7.6 Korrosion und Korrosionsschutz
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Eingeladener Vortrag
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It is well known that cement manufacture, including CaCO3 calcination and clinker formation, is associated with substantial energy consumption and significant greenhouse gas emissions. Alkali-activated binders (AAB) and concretes made from them can significantly contribute to reducing CO2 emissions caused by the construction industry. However, to what extent and for how long concretes made from AAB can protect the steel in reinforced concrete components from corrosion is still unclear. Unlike PC-based binders, AABs are made by reacting alkaline solutions with solid precursors such as fly ash and ground granulated blast furnace slag (GGBFS). Therefore, steel reinforcement corrosion mechanisms in AAMs differ from PC-based binders. The present study aims to evaluate the corrosion behavior of steel rebar in alkali-activated materials (AAMs) synthetic pore solution with different chemical concentrations representing different GGBFS and Flay Ash based binders by performing electrochemical tests to measure the corrosion rate and corrosion potential of steel rebar. This information can be used to improve the design of AAMs in order to ensure a longer service life for these materials.
Blast furnace cements (CEM III) and alkali-activated slags are binders for concretes with several advantageous engineering properties, and their increased adoption in construction industry could contribute to reducing the CO2 emissions associated with cement production and use. However, the current knowledge about how these cements protect steel reinforcement in concretes against corrosion is very incomplete, which impedes their large-scale application. This knowledge gap is mainly due to the fact that these cements release sulfide and other reduced sulfur species into the concrete pore solution, the consequences of which for the state of the reinforcement and electrochemical measurements are not fully understood.
The present contribution first describes peculiarities of electrochemical measurements of steel in sulfide-containing cementitious materials and related solutions as reported in the literature and a recent report by EFC Working Party 11. It is demonstrated that the high sulfide concentrations in these systems lead to low open circuit potentials and low polarisation resistances, which may be incorrectly interpreted as indicating active corrosion of the steel.
Second, preliminary results of an ongoing project [funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – 458297195] related to the passivation and corrosion initiation of steel in sulfide-containing solutions and mortars are presented. Eight mortars based on one alkali-activated blast furnace slag (BFS), three alkali-activated BFS/fly ash blends, one sodium sulfate-activated CEM III/C (‘hybrid cement’), one CEM III/C, one CEM III/B, and one CEM I (ordinary Portland cement, OPC) were produced, and their pore solutions expressed and analysed after 7, 14, 28, and 56 days of curing. The pH values of the solutions differed systematically, with the highest pH values recorded for the CEM I and the alkali-activated BFS/fly ash blends with a high proportion of fly ash, and the lowest pH recorded for the CEM III/B. The redox potentials of the solutions were between −500 mV and −340 mV vs. Ag/AgCl for the alkali-activated binders, approx. +10 mV vs. Ag/AgCl for the CEM I, and in between for the CEM III/B and the CEM III/C. As expected, the electrical conductivity was highest for the alkali-activated binders. These results are explained by the chemical compositions of the pore solutions of the mortars.
Finally, a test set-up to investigate the behaviour of steel in sulfide-containing solutions and the changes on subsequent oxygen and/or chloride addition is introduced. Preliminary electrochemical measurements of steel in sulfide-containing solutions are presented and discussed in the context of the above-mentioned data from the literature and the compositions of the pore solutions of the studied mortars.
In general, pH-sensitive colorants such as phenolphthalein are applied on freshly split surfaces of building structures to determine the state of carbonation. For years, it has been known that the aerosols of phenolphthalein are suspected of being carcinogenic. Hence, more work has currently being done on alternative indicators for the use in research and the use on construction sites. Over the past years, mainly pH indicators from other research areas and natural dyes were investigated for their usability to cementitious systems. This paper presents the experimental results of using phenolphthalein solution bound in agar-agar gel. This gel-bound indicator allows the future use of the already known phenolphthalein dye without the health hazards for the user. To demonstrate the flexibility of the gel-bound colorant, different application forms of the indicator were carried out and assessed. In addition, the results obtained by the different methods were statistically evaluated to ensure validity.
The accuracy of the results was confirmed by thermogravimetric analysis and IR spectroscopic studies as reference methods. It has also succeeded to demonstrate the comparability of the results achieved by the gel-bound colorant with the results of the common sprayed application method. As an additional gain in knowledge, comparative results of indicator tests, TGA analysis, and FTIR investigation are shown.
Chloride contents at the steel–mortar interface that initiate steel corrosion were determined for carbon steel in alkali‐activated fly ash mortar for three different exposure conditions: exposure to 1 M NaCl solution; leaching in deionized water and then exposure to 1 M NaCl solution; and leaching in deionized water, aging in air at 20°C and natural CO2 concentration, and then exposure to 1 M NaCl solution. For comparison, a Portland cement mortar, exposed to 1 M NaCl solution, was studied. The median values of the corrosion‐initiating chloride contents (average over the full length of the rebar) in the alkali‐activated fly ash mortar varied between 0.35 and 1.05 wt% Cl with respect to binder, consistently lower than what was obtained for the Portland cement mortar, but with no clear trend regarding the exposure conditions. For most of the alkali‐activated fly ash mortar specimens, preferential corrosion at the connection between the working electrode and the external measurement setup was observed, while preferential corrosion did not occur for the Portland cement mortar. Scanning electron microscopy and auxiliary experiments in synthetic solutions indicated that this behavior was caused by inhomogeneities at the steel–mortar interface in the alkali‐activated mortar, likely due to its peculiar rheological properties in the fresh state.
Der kritische, Korrosion auslösende Chloridgehalt für die Beurteilung der Korrosions-beständigkeit von Betonstahl in Geopolymerbetonen ist von großer Bedeutung für die Dauerhaftigkeitsprognose für bewehrte Elemente aus solchen Materialien. Es sind kaum experimentelle Daten in der Fachliteratur vorhanden und die vorliegenden Werte unterscheiden sich zwischen den Studien erheblich. In diesem Projekt wurde der kritische, Korrosion auslösende Chloridgehalt für Betonstahl (BSt 500) in flugaschebasierten (Ca-armen) Geopolymermörtel für verschiedene Beaufschlagungskombinationen bestimmt: Chloridbeaufschlagung mit 1 M NaCl-Lösung; Auslaugen in entionisiertem Wasser und anschließende Chloridbeaufschlagung im 1 M NaCl-Lösung; Auslaugen in entionisiertem Wasser, Carbonatisierung in Luft bei 20 °C und natürlicher CO2 Konzentration und anschließende Chloridbeaufschlagung mit 1 M NaCl-Lösung. Für Referenz-Zwecke wurde zusätzlich der Korrosion auslösende Chloridgehalt für eine Portland-Zement Mischung bei Auslagerung in 1 M NaCl-Lösung bestimmt.
Corrosion of steel reinforcement in concrete is one of the major deterioration mechanisms limiting the service life of reinforced concrete structures. While for conventional (Portland cement-based) concretes a great amount of experience exists in this regard, the factors that determine the onset of reinforcement corrosion in alkali-activated materials are incompletely understood yet.
One aspect of corrosion protection is leaching and the accompanying changes of the concrete pore solution. In the present study, alkali-activated fly ash mortars with embedded carbon steel rebars were exposed to de-ionised water for periods up to 330 days, and the electrochemical response of the steel (free corrosion potential, polarisation resistance), the alteration of the mortar (ohmic resistance, mechanical strength, pore size distribution) as well as the pore solu¬tion composition were monitored.
Although substantial alkali leaching was observed, the pH of the pore solution remained at values sufficient to protect the embedded steel from depassivation. The mortar did not exhibit indications of significant deterioration. Thus, the present results suggest that leaching is not critical for pro¬tec¬tion of steel reinforcement in alkali-activated fly ash mortars and concretes.
Corrosion of steel reinforcement in concrete is one of the major deterioration mechanisms limiting the service life of reinforced concrete structures. While for conventional (Portland cement-based) concretes a great amount of experience exists in this regard, the factors that determine the onset of reinforcement corrosion in alkali-activated materials are incompletely understood yet. One aspect of corrosion protection is leaching and the accompanying changes of the concrete pore solution. In the present study, alkali-activated fly ash mortars with embedded carbon steel rebars were exposed to de-ionised water for periods up to 330 days, and the electrochemical response of the steel (free corrosion potential, polarisation resistance), the alteration of the mortar (ohmic resistance, mechanical strength, pore size distribution) as well as the pore solution composition were monitored. Although substantial alkali leaching was observed, the pH of the pore solution remained at values sufficient to protect the embedded steel from depassivation. The mortar did not exhibit indications of significant deterioration. Thus, the present results suggest that leaching is not critical for protection of steel reinforcement in alkali-activated fly ash mortars and concretes.
Corrosion of steel rebars in concrete presents one of the main deterioration mechanisms limiting service life of the reinforced structures. The corrosion is accompanied by an expansion of the corrosion products causing high pressures, concrete cracking and finally spalling of a cover layer. Critical chloride concentration, loss of alkalinity and modeling of the steel corrosion are in researchers' spotlight for decades, however reinforcement corrosion in alkali activated materials is insufficiently described and understood yet.
In this work, the steel reinforcement corrosion in alkali-activated fly ash mortars is investigated in terms of electrochemical behaviour of the reinforced mortars exposed to aggressive environments such as leaching, carbonation and chloride ingress. A selected geopolymer mixture based on hard coal fly ash activated with sodium hydroxide and sodium silicate solutions is used for the steel reinforcement-corrosion experiments. The formation of passive layer on the steel rebars is observed after approx. two weeks of hardening at laboratory temperature. However, alternative heat-treatment at 80°C for several hours leads to immediate formation of the passive layer as well as to a faster strength gain (80 MPa after 24h at 80°C). Chloride-induced corrosion, leaching and carbonation resistance of the alkali activated fly ash-based concrete is studied, where leaching in deionized water or carbonation under natural conditions (~0.04 % CO2) for 300 days did not lead to corrosion of the embedded steel. On the other hand, accelerated carbonation under 100 % CO2 atmosphere lead to depassivation within two weeks.
Alkali-activated fly ash mortars were studied with regard to durability-relevant transport coefficients and the electrochemical behaviour of embedded carbon steel bars on exposure of the mortars to leaching, carbonation and chloride penetration environments. The transport coefficients differed considerably between different formulations, being lowest for a mortar with BFS addition, but still acceptable for one of the purely fly ash-based mortars. Leaching over a period of ~300 days in de-ionized water did not lead to observable corrosion of the embedded steel, as shown by the electrochemical data and visual inspection of the steel. Exposure to 100 % CO2 atmosphere caused steel depassivation within approx. two weeks; in addition, indications of a deterioration of the mortar were observed. The results are discussed in the context of the different reaction products expected in high- and low-Ca alkali-activated binders, and the alterations caused by leaching and carbonation.