Unreinforced concrete or concrete reinforced with noncorrosive reinforcement could potentially be mixed with seawater in locations where potable water is scarce. A fundamental understanding of the properties of concrete mixed with seawater is therefore essential. This paper analyzes the hydration kinetics, hydrate phases, pore solution, and porosity of cementitious pastes made with seawater and compares these results with the corresponding ones from pastes made with deionized water. Pastes were prepared with cement and with a 20% mass replacement of the cement with fly ash. Isothermal calorimetry (to study hydration kinetics), thermogravimetric analysis (to study the hydrated phase assemblage), X-ray fluorescence (to determine pore solution composition and electrical resistivity), and dynamic vapor sorption (to determine the pore size distribution) were performed on the paste samples. Seawater accelerates hydration kinetics at an early age; however, this effect is negligible at later ages. Friedel’s salt formation in systems with seawater at later ages is negligible [0.4% (by mass of paste) at 91 days]. The primary difference between the hydrated phases of pastes made with seawater and those made with deionized water appears to be the absorption of chloride in the calcium silicate hydrate. The pore solution in pastes made with seawater has higher sodium, chloride, and hydroxide ion concentrations. The concentrations of sodium, potassium, and hydroxide ions in pore solutions are lower in pastes with fly ash compared to pastes without fly ash. Pastes with seawater show a lower electrical resistivity than pastes with deionized water due to the higher ionic concentrations. Paste with seawater has a slightly finer pore structure compared to paste with deionized water.