Wang, Danqian; (2021) Effect of microwave curing on the passivity of reinforcing steel in Portland cement pore solution and mortar. Doctoral thesis (Ph.D), UCL (University College London).

Text final-PhD thesis-Danqian.pdf - Submitted Version Access restricted to UCL open access staff until 1 November 2026. Download (8MB)

Abstract

Microwave curing is gaining increased popularity in the precast concrete industry as an alternative low-carbon accelerating curing technique. However, the effect of microwave curing on the passivity of reinforcing steel embedded in concrete is unknown, even though a high degree of passivity is crucial for the durability of reinforced concrete. It is generally believed that microwave radiation interacts with materials via two processes, i.e., thermal and non-thermal effects. When microwave curing is applied to reinforced concrete, there are concerns as to whether the compactness of passive film and the passivity of reinforcing steel could be affected by the thermal and non-thermal effects of microwave curing. Therefore, this study investigated the effect of microwave curing on the passivity of reinforcing steel in Portland cement (PC) system using a customized microwave oven with temperature feedback control. The influence of microwave curing parameters, such as microwave curing temperatures and durations, delay periods, rates of temperature rise and microwave frequencies, on the passivity of reinforcing steel in PC pore solution and mortar were studied. In addition, the passivity of reinforcing steel subjected to microwave curing, steam curing and normal (i.e., room temperature) curing were also compared to identify the thermal and non-thermal effects of microwave curing. The electrochemical methods (including OCP, LPR and EIS) were used to assess the passivation behaviour, whilst SEM, XPS and Raman spectroscopy were employed to characterise the passive film of reinforcing steel. Results show that microwave curing for 4 h resulted in higher passivity of reinforcing steel in pore solution than microwave curing for 2 h and 8 h, which was related to the formation of Fe3O4 in passive film at 4 h and microwave absorption by Fe3O4 at 8 h. As a comparison, the passivity of reinforcing steel in mortar for 2 h of microwave curing was higher than those of 4 h and 8 h. The different results in pore solution and mortar were possibly due to the decreased oxygen availability in mortar, which reduced the formation of iron oxide (especially Fe3O4) on steel surface. Moreover, the passivity of reinforcing steel in both pore solution and mortar was significantly decreased under microwave curing at 90°C for 8 h in comparison to that at 50°C and 70°C due to the formation of porous FeOOH in passive film at 90°C. In addition, the passivity in both pore solution and mortar increased with increasing delay period up to 12 h at 70°C for 8 h, which was attributed to the increased quality of initially formed passive film with increasing delay period. Additionally, the passivity of the reinforcing steel with a rate of temperature rise of 0.38°C/min at 70°C for 8 h was lower than that with 1°C/min due to the reduced formation of iron oxide/hydroxide to fill in the pores in the passive film with 0.38°C/min. Moreover, the reinforcing steel with 0.896 GHz at 70°C for 8 h obtained a higher passivity than that with 2.45 GHz since 0.896 GHz microwave promoted more precipitation of Fe3O4 in the inner layer of passive film and thus a more compact passive film formed. By comparing microwave curing with simulated steam curing and normal curing, it was found that microwave curing leads to lower passivity than simulated steam curing and normal curing. Both the thermal and non-thermal effects of microwave are considered to have contributed to this reduced passivity. Specifically, the thermal effect of microwave accelerated the dissolution of iron and the transport of iron ions towards outer layer of passive film, leading to higher FeOOH concentrations and thus porous passive film structure. On the other hand, the non-thermal effect of microwave promoted more iron ions precipitating into Fe3O4 in the inner layer of passive film by ponderomotive force as well as possibly disorganizing the structure in passive film. Nonetheless, the passivity of reinforcing steel in PC mortar is considered to be acceptable provided that microwave curing is below 70°C. It is anticipated that outcome from this PhD study can lay a foundation for developing the industrial application of microwave curing in precast reinforced concrete.

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