*4.4. Carbonatation Process*

CAC paste is known to react with atmospheric carbon dioxide. Carbonation causes numerous chemical-mechanical changes in the cement paste, but can also improve properties of the cement material [42–45]. For these reasons, the active use of carbonation as a tool to manipulate the properties of CAC-based materials is a very promising research area that could potentially improve fiber-reinforced cementitious compounds and promote concrete recycling and immobilization of waste.

The carbonation of CACs has been investigated in order to find adequate conditions to improve the mechanical properties of CACs. However, the very specific manufacturing conditions of CAC paste, such as water/cement ratios greater than 0.4, hydration in the presence of alkalis and hydration in a high-humidity atmosphere [44,46], produce a paste with very poor mechanical performance for construction and refractories. However, these pastes do have a high CO<sup>2</sup> absorption capacity. Therefore, hydrating CAC in conditions that are unfavorable to its mechanical properties could drastically increase its CO<sup>2</sup> absorption capacity, which could be used to reduce CO<sup>2</sup> contamination elsewhere. After these pastes are saturated with CO2, they could be included in the circular economy as a construction material. While no research has been found in this area, it would be an excellent topic to review. The research presented here describes advances in sustainability and many other specific and important topics related to the circular economy of CAC. However, the lack of progress in these areas is quite evident considering the social demands on modern construction materials. Moreover, the required approaches demand a rethinking of the material and process in the circular economy. The process needs to be not only reused but also optimized to reduce energy, water and material waste. From an academic, innovation and production point of view, this technological lag opens up new opportunities to produce a CAC that fits in with the advances of the 21st century. Such research is important because it would pave the way for not only significant progress towards a more sustainable CAC, but also reveal the limitations and opportunities in the area for a real modern CAC material.
