Multi-technique analysis of seawater impact on the performance of calcium sulphoaluminate cement mortar

The construction of offshore islands and reefs requires the development of new seawater sea-sand concrete materials. This study investigated the impact of seawater on the performance of calcium sulphoaluminate (CSA) cement mortar, with mechanism analyses by the acoustic emission (AE) and digital image correlation (DIC). For all uniaxial compression tests, cumulated AE, average AE frequency, rise angle, b-value, and (1t coefficient were considered to perform the failure analysis. In addition, other techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were also employed to highlight the effects of seawater on the hydration products and microstructure of the CSA cement. The results show that seawater not only promotes the formation and growth of ettringite (AFt) but also interacts with cement hydration products to produce Friedel's salt and various gel formations. Compared to freshwater mixing (0 %), using seawater mixing at normal concentrations (3.3 %) enhances the compactness of the CSA cement, thereby improving the strength and ductility of the CSA mortar. Specifically, the peak load increases from 77 kN to about 86 kN when mixed with seawater at normal concentration, whereas higher seawater concentrations (6.6 %, 9.9 %) resulted in a slight decrease in strength of the CSA mortar, with the peak load reaching approximately 80 kN, yet the ductility is enhanced, as evidenced by the softening post-peak branch of the loading curve. Additionally, AE bursts have a strong correlation with the fracture of the CSA mortar, which can also be forecast by the AE time-scaling coefficient (1t. Therefore, when constructing oceanic island and reef foundations, this type of seawater sea-sand concrete is preferred due to its improved ductility and strength, which enhances the structural integrity of marine constructions and provides early warning for effective marine disaster prediction.