Effect of water environment on mechanical behavior of coral aggregate concrete

Abstract

The complex marine environment often exposes concrete to seawater and acidic conditions during its service life. To investigate the effect and mechanism of special water environments on coral sand concrete, forty-five Φ50 mm × 100 mm cylindrical coral concrete specimens were prepared and cured in freshwater, seawater and oxalic acid in this study. The specimens were cured for 7, 14, 28, 60 and 90 days, respectively. The physical and mechanical properties of the specimens under the lossless state and static failure state were tested through piezoelectric transducer (PZT) test and uniaxial compression test. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed for analyzing the mineral composition and microstructure of the specimens. The results show that the compressive strength of the specimens in seawater and freshwater initially increased and then stabilized with the curing age, whereas in oxalic acid it initially increased and then decreased. The compressive strength of the specimens at 90 days in freshwater, seawater and oxalic acid solution have increased by 54%, 18% and 21%, respectively, compared to that at 7 days. A combination analysis using microscopic and macroscopic test indicated that the completion time required for hydration in freshwater, seawater and oxalic acid solution was 28, 28 and 60 days, respectively. This study suggests that Cl− in seawater accelerates the cement hydration process, improving the early mechanical properties of specimens. During the later hydration process, the soluble CaCl2 and expansile Mg(OH)2 generated during the later hydration process which reduces the strength. In oxalic acid solution, a dense calcium oxalate coating produced through the reaction between oxalic acid and CaCO3 would reduce the degree of hydration. The formation of CO2 would increase the porosity of specimens, thereby weakening the mechanical strength of the concrete.