mass spectrometry (AMS) for ¹⁴C content expressed in pMC (percent modern carbon) units.

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Two approaches were developed to estimate the fraction of atmospheric CO₂ uptake, defined as contamination. The fractionation-based method corrected for isotopic fractionation effects during carbonation reactions using measured δ¹³C and ¹⁴C data, facilitating mass balances of isotopes between initial carbon, supplied fossil CO₂, and atmospheric CO₂.

Source Differentiation Insights

The carbonation experiments verified that relative humidity substantially influences CO₂ uptake in cementitious powders. Carbon fixation was highest near intermediate to high humidity (60–95% RH), consistent with the literature pointing to a balance between sufficient moisture for CO₂ dissolution and adequate diffusion pathways within pores. At low humidity (33% RH), limited pore water availability restricted carbonation rates.

An important observation was the measurable incorporation of atmospheric CO₂ into the carbonated samples during accelerated carbonation, despite the supply of ¹⁴C-free fossil CO₂. This atmospheric contamination arose because the sealed incubation environment still contained residual atmospheric CO₂ with a high ¹⁴C signature, which mixed with the supplied gas, influencing the isotopic composition of fixed carbon.

Model comparisons based on measured vs. estimated radiocarbon content showed good agreement for natural carbonation samples but systematic deviations for accelerated carbonation, reinforcing the occurrence of atmospheric CO₂ mixing under artificial conditions.

The two proposed methods for quantifying atmospheric contamination yielded comparable results. The fractionation-based approach accounted explicitly for isotope fractionation during carbonation, with fractionation factors for ¹³C and ¹⁴C derived and applied.

The simpler radiocarbon-based estimation used weighted averages of radiocarbon signatures to calculate contamination fractions. Both approaches confirmed that even minor atmospheric CO₂ presence can noticeably skew radiocarbon measurements due to the high ¹⁴C content in atmospheric CO₂ versus fossil CO₂.

Enhanced Carbon Accounting

This research provides a pioneering methodology that enhances the precision of carbon accounting related to concrete production by quantifying the origins of CO₂ sequestered within cementitious materials.

These findings have significant implications for the construction industry’s role in carbon management, enabling more transparent and verifiable carbon trading mechanisms and environmental reporting. The ability to identify and measure the uptake of fossil-derived CO₂ in special concrete formulations supports the development of carbon-neutral or even carbon-negative building materials, which is essential for achieving broader climate targets.

This work marks a substantial step toward integrating advanced scientific techniques into sustainable building practices, fostering innovation in low-carbon construction materials that can mitigate the environmental footprint of the built environment.

Journal Reference

Maruyama I., Igami R., et al. (2026). Quantification of sequestered fossil-derived CO₂ in cementitious materials and its atmospheric contamination using carbon isotope measurements. Cement and Concrete Research 207:108290. DOI: 10.1016/j.cemconres.2026.108290, https://www.sciencedirect.com/science/article/pii/S0008884626001602