Hydration and Carbonation of Alternative Binders

Abstract

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, CA 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.

Carbonation behaviour of concretes with alternative binders at different CO2 concentrations

“…Comparable to Ref I, the proportion of the X-ray amorphous phase after 28 d is 41 %. The SEM images and mappings described in [11] showed not completely dissolved slag particles. The X-ray amorphous content consisted of C-(A)-S-H phases.…”

“…Other phases were omitted for clarity. Furthermore, in Kraft et al [11] SEM images of the samples described here can be found. [11], the X-ray amorphous phases are mainly C-(A)-S-H phases and residues of the slag, as expected.…”

“…Quartz occurs as the main crystalline phase, with about 43 %. 51 % of the samples consist of X-ray amorphous phases, which according to the mapping can be assigned to C-A-S-H and N-A-S-H phases [11].…”

“…Trümer [16] and Kraft et al [11] investigated the carbonation of calcined clay cements. They observed a higher carbonation coefficient for mixtures of metaillite than for mixtures of metakaolin, although the mixtures of metaillite had a higher Portlandite content [11; 16].…”

Erhöhung des Carbonatisierungswiderstandes klinkerreduzierter Betone durch alkalische Puffer

“…Die Ergebnisse der LT-DSC in Abbildung 6 zeigen, dass die partielle Substitution des Zements durch calcinierten Ton (Referenz B) zu einer deutlichen Verringerung der Kapillarporosität führt. Die Ergebnisse der Kapillarporosität stehen in Übereinstimmung mit Werten aus der Literatur, nachdem die Verwendung von calcinierten Tonen im Beton zu einer Gefügeverdichtung beiträgt [11,12]. Dies wird auf zwei Effekte zurückgeführt.…”