Carbonation of cement paste under different pressures

•Carbonation causes decrease of porosity of cement paste.•A model of the carbonation coefficient exponentially increase with air pressure is developed.•A chemo-mechanical model for cement paste strength under carbonation is established.•Carbonation leads to the decrease of the splitting tensile strength of cement paste.•Carbonation leads to the increase of the compressive strength of the cement paste. In this study, to reveal the carbonation law of cement under different air pressures, calcium silicate hydrate (CSH) gel and cement paste samples have been prepared. The variations in the material composition, micro-morphology, and structure before and after carbonation were examined. Moreover, the carbonation depth of cement paste under different air pressures as well as the changes in the compressive strength and splitting tensile strength after carbonation were investigated. The results show that a small amount of CSH gel is carbonized, but the main carbonized component is calcium hydroxide. After carbonation, the porosity of the cement paste decreases, and some microcracks appear in the cement paste. The carbonation experiment shows that the carbonation depth monotonically increases with the increase in the air pressure. An exponential model of coefficient of carbonation depth and air pressure was suggested by considering ideal gas law and Arrhenius equation. Finally, based on the carbonation reaction, the evolution model of carbonation strength of cement paste was established, and it was found that the strength of cement paste after carbonation and the carbonation degree are linearly correlated, among which the compressive strength and carbonation degree are positively related, while the splitting tensile strength and carbonation degree are negatively related.