Effect of sulfide and oxygen on the formation and stability of protective surface layers on steel reinforcement in alkali-activated concretes

• Applicants: Dr.-Ing. Gregor Gluth; Professor Dr.-Ing. Michael Raupach
• Subject Area: Construction Material Sciences, Chemistry, Building Physics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 458297195

Alkali-activated binders and concretes produced with these binders (“alkali-activated concretes”) exhibit characteristics that make them promising materials for the construction industry of the future. A barrier to their widespread application, however, is insufficient knowledge about to what extent, and how long, alkali-activated concretes protect embedded steel in reinforced concrete elements from corrosion. Recently, it was demonstrated that the redox conditions in the concrete pore solution are of major importance in this regard, and that these redox conditions are mainly controlled by sulfides in the ground granulated blast furnace slag of the binder. Here, a high sulfide concentration leads to inhibition of steel corrosion via reduction of oxygen in the pore solution, even at very high chloride concentrations. It is, however, not known whether, or to which extent, the same effects occur for mixed binders (e. g., ground granulated blast furnace slag/fly ash), and how subsequent access of oxygen affects the corrosion protection of the reinforcement. Therefore, the project comprises investigation of the influence of the binder composition on the composition of the concrete pore solution, the influence of the composition of the pore solution on the formation of a surface layer on steel, and the changes that take place when oxygen and chloride enter the system. A wide spectrum of electrochemical analytical methods will be applied for the investigations; the formation and stability of the surface layers will be studied inter alia by time-of-flight secondary ion mass spectrometry at nanometre-scale resolution. Experiments will be performed in simulated pore solutions in electrochemical cells in which the solution composition and other conditions can be selectively controlled, as well as in concrete specimens with varied access of air to reproduce real-world conditions. Complementary experiments are done to determine the ohmic resistance of alkali-activated concretes. Based on the results, a quantitative description of the influence of sulfides on the passive layer and the cathodic reaction as well as the corrosion rate in alkali-activated concretes will be developed. By this means, the project will significantly contribute to the current lively discourse on novel cements and their potential role in a more sustainable construction industry.

• DFG Programme: Research Grants
• International Connection: Australia
• Cooperation Partner: Dr. William Rickard