Carbonation-cementation of recycled hardened cement paste powder

Abstract

Well-hydrated cement paste powder was prepared to imitate recycled hardened cement paste powder. Compacted blocks were formed with the powder, and subjected to carbonation at different CO2 pressures. The effects of water/powder (w/p) ratio and compacting pressure in block preparation, and the CO2 pressure on the carbonation degree and compressive strength of the carbonated blocks were studied. The mineral compositions, microstructures and pore structures of the carbonated blocks were analyzed by XRD, SEM and MIP. The results show that the hardened cement paste powder in the blocks is able to re-cement and produce high level strength by carbonation. In the CO2 pressure range of 0.05–0.40 MPa, the higher the CO2 pressure was, the higher the carbonation degree and compressive strength of the blocks at all the carbonation times. Carbonated blocks with the compressive strength as high as 62.6 MPa were prepared by carbonation in 0.4 MPa CO2 for 24 h. The calcium hydroxide, calcium silicate hydrates and the un-hydrated β-C2S in the hardened cement paste powder were almost fully carbonated in 24 h. There was no new well-crystallized mineral other than the three polymorphs of calcium carbonate, mainly calcite, formed during the carbonation. The microstructures became more compacted due to the filling of the pores by CaCO3 and the other carbonation products such as low calcium silicate hydrates, companied with the decrease of the porosity and the pore sizes.