Mineral carbonation facilitated by aqueous amines is a carbon capture technique applied in global CO₂ emissions controls, featuring enhanced CO₂ dissolution plus mineral carbonate deposition and amine regeneration. As a novel approach, amines blend into cement-based materials can be worked as ‘synthetic trees’ in direct air capture (DAC). Extending this concept, the current work was initially conducted to identify the most potential amine to absorb a high amount of CO₂ using Ca(OH)₂ as the cement-based material. Thermogravimetric Analysis was conducted to analyse the CO₂ capture by CaCO₃ weight %. 2-(methylamino)ethanol (MAE), N-methyldiethanolamine (MDEA), 2-amino-2-methyl-1-propoanol, ethane-1,2-diamine, and 2-(2-aminoethylamino)ethanol were selected as amines. Solutions with 300 mL of 1 M aqueous amines plus Ca(OH)₂ in 1:10 of solid: liquid ratio were carbonated 2 h at 2.0 L/min of 5 % CO₂ gas flow rate at 20 °C. There, MAE was observed with the highest CO₂ absorption. Yet, considering the chemical structures, blends of amines are expected to deliver further enhancement of CO₂ absorption in the presence of cement-based materials. This study was further focused on the effect of PZ and MAE blends on CO₂ absorption behaviour with Ca(OH)₂. The PZ/ MAE blends were prepared in different ratios of PZ: MAE as 1.0:0, 0.2:0.8, 0.5:0.5, 0.8:0.2, and 0:1.0 maintaining total amine concentration at 1.0 M. Comparison was made with Ca(OH)₂ without any amine inclusion as the control. As a result, augmented CO₂ capturing for PZ/MAE blends at 0.5:0.5 and 0.2:0.8 PZ: MAE ratios were observed compared to control, as well as to pure PZ and MAE. The highest CaCO₃ weight % was noted for 0.5:0.5, while the second highest was recorded for 0.2:0.8 PZ: MAE ratios. Understanding the reaction mechanism in PZ/ MAE blend with CO2, identification and confirmation of CaCO₃ polymorphs and their microscopic structures, and DAC experiments will be conducted to standardise these spotlight findings.

Using recycled concrete aggregate (RCA) derived from substantial volumes of global construction waste represents an environmentally and economically friendly solution for the CO₂ crisis. RCA is an eco-friendly calcium source due to the presence of hydrated cement paste and exhibits potential for CO₂ sequestration via CaCO₃ precipitation. Ca²⁺ from cement hydration products in RCA can be extracted by leaching in solutions, and subsequently, react with atmospheric CO₂ to form CaCO₃. Nonetheless, the process of CaCO₃ precipitation may be impeded by various factors, with the hydration reaction of CO₂ serving as one of the limiting steps. To accelerate CaCO₃ precipitation, the application of carbonic anhydrase (CA), a class of enzymes that catalyze the hydration of CO₂ into H⁺ and HCO₃^- up to 10⁷ times faster than the natural process, can be employed. Studies have shown that the CaCO₃ precipitation onset is faster with CA. Therefore, larger quantities of CaCO₃ can precipitate in a shorter duration with the enzyme CA.

Our research aims to achieve efficient CO₂ sequestration by utilizing calcium sources from RCA with the help of CA. Our preliminary results indicate that CA can significantly accelerate CO₂ sequestration in alkaline solutions (Fig. 1a and b), in which solutions with CA showed faster pH decreases due to the faster reaction between CO₂ and OH^-. CA enzyme activity is essential for the efficacy of CO₂ sequestration. Therefore, CA enzyme activity in RCA leaching solution is investigated by using p-NPA assay. Parameter studies, covering pH, ionic strength, and different ions such as Ca²⁺, Mg²⁺, Al³⁺, and SO₄^2-, are used to evaluate CA activity in RCA leaching solutions. Our preliminary results (Fig.1c) indicated that high pH is critical for CA activity, and high ionic strength of the solution has some minor effects on CA activity. The leaching scheme used in this study has a negligible effect on CA activity. The extraction of precipitates at different intervals during experiments allows diverse characterization techniques, including XRD and SEM, to elucidate the fundamental mechanisms of CA-facilitated CaCO₃ precipitation involving calcium leached from RCA.

Acknowledgment

This work was done within the framework of the ALIVE initiative (Advanced Engineering with Living Materials) and funded by the SFA-AM program (Strategic Focus Area – Advanced Manufacturing). The enzyme activity test was supported by the group Complex Materials, Prof. Dr. André R. Studart

This study investigated the effect of triethanolamine on the carbonation and hydration of Class C fly ash. As the international agreement against climate change proposed that the world must achieve net-zero CO₂ emissions by 2050, the development of technology to reduce CO₂ is necessary. Fly ash, a byproduct from coal fire plants, has been used as supplementary cementitious materials (SCMs) to lower clinker, however, the clinker production should be further reduced and the technology on carbon capture, utilization, and storage should be introduced in construction materials area to achieve carbon neutrality. Because fly ash itself can gain compressive strength without clinker involvement through carbonation [1], it was considered a green construction material.

Since carbonation is affected by the calcium content and TEA can enhance the calcium solubility of fly ash, the addition of TEA led to a noticeable increase in the compressive strength of carbonated fly ash pastes. For instance, in the addition of TEA 5 wt.%, compressive strength was improved up to 70 MPa on 7 days of carbonation (Note Figure 1). Meanwhile, TEA could also improve the compressive strength of moisture-cured fly ash pastes, but not as much as carbonated fly ash at the same amount of TEA addition. In the presence of TEA, ettringite and AFm phases were formed in moisture curing, while C(-A)-S-H and amorphous calcium carbonate were produced in carbon curing, which led to the refinement of porosity in both curing conditions.

Reference

1. Wei, Z., et al., Clinkering-free cementation by fly ash carbonation. Journal of CO2 Utilization, 2018. 23: p. 117-127.