Modified natural seawater sea-sand concrete: Linking microstructure to mechanical performance

The rich availability of seawater and sea sand offers an alternative material resource for concrete production. However, the mechanical performance and durability of such concrete might not satisfy the construction requirements. Due to this, this study has first investigated the effects of varying dosages of ultrafine metakaolin (UMK) and nano-TiO2 (NT) as the supplementary cementitious materials on the mechanical properties of concrete produced using natural, untreated seawater and sea sand. The workability, compressive strengths, elastic moduli and flexural strengths have been explored for the concrete using the unary (ordinary Portland cement, OPC), binary (OPC and UMK) and ternary (OPC, UMK and NT) mixtures. The experimental results indicated the significant enhancement in the mechanical properties of the modified concrete. The cube compressive strength, axial compressive strength, the splitting tensile strength, the elastic modulus and the flexural strength have increased by 22.29 %, 22.82 %, 9.76 %, 16.02 % and 44.44 %, respectively. After that, the microstructural aspects expressed by SEM and XRD were also investigated for revealing the contributions of the NT and the UMK to the macroscopic mechanical performance of the seawater sea-sand concrete. The SEM analysis revealed a reduction in porosity and improved interfacial zones in the concrete containing the UMK and NT. The XRD analysis confirmed that the addition of UMK and NT promoted the calcium silicate hydrate (C-S-H) gel formation, mitigating the alkali-aggregate reactions. It was found that the addition of UMK and NT could improve the microstructure of the seawater sea-sand concrete, thereby enhancing the mechanical properties. Subsequently, the corrosion test conducted in a natural marine tidal environment revealed that, after the 360 tidal corrosion cycles, only the ternary mixed concrete maintained its structural integrity without the strength degradation, highlighting its superior durability in a marine condition. Lastly, analysis of variance was also performed to statistically evaluate the effects of UMK and NT as the single or combined mixtures on the modified concrete's performance. UMK has a significant impact on the short-term mechanical performance, while NT showed long-term contribution to the performance under the corrosive environment.