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.
Alkali-activated materials (AAMs) are binders that can complement and partially substitute the current use of conventional cement. However, the present knowledge about how AAMs protect steel reinforcement in concrete elements is incomplete, and uncertainties exist regarding the application of electrochemical methods to investigate this issue. The present review by EFC WP11-Task Force 'Corrosion of steel in alkali-activated materials' demonstrates that important differences exist between AAMs and Portland cement, and between different classes of AAMs, which are mainly caused by differing pore solution compositions, and which affect the outcomes of electrochemical measurements. The high sulfide concentrations in blast furnace slag-based AAMs lead to distinct anodic polarisation curves, unusually low open circuit potentials, and low polarisation resistances, which might be incorrectly interpreted as indicating active corrosion of steel reinforcement. No systematic
Vor dem Hintergrund der Klimadebatte werden zur Reduzierung des CO2‐Austoßes vermehrt Alternativen zu Portlandzement als Bindemittel im Beton entwickelt. An diese alternativen Bindemittel werden die gleichen Ansprüche hinsichtlich ihrer Dauerhaftigkeit wie an Portlandzement gestellt. In der vorliegenden Studie wurden Mörtel mit Bindemittel aus den Bindemittelgruppen Portlandzemente, Kompositzemente, CSA‐Zemente, alkaliaktivierte Bindemittel sowie Niedrigtemperatur C‐S‐H‐Binder hinsichtlich ihres Carbonatisierungs‐ und Chlorideindringwiderstands untersucht. Dabei wurden Analysen zu dem pH‐Wert der Porenlösungen, dem Porenvolumen sowie dem Grenzradius mit in die Betrachtungen einbezogen. Es wurde festgestellt, dass für die ausgewählten alternativen Bindemittel die für Portlandzement bekannten Zusammenhänge zwischen Materialeigenschaften und Eindringverhalten von Chlorid und CO2 nur mit Einschränkungen gelten. So konnte zwar ein genereller Zusammenhang zwischen Chloridmigrationskoeffizient und Porenvolumen bestätigt werden. Bei dem Carbonatisierungsfortschritt war dieser Zusammenhang allerdings nicht feststellbar. Im Gegensatz zu konventionellen Zementen konnte kein allgemeingültiger Zusammenhang zwischen dem pH‐Wert und der Carbonatisierungstiefe nachgewiesen werden.
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