The carbonation degree determines the carbonation capacity of materials and the actual amount of CO₂ absorption. Accurate and efficient assessment of the carbonation degree of materials can provide guiding suggestions for the selection of materials in actual carbonation applications and the optimization of carbonation environmental conditions.Therefore, methods to evaluate the carbonation degree were analyzed and compared from the perspectives of carbonable reactant consumption, product generation and mass change during carbonation in this study. Using these methods, the carbonation degree of steel slag, C₃S and calcium carbide slag at l/s = 0 and l/s = 0.15 was measured by QXRD, TG or weighing.

The results demonstrated a similar variation trend in the carbonation degree of the three raw materials under different liquid-solid ratios, with a variance of ±1.07%. In addition, both XRD and TG analyses yielded highly comparable carbonation results, exhibiting linear fitting correlation coefficients greater than 0.98. However, slight differences were observed in the carbonation values obtained by the dry mass change method, particularly in the presence of liquid water. Nevertheless, all methods investigated in this study exhibited strong consistency and reliability, including the dry mass change method which displayed an acceptable deviation from other methods and was quickly and conveniently testable. Furthermore, a carbonation degree of 24 hours at l/s = 0.15 is proposed as an index to evaluate carbonation activity due to its ability to rapidly and effectively difference between different materials’ carbonation potential. Finally, carbonation activity inventory was established, in which calcium-rich materials were categorized into five groups in terms of their activity. These evaluation methods for carbonation are conducive to select highly active materials and are expected to support improvements in environmental benefits associated with material carbonation treatment.

The French Fastcarb project objectives included the storage of CO₂ in recycled concrete aggregates (RCA). As part of theses, it was necessary to evaluate techniques for measuring CO₂ captured during accelerated carbonation. In order to assess the effectiveness of a process and determine any associated environmental benefits, an accurate assessment of the CO₂ mineralised during carbonation is required.

The aim of this study was to carry out collaborative experiments (lab1, lab2, lab3) to measure the rate of CO₂ captured after accelerated carbonation using a calcimetric method. This chemical method for quantifying CO₂ is based on reacting the material with an acid (HCl). It consists of using a Bernard or Dietrich-Frühling calcimeter to measure the volume released during the reaction with the acid at constant atmospheric pressure and temperature. This study was carried out on RCA (0/4 and 4/16) from the same source. These are aggregates produced from residual concrete returned to ready-mix concrete plants. A semi-industrial process was used on the site of a cement plant to achieve accelerated carbonation of RCA. A calcimetric analysis of RCA before (NC) and after carbonation process (C) was carried out in laboratories.

It has been shown that the calcimetric method is suitable when it is carried out in the laboratory in a controlled environment. For a same sample preparation and storage (st) the repeatability (within the same lab.) and the reproducibility (between lab.) of the measurement are very satisfactory. The repeatability of the sample preparation during the production of the powder is very satisfactory, while the reproducibility is moderate. This study contributed to the proposal of tests recommendations in the context of Fastcarb.

In the scope of Carbon Capture, Utilization and Storage (CCUS), carbonation of recycled concrete aggregates (RCA) is one way of sequestering CO₂ from industrial facilities as well as atmospheric CO₂. This kind of process can be seen as a low-cost and low-tech solution. The effectiveness of such a solution depends mainly on the CO₂ binding capacity of RCA. Thus, test methods are needed to assess it properly. For this purpose, thermogravimetric analysis (TGA) is a usual method used to quantify the amount of CO₂ bound by carbonation. However, TGA has limitations due to the small sample size (a few hundred mg), which requires careful sampling and preparation. In the present paper, we introduce the principle of a new method allowing for testing larger masses of RCA. This method consists in determining instantaneous CO₂ binging rate (IBR). A sample of RCA is placed in a closed cell where the CO₂ concentration is monitored by a sensor. As the CO₂ is consumed during carbonation of the sample, regularly, the cell is refilled by CO₂ to keep the initial concentration. IBR are deduced from the decreases of CO₂ concentration. The quantity of bound CO₂ is calculated by integrating the time-evolution of IBR. Based on the same principle, two versions of this test method are available. The first one is manual and deals with atmospheric carbonation (Figure 1). The second one is automated and simulates an accelerated carbonation at regulated high CO₂ concentration. To validate the proposed methods, samples of crushed mortar were tested, and the CO₂ binding capacities obtained through different methods, IBR, TGA and loss on ignition (LOI), were compared.

Carbonation of cementitious systems is a growing field of study due to cement’s potential to permanently store CO₂ via mineralization. Moreover, carbonation is a well-known degradation mechanism that lowers the pore solution pH, potentially promoting corrosion in reinforced concrete systems. Currently, several methods are available for recording the extent of carbonation and studying carbonation kinetics. These range from observing the surface pH change via a phenolphthalein indicator to quantifying calcite via bulk methods (XRD, TGA). However, there are fine microstructural details at the carbonation front that these traditional techniques may not be able to record. Recently, Raman imaging has been reported to be a potential tool for studying microscale precipitation and growth of calcium carbonate on cementitious substrates subject to carbonation.¹ We are now interested in exploring if this technique can be extended to track the extent of carbonation and if any new insights can be gained on the nature of the carbonation front.

In this study, we report our preliminary results with Raman imaging on a series of cementitious pastes subjected to CO₂ exposure. We subject prismatic specimens of varying porosity (w/c ratio ranging from 0.4 to 0.7) to unidirectional CO₂ ingress at 20% CO₂concentration at a relative humidity of 60% and a temperature of 50 C for 3 days. As expected, the sample's porosity directly affected the extent of carbonation, which was verified by surface pH change measurements conducted by the phenolphthalein indicator via optical imaging. However, the exact extent of carbonation, as measured by calcite precipitation via Raman imaging, portrays a slightly more complex picture than the sharp front often visible by the pH change measurements. In the talk, we will elucidate the nature of this complex microstructure modified and shaped by the active growth of calcite. In summary, optical imaging measurements, when combined with Raman imaging, offer a rich and detailed view of the complex, evolving carbonation front.

This study investigates the wet carbonation of C₂S in a closed system at room temperature, employing techniques such as quantitative X-ray diffraction (QXRD) and thermogravimetric analysis (TGA) to monitor the carbonation process. It also involved measuring pH, calcium and silicon concentrations, modeling carbon species in solutions, and determining the saturation indices of phases.

The new innovative approach in the study to understanding the carbonation reaction comes from the measured stable carbon isotopes in the system. We measured δ¹³C in CO₂, dissolved inorganic carbon (DIC), and carbonate phases to explore the potential fractionation of δ¹³C isotopes during carbonation and whether this fractionation could be linked directly to the carbonation reaction. The findings indicate that δ¹³C isotopes undergo fractionation throughout the carbonation process, which spans 48 hours in our experiment, transitioning from the gaseous phase to the solution and finally to the solid phase, with isotopic fractionation correlating with the carbonation's progress.

The research highlighted the formation of three carbonation products within 48 hours: calcium carbonate (in calcite and vaterite forms) and amorphous silica gel. The carbonation process was divided into three stages measured through CO₂ consumption and the phase content of C₂S and CaCO₃, revealing detailed insights into the reaction dynamics and the potential of δ¹³C isotopes for tracing carbonation pathways and reaction timelines.

Future studies delved into the distinctiveness of δ¹³C fractionation during carbonation and its ability to differentiate between CaCO₃ sources, which is crucial for accurately determining CO₂ uptake in carbonation, particularly in materials that already contain CaCO₃ as aggregate before carbonation. This research opens new avenues for understanding and quantifying carbonation processes, with implications for carbon capture and storage technologies.

Great interest in carbon uptake in concrete has emerged in recent years. The hydration products from cement can react with the CO₂ in the atmosphere, with calcium carbonate as the chemical reaction's final product. In connection with this, there are many tons of concrete demolition waste standing in landfills, which could be used as a CO₂ sink and potentially material to produce new concrete. Therefore, the development of methods to determine the degree of carbonation of concrete elements and crushed concrete particles are essential to assess the best storage and processing actions for crushed concrete between the time of demolition/crushing to the time of recycling/application in new concrete.

The objective of the study was to find a suitable method to determine the carbonation degree of crushed concrete. Three samples were evaluated: crushed mortar cast in the laboratory with known composition (reference material), crushed concrete from an industrial concrete plant exposed to carbonation due to full outdoor exposure, and the same crushed concrete without full exposure to carbonation. The carbonation degree of crushed concrete was evaluated using three methods: Selective acid dissolution combined with Ca²⁺ titration (SACT), petrographic analysis with pH indicator, and thermal treatment.

To determine the carbonation level, it is essential to have information about the cement paste and/or the cement content. The cement paste content was determined by acid dissolution (2M HNO₃ solution) for methods 1 and 3. For method 2, point counting was conducted on thin sections. All methods for determining the cement paste content showed disadvantages. The primary difficulty with acid dissolution is the insoluble residue of cement and the soluble phase of the aggregates. In the case of point counting, it is challenging to obtain a representative sample and count the fine particles around the aggregates.

The carbonation degree results varied significantly depending on the method used. For crushed concrete not exposed to direct carbonation, the measured carbonation degree was 67% using method 1, 19% using method 2, and 45% using method 3. In contrast, the carbonation level of crushed concrete samples exposed to direct carbonation was 95% using method 1, 62% using method 2, and 101% using method 3. This variation could be attributed to the challenge of distinguishing the CaCO₃ content originating from the aggregates and that resulting from the carbonated cement paste. On the other hand, method 2 yielded the lowest carbonation degree values, possibly because the quantification of the carbonated paste is based on the areas not colored by phenolphthalein spray (a change in pH < 8.2) and not the CaCO₃ content directly. Consequently, results obtained by methods 1 and 3 may not be directly comparable to those obtained by method 2. Moreover, the carbonation degree determined in the non-carbonated reference sample was 24% using method 1 and 6% using method 3. This suggests that the SAM solution may not fully dissolve the CSH and Ca(OH)₂ phases. Additionally, the weight loss observed between 550-850ºC could be linked to chemically bound water.

The authors conclude that each method has distinct limitations and recommends employing multiple methods to evaluate the carbonation degree of crushed concrete. Additional research is required to determine the reliability of using SACT to measure the carbonation degree of crushed concrete.