In this paper, the types, formation time, structural morphology, and influence of steel corrosion products in seawater and sea-sand concrete were studied, and the intermediate and final products of steel corrosion under different conditions were determined. The corrosion products of steel in these concrete specimens under two curing methods were studied separately by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). Due to the presence of a large amount of chloride ions in the concrete, the rust layer on the surface of a steel bar contained many intermediates, such as lepidocrocite (γ–FeOOH) and aka-ganeite (β–FeOOH). Under wet/dry cycles, with the addition and loss of moisture in concrete, various corrosion products were also dynamically converted into each other. In the specimens immersed in seawater for a long time, the intermediates of corrosion were lepidocrocite (γ–FeOOH) and aka-ganeite (β–FeOOH), which were substituted for oxygen as the new depolarizers of cathode reduction reaction, and consumed themselves to ensure smooth corrosion.

In this paper, the effect of chloride ions on the development of corrosion pits in different reinforced concrete under different environmental conditions is studied. A fitting model for the size distribution of pits in seawater and sea-sand concrete (SSC) under different curing modes is established, and time-dependent fractal features are examined. Under wet/dry chloride cycles, the weight loss rate and corrosion rate of steel bars are higher, and the number of corrosion pits appears to increase on a small scale within 60-day. A majority of the corrosion is metastable pitting, and we propose a model to describe the size distribution of pitting in different periods. The good agreement between the proposed model and the available data illustrates that the proposed model is reliable and accurate. Macroscopic pitting occurs first in wet/dry chloride cycles. With the increase of concrete age, the size distribution of pits under wet/dry chloride cycles is uneven, and the pit sizes in submerged concrete tend to be equal.

Potassium magnesium phosphate cement (MKPC) is a type of chemically bonded ceramic material that has higher performance compared to traditional Portland cement. To develop the spraying and crack pouring process of MKPC, the mechanical properties, volume deformation, hydration temperature, and water stability of the high-fluidity MKPC with different mineral mixed materials and their influence laws were studied. The effects of phase composition and micromorphology of hydration products on the properties of MKPC and its mechanism were analyzed using X-ray diffraction (XRD), thermogravimetry/differential thermal analysis (TG/DTA), and scanning electron microscopy (SEM). The results show that fly ash and metakaolin will not reduce the fluidity of MKPC paste because of their material properties, and silica fume will reduce the fluidity of MKPC paste because of its large specific surface area and high water absorption. Metakaolin can react with phosphate to form aluminum phosphate gel and fill the pores between the crystals because it has a higher activity, which can significantly improve its compressive strength. However, during the later stage of hydration, there will be slight expansion, which would reduce its bonding flexural strength. The MKPC-hardened paste mixed with silicon ash has optimal stability: therefore, it has the highest bonding flexural strength. Microcosmic analysis shows that mineral mixed material plays a physical filling role and participates in the hydration reaction as an active ingredient to improve the early hydration degree, which can change the crystal size and micromorphology of MKPC-hardened paste and make the structure more compact.

Magnesium ammonium phosphate cement (MAPC) with a ceramic-like structure has good anti-corrosion properties. As an anti-corrosion coating, it can resist steel corrosion and reduces economic losses caused by steel corrosion. To further improve the anticorrosive properties of the coating, this study modified MAPC by partially replacing MgO with metakaolin (MK). Electrochemical tests analyzed the effect of MK on the anticorrosive properties of the coating. The results showed that the incorporation of MK significantly improved the anticorrosive properties of the coating, and the maximum improvement was obtained when the substitution amount was 15%. The compositions and microscopic morphology of the coatings were analyzed by X-ray diffraction (XRD), thermogravimetric analysis (TG-DTG), and scanning electron microscopy (SEM). According to the results, MK addition did not significantly alter the main composition of the coating, but the MK promoted the formation of MgNH₄PO₄·6H₂O(MAP) to some extent. The microscopic morphology of the coating surface showed that the crystals were stacked together in flower-like layers before soaking, and the crystals grew more entirely after soaking. The internal microstructure of the coating is very dense, which helps to resist the erosion of corrosive media. In addition, the changes in coating pores before and after the incorporation of MK were tested by low-field nuclear magnetic resonance (LF-NMR). Based on the results, the doping of MK can optimize the pores of the coating and reduce its porosity. It is feasible to replace part of the MgO with MK to improve the corrosion resistance of magnesium phosphate cement-based coating, and the optimal dosage of MK is 15%. This research is helpful to improve the anti-corrosion performance of the coating and reduce corrosion costs.

The actual protective performance of the coated reinforcement in coral concrete was investigated by measuring the chloride ion diffusion coefficient, electrochemical analysis, and numerical simulation. The test results show that the corrosion rate of coated reinforcement in coral concrete under the action of wet and dry cycles is kept at a low level, and the Rp value is always greater than 250 kΩ·cm² during the test period, which is in the uncorroded state and has good protection performance. Moreover, the chloride ion diffusion coefficient D is in accordance with the power function relationship with the wet and dry cycle time, and a time-varying model of chloride ion concentration on the surface of coral concrete is established. The surface chloride ion concentration of coral concrete reinforcement was modeled as a time-varying model; the cathodic zone of coral concrete members was the most active, increasing from 0 V to 0.14 V from 0 to 20 years, with a large increase in potential difference before the 7th year, and a significant decrease in the increase after the 7th year.