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This work examined the effects of seawater (SW) on the hydration of tricalcium aluminate (C3A) in C3A–gypsum and C3A–gypsum–Ca(OH)2 systems through the characterization of hydration heat release, the evolution of aqueous phase composition and hydration products with the hydration time. It was found that SW increased the dissolution driving force of C3A and solubility of gypsum, which accelerated the early hydration of C3A and the formation of ettringite (AFt), leading to a higher hydration degree of C3A at an early age compared with the deionized (DI) water–mixed pastes. After gypsum depletion to form AFt, and in the absence of Ca(OH)2, the formation of chloroaluminate hydrates was slower due to the insufficient Ca resulted in an accumulation of Al in solution. This would delay the subsequent transformation of AFt to monosulfate (SO4–AFm) and the formation of hydrogarnet (C3AH6), which would further reduce the hydration degree of the C3A at the later ages. However, in the presence of Ca(OH)2, the hydration degree of C3A–gypsum–Ca(OH)2 at later ages was increased, which was similar to that of the corresponding DI pastes. This can be inferred that the amount of Ca available in SW‐mixed cement concrete can affect the hydration degree of C3A in cement. [ABSTRACT FROM AUTHOR]

This work examines the micromechanical properties of AFm phases formed in seawater and deionized water. As the interlayer SO 4 2− in the AFm phase is gradually replaced by Cl− in seawater, the basal spacing of the AFm crystal narrows, which promotes the packing density of nanocrystals and their indentation modulus and hardness, i.e. , Cl-AFm (Friedel's salt) > SO 4 -Cl-AFm (Kuzel's salt) > SO 4 -AFm (monosulphate). It is found that Friedel's salt formed in seawater feature multiple structural and compositional defects. First, the anions (Cl−, OH−, SO 4 2−) are bound in the interlayer of Friedel's salt, and the bound OH− contents have negative relations with the Cl− concentration in the seawater. Second, the incorporation of Mg ions in the Friedel's salt barely changes its micromechanical properties based on the experimental data and molecular dynamics simulations. Third, when portlandite is present in the seawater, some Ca vacancies would be formed in the Friedel's salt, which would decrease its Young's modulus significantly. This accounts for the decrease in the indentation modulus of Friedel's salt as observed in the experiments. These findings enable us to better understand and control the micromechanical properties of Cl-containing AFm phase formed in seawater-mixed cementitious materials. [ABSTRACT FROM AUTHOR]

This study explored the mechanism of the effect of seawater on tricalcium aluminate (C 3 A) hydration. The results showed that seawater retarded C 3 A hydration and reduced the reaction degree of C 3 A. The co-existence or ion pairing of Ca2+ and SO 4 2− onto the surface of C 3 A poisoning the reactive sites is the main reason for this retardation. Besides, the precipitation of Mg(OH) 2 on the surface of C 3 A would prolong the induction period for another 30 min and then consequently decrease the dissolution rate of C 3 A hydration. Trace amounts of MgSO 4 0 and Mg2+ present in the alkaline solution had little retardation effect. It was also found that Cl− would preferentially react with C 3 A to form Friedel's salt, rather than forming hydroxy-AFm as that in the C 3 A-deionized water paste. The direct formation of Friedel's salt resulted in an accumulation of Al3+ in solution, which would also hinder the subsequent dissolution of C 3 A. [ABSTRACT FROM AUTHOR]