Mechanisms of carbonation hydration hardening in Portland cements

Carbonation hardening of Portland cement concrete can contribute to lower its CO2 footprint and to improve its strength. The reaction mechanisms involved in carbonation curing were investigated on fresh cement pastes. The quantitative results gained allowed to link the curing conditions to the reactions kinetics, phase assemblage and microstructure evolutions. During the early carbonation curing, alite and belite react with dissolved CO2. The main carbonation products are calcium carbonate, an alumina-silica gel and a C-S-H phase with a low Ca/Si ratio. Hydration curing following the carbonation transforms the silica-rich phase into a C-S-H phase typical of hydrated Portland cements. Additionally, anhydrous calcium aluminates from clinker react to form ettringite and monocarbonate phases. The structure and composition of these phases is governed by the local equilibria in the dense microstructure. The carbonation and hydration reactions decrease the pore volume and refine the porosity resembling the microstructure development in the composite cements.