Subject matter

The carbonation of cementitious materials such as concrete or mortar is important in at least two respects: First, carbonation induces a decrease of the pH of the concrete pore solution that can potentially lead to the corrosion of reinforcing steel, which in many cases determines the lifetime of reinforced concrete structures. Second, carbonation of cementitious materials involves uptake of atmospheric CO₂ in solid reaction products, which partly compensates for the CO₂ released by the production of the cement used to produce concrete or mortar, a process also referred to as ‘recarbonation’. Thus, carbonation is a process which occupies a central position in determining the sustainability of cementitious materials.

The carbonation rate of cementitious materials depends on many parameters, some of the most important being intrinsic to the materials such as the water/cement ratio and the type of cement, and others linked to the in-service environment such as temperature, the relative humidity (via the degree of water saturation of the pore system), and the CO₂ concentration of exposure. Thus, the carbonation rate (or the coefficient of carbonation, k_c) of concretes or mortars varies widely between different materials and exposure conditions, and it is difficult to predict. Early reports provided a first collation of recommended values to be used for k_c to estimate the CO₂ uptake by cementitious materials during lifetime and after demolition. Though these reports advised, about two decades ago, that these values be verified or adjusted in the future, the k_c originally proposed have propagated into the most recent assessments of global CO₂ uptake by cement carbonation. There is, thus, an urgent need to review and, if necessary, revise and extend this compilation of values, to revise how k_c are being determined, and to assess how representative such values are when used to simulate behaviour in service con­ditions.

The increasing use of new blended cements and non-Portland binders, for which pertinent field data cannot be obtained, necessitates that such a revision is, at least partly, based on results of laboratory tests, including accelerated tests performed at elevated CO₂ concentrations. This requires examination of relationships that have been used to convert carbonation rates obtained under accelerated conditions to rates identified under natural conditions, and to create guidelines about how to predict the carbonation rate of materials based on blended cements or non-Portland binders. In addition, relationships between CO₂ concentration, relative humidity and carbonation rate are needed to estimate k_c in different environments in which CO₂ concen­tra­tions and humidity can differ considerably (e.g., rural areas versus urban or industrial areas; demolished concrete/recycled aggregates below ground).

Another crucial parameter for the estimation of the CO₂ uptake by cementitious materials is which fraction of the CaO of cement in the carbonated layer of a concrete element or mortar is converted to CaCO₃, i.e., the type and abundance of carbonation products. Again, early reports gave an estimate for the fraction of CaO that carbonates, based on simplified calculations for Portland cement and on data obtained under elevated CO₂ concentrations, but indicated that patterns under field conditions might differ. Indeed, it is well-known that the amount of CO₂ bound in the carbonated layer, referred to CaO, differs drastically between different materials and environmental conditions. This might also have a bearing on the protection of steel reinforcement in concrete, as different phase assemblages might induce different pH values of the pore solution with associated consequences for the passivation/depassivation of steel. Recent studies using novel analytical techniques have, indeed, shown that the pH in the carbonated zone can differ by more than one pH unit between different mortars.

The objectives of the Technical Committee (TC) are thus:

• Conduct a comparison of the coefficients of carbonation and related assumptions employed in previous and present assessments of CO₂ uptake by cement carbonation.
• Conduct a comparison of the fraction of carbonated CaO in the carbonated layer and related assumptions employed in previous and present assessments of CO₂ uptake by cement carbonation.
• Determine the influence of the CO₂ concentration on the carbonation rate of concretes as a function of cement/­binder types and the mechanical performance of concretes. This will enable to develop a better understanding of the influence of cement type or concrete compressive strength on recarbonation. General reviews of these relationships are available; however, new data in this regard, particularly for new binder types, becomes available at an increasing rate, which will be utilised by the TC.
• Elucidate the influence of the relative humidity on the carbonation rate of concretes or mortars, beyond analysis of only service-life exposure conditions. This includes carbonation of wet/submerged cementitious materials, which is relevant in the context of CO₂ uptake after demolition but have received less attention than carbonation at values of the relative humidity that are representative of atmospheric conditions.
• Identify the type and abundance of the reaction products in the carbonated layer of cementitious materials, depending on the cement/binder type, environmental exposure conditions, and other relevant parameters. This includes an assessment of the fraction of CaO of the cement that has been carbonated under specific conditions.

The above assessments are expected to lead to an update, including a more detailed classification according to cement type etc., of crucial parameters that are used to estimate the service lifetime of reinforced concrete structures as well as the CO₂ uptake during and after the service life of structures made from cementitious materials.

The induced (or enforced) carbonation of recycled concrete aggregates and concrete fines, and mineral carbonation processes in general, usually take place at CO₂ partial pressures and temperatures that are very different from those encountered under field conditions, and these will not be considered in the work of the TC.

Terms of reference

The TC will mainly conduct bibliographical research. In addition, the TC work involves exchange of good practices information and lab protocols, and possibly experimental data. Optionally, experiments will be performed to obtain additional data.

The TC will be active for five years. This includes a launch phase (year 1), the collation and processing of literature and data (years 2–4), and the finalisation of the outcomes (year 5) (see Section ‘Detailed working programme’).

The number of TC members should be around 40; members might be from academia, public research institutes and industry. It is envisaged to attract TC members from different continents, countries (language areas) and climatic regions, as these may be able to contribute relevant reports and data that are not accessible to other TC members. Young scientists and PhD students are encouraged to join the TC. A prerequisite for TC members is the commitment to actively contribute to the TC work.

The outcomes of the TC work can be assumed to be of high relevance for the construction industry and beyond, as they relate directly to service lifetime and CO₂ uptake predictions/estimates for cementitious materials.

Detailed working programme

Year 1: Launch

The launch involves the kick-off meeting, comprising the introduction of the TC members; overview of members’ competences and experience with concrete or mortar carbonation; suggestion of new members; presentation of the current status and planned activities; and formation of topical working groups (WGs).

The current status is mainly defined by published assessments of the CO₂ uptake by cement carbonation; the data collated by TC 281-CCC and subsequent relevant publications; and additional published work pertinent to the reaction products in the carbonated layer of cementitious materials.

The number and scope of the working groups are planned to correspond to the above-stated objectives of the TC:

(1) overview of the parameters (carbonation rate, fraction of carbonated CaO) employed in previous and present assessments of (global or regional) CO₂ uptake by cement carbonation;

(2) parameters influencing the reaction products and ‘degree of carbonation’ in the carbonated layer;

(3) influence of the humidity, including wet/submerged conditions, on the carbonation rate and CO₂ uptake during lifetime and after demolition;

(4) influence of CO₂ concentration on the carbonation rate and CO₂ uptake during lifetime and after demolition.

---

Year 2–4: Data collation and analysis

The operation phase involves, in each of the working groups, the identification and collation of relevant literature and additional data; review and classification/analysis of the data; and evaluation regarding the significance of the data. It is planned to process the data in a way that it can be provided in digital form, ideally being consistent (i.e., mergeable) with previous databases. This phase also includes presentation of preliminary results at conferences to obtain feedback from the scientific community outside the TC (and, in the beginning, to attract further expert members).

---

Year 5: Finalisation of outputs

In the final phase, the completion and submission of a series of review papers and possibly recommendations, preferably as topical collection in Materials and Structures, will take place. The datasets generated in this TC will be made available (open access) either via publication as electronic supplementary material accompanying open-access journal articles or deposition on the EU-funded data repository Zenodo. The results will also be presented at a dedicated special session at a RILEM conference (cf. Section ‘Expected achievements’).