The influence of highly nitrogen-doped mesoporous carbon spheres (NMCSs) (internal pore size of 5.4–16 nm) on the electrochemical response of low carbon steel (St37) in model alkaline solutions of pH 13.9 and 12.8 was studied, using Open Circuit Potential (OCP) monitoring, Electrochemical Impedance Spectroscopy (EIS) and Cyclic Voltammetry (CV). Prior to adding the NMCSs in the relevant solutions, they were characterized in the same model medium by measuring their Zeta-potential, hydrodynamic radius and particle size distribution, using dynamic light scattering (DLS) and transmission electron microscopy (TEM). In alkaline environment of pH 13.9 and 12.8, which simulates the concrete pore water of fresh and mature concrete, the DLS measurements indicated that the hydrodynamic radius of NMCSs particle varied from 296 nm to 183, respectively. According to the Zeta-potential measurements in the same solutions, the NMCSs were slightly positively charged.

The addition of 0.016 wt.% of NMCSs to the model medium induced certain variation in the electrochemical response of the tested steel. In alkaline solutions of pH 12.8, the presence of NMCSs in the passive film/solution interface induced a delay in the formation of a stable passive film. On the other hand, in solutions of pH 13.9, the higher corrosion activity on the steel surface, enhanced by high pH, was limited by adsorption of NMCSs on the film/substrate interface. In addition competing mechanisms of active state, i.e., enhanced oxidation on the one hand, and particles adsorption on anodic sites and oxidation limitation, on the other hand, was relevant in solution of pH 13.9 inducing larger fluctuations in impedance response and stabilization only towards the end of the testing period.

Except steel electrochemical response, the properties of the cement-based bulk matrix were investigated in the presence of the aforementioned additives. The mortar bulk matrix properties were highly affected by NMCSs. The lowest electrical resistivity values were recorded in mortar specimens with mixed-in 0.025 wt.% NMCSs (with respect to dry cement weight). Furthermore, the addition of 0.025 wt.% NMCSs increased the compressive strength when compared to control specimens. The presence of F127 as a dispersing agent for NMCSs was found to be not suitable for reinforced concrete applications, which is in view of the reduced mechanical strength and electrical resistivity of the cement-based bulk matrix. This is in addition to the adverse effect on the formation of electrochemically stable passive layer on steel surface in alkaline medium in its presence.
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