Chloride penetration resistance is one of the most important performance measures for the evaluation of the durability of concrete under a chloride environment. Due to differences in theory and experimental conditions, the durability index (chloride diffusion coefficient) obtained from laboratory accelerated migration tests cannot reflect the real process of chloride ingress into concrete in the natural environment. The difference in test methods must be considered and the transfer parameter kt should be introduced into the service life prediction model when the test results of accelerated methods are used. According to the test data of coastal exposure in South China, the attenuation rule of the chloride diffusion coefficient of different cement-based materials changed with time and was analyzed in this paper. Based on the diffusion coefficient–time curve, the theoretical natural diffusion coefficients of 28 d and 56 d were deduced, which were compared with the chloride diffusion coefficients obtained from the non-steady-state rapid migration method (RCM) at the same age. Therefore, the transfer parameter kt that expounds the relationship between concrete resistance to chloride permeability under a non-stationary electrical accelerated state and natural diffusion in the marine environment can be calculated; thus, the RCM testing index can be used to evaluate the long-term performance of the concrete structure in the marine environment. The results show that the value of kt was related to environmental conditions, test methods, and binder systems.

Traditional Portland cement is widely used in the preparation of various hydraulic concrete. However, the high alkalinity produced by cement hydration threatens the survival of aquatic animals and plants. In this paper, a new eco-friendly, ultra-low alkalinity, cementitious material was prepared with industrial waste phosphogypsum, granulated ground blast slag (GGBS) and sulphoaluminate cement. When appropriate proportions are used, the pH value of the test blocks’ pore solutions at different ages were all less than 9, showing the remarkable characteristic of ultra-low alkalinity. The XRD and SEM analyses showed that the 56 d hydration products were mainly ettringite and hydrated calcium silicate, and the content of Ca(OH)2 was not detected. The new cementitious material also has the advantages of short setting time, low heat of hydration, high strength of cement mortar and the ability to fix harmful substances in phosphogypsum, such as phosphate, fluoride and Cr and Ba elements. It has a broad application prospect in the construction of island and reef construction, river restoration and so on.

The chloride diffusion of marine high-performance concrete under a couple of actions, flexural load and freezing–thawing, was investigated by a fast freezing and thawing test in NaCl solution. Concrete specimens of 100 × 100 × 515 mm were tested under bending load and 300 freeze–thaw cycles under the stress levels of 15%, 30%, and 50% of the ultimate fracture modulus. The change in the microstructure of the concrete was analyzed by SEM and MIP. The results indicated that the chloride diffusion coefficient of concrete under the coupling effect of flexural load and freezing–thawing or simple flexural load increased with the increasing in the flexural stress level, and the chloride diffusion coefficient was approximately exponential to the flexural stress level, as D = 0.8777e1.668σ for a couple of actions of flexural load and freezing–thawing, and D = 0.8336e1.3231σ for a simple flexural load. The resistance ability of concrete to chloride diffusion was reduced by the freezing–thawing procedures, the resisted ability dropped more severely under a couple of actions of flexural load and freezing–thawing than simple flexural load at the same stress level. Micro-cracks at the interfacial transition zone between the aggregate and the paste matrix in concrete was induced under a couple of actions of flexural load and freezing–thawing, which increased the average pore size and total pore volume, resulting in the modification of the pore size distribution in the concrete. The influence of a couple of actions of flexural load and freezing–thawing on the concrete was greater than that of simple flexural load.