| Summary: | Applying positive potential to a steel electrode immersed into a cement changes the packing of cement particles in the vicinity of the electrode surface. The electrophoresis-induced packing enhancement at anode has promising applications in oil & gas and CO<sub>2</sub> storage industries since it could be used to improve the mechanical and hydraulic cement-casing bonding in wells and thereby improve the well integrity, both in short and long term. In this experimental study, we use synchrotron radiation microtomography (µ-CT) and X-ray diffraction (XRD) analyses of the interfacial transition zone (ITZ, a 20–100 µm wide near-wall zone depleted of large particles) to find out what structural changes are responsible for different cement-steel adhesion at anode and cathode. Particle size distribution analysis reveals that the ITZ is enriched with large (equivalent diameter > 10 µm) cement particles near anode. On the contrary, near cathode, cement is depleted of large particles, which results in poor adhesion to the electrode. XRD analysis reveals that cement near anode is enriched with tricalcium silicate (Ca<sub>3</sub>SiO<sub>5</sub>). These findings suggest that electrophoresis-enhanced cement-steel adhesion is due to large (>10 µm) negatively-charged tricalcium silicate particles being attracted to anode.
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