Effect of alkalis on enforced carbonation of cement paste: Mechanism of reaction

In the present research, an alternative approach to produce supplementary cementitious materials (SCMs) based on the carbon capture and utilization is developed. The approach focuses on enforced carbonation of cement pastes obtained from recycled concrete and its application as SCM. This work focuses on the effect of alkalis in the starting solution on the carbonation mechanism of ground hydrated cement pastes in a wet reactor. The enforced carbonation of cement paste is a rapid process at ambient temperature and pressure being close to complete reaction within a few hours of carbonation, independently on alkali concentration. However, alkalis have a complex impact on the carbonation reaction as they accelerate the initial stages of the carbonation reaction, while the kinetics of the middle stages is retarded. The final degree of carbonation is only slightly affected by the alkali concentration in the solution. The origin of these effects can be explained by relating them to the evolution of solid and solution properties. Additionally, the alkali concentration has an impact on the morphology of the main carbonation products, that is, calcite and the alumina-silica gel.