Influence of ultrafine metakaolin and nano-TiO₂ on the durability and microstructure of seawater sea - sand concrete

Abstract

The increasing demand for sustainable construction materials has spurred interest in seawater sea-sand concrete (SWSSC) as a substitute for natural resources. However, SWSSC's durability faces challenges from aggressive chloride and sulfate ions existing in seawater, causing structural degradation. Therefore, this study has prepared seven SWSSC formulations with the different ultrafine metakaolin (UMK) and nano-TiO₂ (NT) dosages using untreated seawater and sea sand. The SWSSC specimens fabricated using these formulations were evaluated through the wet-dry sulfate cycling, chloride permeability and water permeability tests to assess their durability performance. The advanced microstructural analyses, including the Fourier transform infrared spectroscopy, the scanning electron microscopy and the X-ray diffraction, were also employed to examine the effects of the UMK and NT on the pore refinement, the phase evolution and the functional group changes within the concrete matrix. The test results have revealed that a combined addition of 15 wt% UMK and 0.5 wt% NT significantly reduced the chloride ion permeation and the water permeability, enhancing the initial impermeability of the modified concrete. However, after the 60 cycles of sulfate exposure, the specimens with the 15 wt% UMK addition (with or without NT) lost their strengths, while the unmodified concrete specimens retained the higher residual strengths. The formulation with the addition of 5 wt% UMK and 0.5 wt% NT demonstrated the resistance improvement up to the 60 cycles, although the specimen strength was slightly lower than that of unmodified SWSSC specimen. These findings highlighted the need for optimizing the UMK and NT dosages in order to balance initial impermeability and long-term durability of SWSSC.