Effect of maintenance environment on the mechanical property and pore structure of cement paste mixed by seawater

Abstract

This study aims to examine the effects of macroscopic mechanical properties and pore structure of cement paste mixed with seawater and explore the relationship between pore structure and macroscopic properties. The changeable law of mechanical properties and pore microstructure of hardened cement paste under seawater mixing is methodically investigated based on the compressive and flexural strengths using X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) tests. The relationship between the fractal dimension of the pore structure of cementitious materials and the concentration of chloride ions in simulated seawater, the curing age, and the fly ash dose was obtained based on the thermodynamic fractal model. The obtained results reveal that with the increase in chloride ion concentration of the simulated seawater, the compressive and flexural strengths of the samples substantially grow in the pre-maintenance period, and the strength of the samples tends to stabilize at the age of 28 days. For the case of the chloride ion concentration of 1.3%, the compressive strength of the cement slurry exhibits a growth of 24.08%, 24.83%, and 28.81% compared to the control cement sample after curing at 5 °C, 20 °C, and 50 °C for 3 days, respectively. Additionally, the flexural strengths in order are 41.86%, 46.57%, and 38.89% higher than those of the control group. The interaction of Cl- with C3A, C4AF, or AFm in seawater mixed at high-temperature to generate Friedel's salt stimulated the volcanic ash reaction of fly ash, promoted the generation of C-S-H gels, and improved the internal pore structure of cementitious materials. The results indicate that at 50 °C curing, the most probable pore diameter of cement paste with 1.3% chloride ion concentration is 39.51% less than that of the control cement sample, and its harmless pores are 23% higher than that of the control group. At 5 °C curing, the total specific pore volume of cement paste with 1.3% chloride ion concentration is 30.18% less than that of the control cement sample, and the total specific pore volume is 22.9% less than the control group for the fly ash content of 25%. The obtained results reveal that there exists a fairly low correlation between the fractal dimension of the pore surface area of the cement paste and porosity, compressive strength, and flexural strength. However, the achieved correlation with the mean pore diameter, median pore diameter, and pore surface area is good, which are all above 75%. The fractal dimension fitting curve of high-temperature curing possesses a range of singular points.