Effect of seawater on the morphology, structure, and properties of synthetic ettringite

This work investigated the influence of seawater on the growth rate, morphology, and properties of ettringite (AFt). The results showed that, in the absence of calcium hydroxide (CH), a part of the Mg ions in seawater preferentially formed Mg-Al layered double hydroxide containing Cl- (Mg-Al-Cl LDH), which could accelerate the subsequent precipitation of AFt in seawater. However, in the presence of CH, some of the Mg ions in seawater formed brucite, and there was no significant acceleration for the precipitation of AFt in the seawater-CH solution, compared with the AFt formed in seawater without CH. Besides, in both cases with and without CH, some Mg ions in seawater could enter the sites of Ca in the AFt structure, leading to an unstable structure. This would cause a relatively lower packing density compared with the corresponding AFt samples synthesized in deionized (DI) water, further leading to a relatively lower mechanical property. When the Mg ions were excluded from seawater, the AFt samples synthesized seemed to have a relatively higher packing density compared with the corresponding AFt formed in DI water, resulting in a relatively higher mechanical property. This finding could lead to a better understanding of the degradation of the micromechanical properties of AFt in seawater-mixed cementitious materials caused by the Mg ions.