Effects of Seawater, NaCl, and Na2SO4 Solution Mixing on Hydration Process of Cement Paste

The use of seawater as mixing water for concrete production is one of the most efficient strategies to solve freshwater shortage problems. The purpose of this paper is to investigate the effects of seawater mixing on hydration, microstructure, and strength development of cement pastes. Multiscale tests were used for cement pastes mixed with seawater, NaCl, and ${\mathrm{Na}}_2{\mathrm{SO}}_4$ solutions, respectively. The chloride ions in seawater increased the hydration degree during the process for nucleation and crystal growth and shortened the period for phase boundary reactions as a dominant process. At the age of 28 days, ettringite and Friedel’s salt coexisted for the seawater mixing paste, whereas the ettringite almost disappeared for the deionized water mixing. The seawater mixing led to a less porous cement matrix with much finer pores. Furthermore, seawater mixing enhanced the compressive strength and bending strength at early ages but later the enhancement decreased with curing time. ${\mathrm{Na}}_2{\mathrm{SO}}_4$ in the seawater enhanced the strength of the cement paste before 1 day but later inhibited the strength development.