This study investigates the pore and strength characteristics of alkali-activated slag paste, using seawater (SW) and tap water (TW) as the mixing water. During the investigation, the water/binder ratio was 0·45 and the activator used was sodium hydroxide (NaOH). Activator concentrations of 2, 4 and 6% of the binder weight were used. The compressive strength was measured and X-ray diffraction, mercury-intrusion porosimetry (MIP) and scanning electron microscopy analyses were conducted to determine the cause of the change in strength and pore structure. The results indicate that the compressive strength for early and later ages improved with the use of SW instead of TW. When SW was used, calcium–silicate–hydrate gel and Friedel's salt were identified as the hydration reactants. Based on the MIP measurements, when the sodium hydroxide concentration was increased, the pore size and number of pores decreased to form a denser matrix. In particular, samples containing SW had an increased number of gel pores (<0·01 μm).

Previous studies of alkali-activated slag cement (AASC) using nano-silica have mentioned mostly powdered nano-silica and binder weight replacement methods, which have a rapid decrease in fluidity, a short setting time and a low nano-silica replacement rate (< 5%). In this study, colloidal nano-silica (CNS) was used and the mixing-water weight substitution method was applied. The substitution method was newly applied to improve the dispersibility of nano-silica and to increase the substitution rate. In the experiment, the CNS was replaced by 0, 10, 20, 30, 40, and 50% of the mixing-water weight. As a result, as the substitution rate of CNS increased, the fluidity decreased, and the setting time decreased. High compressive strength values and increased rates were also observed, and the diameter and volume of pores decreased rapidly. In particular, the increase of CNS replacement rate had the greatest effect on decrease of medium capillary pores (50–10 nm) and increase of gel pores (< 10 nm). The new displacement method was able to replace up to 50% of the mixing water. As shown in the experimental results, despite the high substitution rate of 50%, the minimum fluidity of the mixture was secured, and a high-strength and compact matrix could be formed.