Yang, SiYu;
(2021)
Passivation and De-passivation Behaviour of Steel Reinforcement in Alkali-Activated Slag.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
Due to the concerns of environmental issues and claim of green construction materials, using the alkali activated slag (AAS) as a binding material starts to attract more and more attention. However, the studies on the corrosion of reinforcing steel in AAS concrete are still limited, even though steel corrosion has been considered as a great threat to the durability of reinforced concrete structures. Due to the distinct difference in the raw materials composition between PC and AAS (e.g. the presence of sulfide species), as well as the potential effect of using different AAS activators, the durability performance of reinforced AAS is expected to be different from that of PC. Considering the chloride induced corrosion accounts for 80% cases of deterioration of concrete structures, a good understanding on the mechanisms of chloride-induced corrosion in AAS is, hence, essential for the future applications of reinforced AAS concrete in practice. This PhD thesis, therefore, mainly focused on investigating the passivation (formation of a dense/protective oxide layer that inhibit further oxidation) and chloride-induced depassivation (destruction of this protective oxide layer by aggressive chlorides) of reinforcing steel reinforcement in AAS manufactured with four different activators (namely NaOH, waterglass, Na2CO3 and Na2SO4). The electrochemical techniques, including open circuit potential (OCP), linear polarisation resistance (LPR) and electrochemical impedance spectroscopy (EIS), were used in characterising the passivation and de-passivation behaviour of reinforcing steel, whilst the properties of passive films were characterised via Mott- Schottky (M-S) curve, anodic polarisation (AP), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) to understand the mechanisms involved during the passivation and chloride-induced depassivation processes of reinforcing steel. The results show that the activator used for AAS has huge impacts on the passivation and chloride-induced de-passivation of reinforcing steel. For steel passivation, the presence of sulfide originating from slag (AAS precursor material) inhibited the passivation process via competitive adsorption against OH- . Additionally, the formation of Si-based substance (SiO2 and Al-Si gel-like species) on the passive films in waterglass and Na2CO3 activated AAS systems could enhance the quality of passive film. However, Na2SO4 was found to result in unstable passivation, and the corrosion was even noticed during the passivation stage. In the case of NaOH activated AAS system, Fe-S based compounds were formed on passive film. Based on the results from the electrochemical measurements, the quality of the passive films formed in different activator systems can be ranked as: (PC) > waterglass > Na2CO3 > NaOH >> Na2SO4. For de-passivation, corrosion behaviour of steel were not observed in NaOH, waterglass and Na2CO3 activated systems due to the presence of sulfide in the pore solutions (the threshold concentration of sulfide was not investigated in this study). However, in the absence of sulfide, the corrosion of steel occurred in all these systems. Based on the chloride threshold values, a ranking order of the protective capacity of different AAS systems provided to steel can be established as: waterglass (not corroded) > Na2CO3 (4.5 mol/L) > NaOH (3.5 mol/L) > (PC) >> Na2SO4.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | Passivation and De-passivation Behaviour of Steel Reinforcement in Alkali-Activated Slag |
| Event: | UCL (University College London) |
| Language: | English |
| Additional information: | Copyright © The Author 2021. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request. |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Civil, Environ and Geomatic Eng |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10138954 |
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