The new cement standard EN 197-6:2023 defines cements with recycled concrete fines (RCF) as main constituent, which is an essential factor in achieving the overall goal of circular economy. Additionally, CO₂ mineralization of hardened cement pastes (HCP) in RCF is a promising carbon capture and utilization as well as storage (CCUS) approach to reduce CO₂ emissions, which may even improve the properties of the resulting supplementary cementitious material (SCM). However, the HCPs to be subjected to CO₂ mineralization are derived from different types of cement. The present study deals with systematic findings about the CO₂ binding capacity of HCPs based on cements with different compositions and their effects on pozzolanic reactivity.

Accelerated carbonation tests were carried out in a laboratory scale on mortars made from different cement types (acc. EN 197-1) with a water/cement ratio of 0.50 and natural siliceous sand. The mortar samples (age > 90 days) were crushed and ground to obtain analogues of recycled concrete fines. The accelerated aqueous carbonation experiments were carried out with a newly developed cold carbonation technique (CCT) in a custom-built continuously stirred reactor with pure CO₂ injection at a constant temperature of 5 °C. CO₂ uptake and pozzolanic reactivity were evaluated using a multi-analytical approach. This includes thermogravimetric analysis, isothermal calorimetry according to ASTM C 1897-20, powder XRD analysis in combination with Rietveld refinement (internal standard method) and mid-infrared spectroscopy in ATR mode.

The experiments carried out using CCT showed that the CO₂ mineralization of HCP leads to the formation of crystalline calcium carbonate and amorphous products from decalcified hydrate phases, irrespective of the cement composition. It was found that the estimated CO₂ uptake depends on the composition of the cement type used (type and amount of SCM). Reactivity tests according to ASTM C 1897-20 indicate that CO₂ mineralization of HCP can improve pozzolanic properties of the analogous fines, with the degree of improvement depending on the cement composition. It should be noted that the non-hydrated residual particles within the analogous fines may contribute to the assessment of pozzolanic reactivity, especially in cements containing granulated blast furnace slag.

The conducted study has shown that CO₂ mineralization of HCP by CCT is a promising approach to obtain value-added RCF with enhanced pozzolanic reactivity. However, RCF as a by-product of aggregate recycling from concrete demolition is complex and can be very different in composition compared to the materials used in this study. This circumstance may require an individual assessment of the maximum CO₂ sequestration potential of RCF and the resulting pozzolanic reactivity.

Carbonated recycled cement pastes (cRCP) are a new material class with potential application as supplementary cementitious materials (SCM). Wet carbonation of hydrated cement fines gives easy access to cRCPs. Their pozzolanic reactivity as SCM is attributed to the alumina-silica gel, which forms beside calcite during carbonation [1, 2]. The amorphous character of the gel impedes deeper structural characterization by quantitative X-ray diffraction (XRD) analysis. Therefore, we applied solid-state nuclear magnetic resonance (NMR) spectroscopy to study the connection between the composition of the carbonated product and the original cementitious binder chemistry, using three Portland cements (2x CEM I 42.5 R and 1x CEM I 52.5 N).

After characterization by QXRD, XRF, LDA, and BET, the three cements were hydrated for 35 days at 50°C, finely ground, and subsequently carbonated by bubbling pure CO₂ through a suspension of the ground, hydrated cement for 2 h. The resulting solids – hydrated and carbonated – were subjected to ²⁹Si and ²⁷Al solid-state NMR analysis. To quantify all silicon species present, the deconvolution approach, described by Skibsted [3] and Neto [4], was adopted.

Using the relative quantities from the deconvoluted Si-spectra, we report on the composition of the hydrated binder, e.g., the hydration degree, the average chain length of the silicate chains, and the amount of Al in the C-S-H phases. Not surprisingly, the cement with the smallest particle size distribution showed the largest degree of hydration, as determined by ²⁹Si NMR. Furthermore, the peaks in the ²⁷Al spectra confirm the presence of aluminum species in the CSH phase (Al_IV, Al_V, and Al_VI), ettringite, and AFm. After carbonation, the silicate and aluminate ions are mostly found in the amorphous gel. After 2h of carbonation, the reaction is not complete, and residual AFm, CASH, and anhydrous clinker phases are also detected. Again, the cement with the greatest surface area, CEM I 52.5 N, exhibited the largest gel content. We assign the faster decalcification process of this binder to a higher hydrate phase content, initially caused by finer particles.

In summary, we describe how the raw material properties of three Portland cements and their hydration products influence the composition of the final carbonated product. Ultimately, we are interested in the influence of admixtures on the kinetics and morphology of the carbonated cement hydrates in future studies.

Bibliography

[1] M. Zajac, J. Skibsted, J. Skocek, P. Durdzinski, F. Bullerjahn, M.B. Haha, Cem. Concr. Res. 130 (2020): 105990.

[2] M. Zajac, J. Skocek, P. Durdzinski, F. Bullerjahn, J. Skibsted, M.B. Haha, Cem. Concr. Res. 134 (2020): 106090.

[3] K. Scrivener, R. Snellings, B. Lothenbach, A Practical Guide to Microstructural Analysis of Cementitious Materials. CRC Press, 2018.

[4] F. M. Neto, R. Snellings, J. Skibsted, Cem. Concr. Res. 177 (2024): 107428.

The carbonation of mineral oxides/hydroxides like lime and slaked lime holds promise for the creation of net-neutral or net-negative carbon construction materials, providing a sink for atmospheric CO2 in the process. The carbonation extent and kinetics are influenced by factors equally dependent on the base material physicochemical properties and the conditions employed for carbonation. Optimizing these conditions and understanding the instantaneous carbonation mechanisms are the premises for a fast carbonation process.
In this work, we tested the hydration of CaO and subsequent carbonation at various relative humidities and temperatures. Higher relative humidity (~60%) and higher temperature (40 oC) can improve the carbonation degree. However, high levels of relative humidity and temperature influence the diffusion and solubility of CO2, respectively. The carbonation mechanisms were studied by in situ Raman during the reaction process. This study provides detailed methodologies for studying the carbonation process and unveils the reaction mechanism, which is of significant importance for both academic and practical applications.

The amorphous alumina-silica gel, produced through enforced carbonation of recycled concrete fines from end-of-life concrete, exhibits rapid pozzolanic properties as an SCM in composite cements. It contains different types of SiO₄ species, with AlO₄ sites distributed in the alumino-silicate network. The objective of this study is to determine the maximum aluminum uptake in the alumina-silica gel, and thereby its boundary composition, and to establish a thermodynamic model for the alumina-silica gel. The latter will be useful for predictions of the impact of parameter variations and compositional changes under different carbonation conditions and for different starting materials.

Aqueous carbonation of synthesized C-S-H phases and ettringite is used as a model system for the carbonation of hydrated Portland cement. Synthesized C-S-H with Ca/Si ratios of 0.83 and 1.4 were prepared and mixed with synthesized ettringite. The mixtures were carbonated for 6 hours under wet conditions and studied by in-situ pH, TGA and ²⁷Al, ²⁹Si NMR measurements. After carbonation, the aluminum from ettringite is preferentially present in tetrahedral coordination in the alumina-silica gel. However, a secondary alumina-gel containing aluminum in octahedral coordination Al is also present and the fraction of this gel increases with increasing ettringite content of the C-S-H – ettringite blends (e.g., from 1 wt% to 20 wt% as investigated here). The fraction of alumina gel is less affected by the Ca/Si ratio of the C-S-H phase used in the experiments.

A thermodynamic model for the alumina-silica gel is established based on different compositions of alumina-silica gels formed in cementitious systems, using the GEMS 3.9.6 software and the Cemdata 18.1 database. The solubility products, determined by GEMS, show a decreasing linear relationship with the Al/(Al+Si) ratio of the alumina-silica gel over the range from 0 to 0.5. This suggests that the gel can be stabilized by an increasing amount of AlO₄ sites.