## 8 Conclusions  
Our previous research on the CO\({}_{2}\) absorption performance of fresh cement slurry demonstrated that ultrasonic vibration could more effectively improve CO\({}_{2}\) absorption than mechanical mixing. Moreover, the fluidity of the slurry decreased with the increase in CO\({}_{2}\) absorption. In addition, ultrasonic vibration can effectively improve the fluidity of cement slurry followed by CO\({}_{2}\) absorption [21]. If a water-reducing agent is added to the slurry, it not only increases its fluidity but also restores the working performance that has lost fluidity [12]. Therefore, this paper focused on the dynamic analysis through the principle of rheology and establishing the molecular model while observing the change in internal microstructure.  
Under ultrasonic vibration, the fluidity of fresh paste decreases with the increase in CO\({}_{2}\) absorption and with the addition of a superplasticiser, the fluidity of the decreased slurry could be restored, which indicates an unclear principle of change. Therefore, in this paper, the rheological principle was analysed. Moreover, the internal microstructural change was observed, followed by the establishment of the rheological structure model. Through the development of a molecular model and dynamic analysis, the internal workability-change mechanism of fresh slurry along with superplasticiser was intensively explored while subjected to ultrasonic vibration. The results indicated that ultrasonic vibration can increase the shear stress and plastic viscosity of cement paste more effectively than the cement paste subjected to only CO\({}_{2}\) absorption via mechanical stirring. In addition, the shear stress and viscosity of cement paste could increase with the increase in CO\({}_{2}\) absorption. At the same time, the addition of polycarboxylic-acid superplasticiser to the cement paste along with CO\({}_{2}\) absorption can more effectively reduce the shear stress and plastic viscosity of the fresh paste while restoring and improving its fluidity. In addition, the rheological model of cement paste subjected to CO\({}_{2}\) absorption by mechanical stirring fitted well with the H-B rheological model, while the rheological model under ultrasonic vibration was within the range of the traditional rheological model. However, it does not completely accord with the actual rheological characteristics. Therefore, an ultrasonic rheological model was proposed for the cement-based materials which could suitably absorb CO\({}_{2}\) under ultrasonic vibration. In addition, both CO\({}_{2}\) and superplasticiser could reduce the intermolecular binding force and increase the driving force between the intermolecular particles. As a result, the dispersion effect of C-S-H gel could enhance, if the gel groups are effectively distributed. Finally, the rheological properties of the slurry could improve.  
Due to the lack of research on the absorption of CO\({}_{2}\) by cement slurry and the limitations of experimental equipment, at present, we are only able to report the CO\({}_{2}\) absorption improvement process and its impact on fluidity. The next step will be investigating the effect of CO\({}_{2}\) absorption on the carbonation resistance of concrete and its corresponding mechanism.  
Study design, Y.J.; conduct of the study, L.L. ([PERSON]); conceptualisation, J.Z.; data collection, L.L. ([PERSON]); methodology, L.L. ([PERSON]); writing--original draft preparation, L.L. ([PERSON]) and Y.J. All authors have read and agreed to the published version of the manuscript. This word was funded by the National Natural Science Foundation of China (51972337) and Science and Technology Innovation Project of Xuzhou (Grant No. KC21014).