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This study aimed to investigate the durability performance of marine concrete incorporated with nano–SiO2 and nano–Al2O3 under the joint effect of dry–wet cycles, Cl− erosion, and carbonation. Free Cl− content and carbonation depth were used as durability evaluation indexes of concrete with nanoparticles under the joint effect of the three factors. Scanning electron microscope (SEM), energy dispersive spectrometer (EDS), backscattered electrons (BSE), and X-ray diffraction (XRD) were used to analyze the morphology and phase composition of the concrete. The test results showed that the free Cl− content and carbonation depth of the concrete were significantly reduced after incorporating nanomaterials, indicating that nanomaterials improved the resistance of the concrete to Cl− erosion and carbonation under the joint effect of dry–wet cycles, Cl− erosion, and carbonation. Compared with plain concrete at 56 cycles, both nanomaterials showed the most significant improvement at a dosage of 1.8%, with a 20% and 37.5% reduction in free Cl− content and carbonation depth, respectively, for nano–SiO2 concrete, and 16.67% and 31.66% reduction, respectively, for nano–Al2O3 concrete. The improvement effect of nano–SiO2 was better than that of nano–Al2O3 , and the free Cl− content and carbonation depth of nano–SiO2 concrete were reduced by 3.33% and 5.84%, respectively, compared with those of nano–Al2O3 concrete at the optimal dosage. The microscopic test results showed that incorporating nanomaterials promoted the hydration process of cement and refined the pore structure of the concrete, improving the durability of the concrete under the joint effect of dry–wet cycles, Cl− erosion, and carbonation. [ABSTRACT FROM AUTHOR]

In order to study the influence of nanomaterials on the carbonation resistance of marine concrete under bending loads, an appropriate amount of nano-SiO2 was added to plain concrete, and a self-developed carbonation box and bending loading device were used to conduct a coupling test. Four different stress levels were set to measure the carbonation depth of nano-concrete at different ages. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to analyze the concrete interfacial transition zone. The carbonation depth was used as the test index to evaluate the durability of nano-SiO2-based concrete under the combined action of bending load and carbonation. The test results showed that the compressive and flexural strengths of concrete remarkably improved when the nano-SiO2 concentration was 2%. Compared with regular concrete, the compressive and flexural strengths of nano-SiO2 based concrete improved by 15.5% and 15.3%, respectively. When the stress level was 0.15 and 0.6, the carbonation depths of NS20 were 20.5 and 18.4% lower than those of PC in the tensile zone and 18.9 and 23.7% lower than those of PC in the compression zone. The carbonation depth of the NS20 tensile zone was lower by 31 and 18.4% at 3 and 28 days than that of PC. Compared with PC, the carbonation depth in the compression zone of NS20 decreased by 50 and 23.7%, and the carbonation depth of nano-concrete was significantly lower than that of conventional concrete under the same stress level and age. When the stress level is constant, the carbonation depth of the tension zone and compression zone increases gradually with the increase in age, and the carbonation depth of the concrete in the first 7 d was 50% that at 28 days. Under the same age, the carbonation depth in the tension zone increased with increasing stress levels, while the carbonation depth in the compression zone decreased with increasing stress levels. When the stress level was 0.3–0.45, the slope of the carbonation depth curve significantly increased. SEM and XRD analysis results revealed that nano-SiO2 significantly improved the internal structure of concrete by reducing the width of the microcracks, the number of pores, and the number of microcracks. The number of C3S/C2S and CaCO3 crystals in nano-SiO2 based concrete was significantly less than that in plain concrete, and the amount of C-S-H gel was more than that in plain concrete. Under bending loads, the nano-SiO2 significantly improved the carbonation resistance of concrete. When the dosage of nano-SiO2 was 2%, its improvement effect was the most significant. [ABSTRACT FROM AUTHOR]