Phase assemblage and microstructure of cement paste subjected to enforced, wet carbonation

Abstract

Carbonation of fines from recycled concrete can significantly lower the CO2 footprint of concrete. Benefits of this approach come from fines' abundance, easiness of carbonation and their potential reactivity upon carbonation. The mechanisms of enforced carbonation have been investigated in this work. By carbonating well hydrated, dried and ground cement paste in an aqueous solution, it is found that portlandite initially reacts with the dissolved CO2 and upon its depletion, other hydrates progressively decalcify. The main carbonation products are calcite and an alumina-silica gel rich in alkalis. The gel has an amorphous structure similar to silica gel, including a range of different Qn(mAl) environments according to 29Si NMR. Calcite precipitates mainly in the space occupied by the solution and on the surface of grains of the paste. Al and Si from the hydrates do not diffuse out of the grains and remain in the space occupied initially by the hydrates.