Hydration and Carbonation of Alternative Binders

The cement industry gains increasing public attention because of the high CO₂ emissions for which it is accountable. One approach to improve the carbon footprint of cement production is the development of alternative binders, which can be produced with less energy consumption. These alternative binders have a reduced clinker to cement ratio or contain no Portland cement at all. For the corrosion protection of the reinforcement, the same requirements apply as for standardized types of cement. To perform this evaluation, a basic understanding is necessary of the products formed during hydration, the composition of the pore solutions and pore structure, and their influence on the resistance to carbonation. In this study, various alternative binders, such as novel types of composite cement with calcined clays or modified steel mill slag, alkali-activated materials, C<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mover><mi mathvariant="normal">S</mi><mo>¯</mo></mover></mrow></semantics></math></inline-formula>A cement, and a C-S-H binder, are presented. The conducted investigations at mortars and cement pastes show the differences in hydration, microstructure development, and pH. The changes in microstructure and phase assemblage due to carbonation and the suitability of carbonation testing at an elevated CO₂ content of 1 vol.-% for the different binders are also presented.