Properties of Alkali-Activated Slag Paste Using New Colloidal Nano-Silica Mixing Method

Abstract

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.

Influence of Nanosilica on Mechanical Properties, Sorptivity, and Microstructure of Lightweight Concrete

“…Simultaneously, the use of NS tends to increase the proportion of smaller pores, by disconnecting continuous pores from discontinuous ones and subdividing larger pores into smaller ones; thus producing a material with a finer pore structure, which exhibits lower transport properties. The MIP measurements in the present study, as well as those of other authors [36,46,47,48], confirm that total porosity decreases significantly in the presence of NS in the cement matrix, when the optimal amount of NS is incorporated. Therefore, as reported in this study, NS-modified composites exhibit denser microstructures with lower water absorption, which might potentially prevent the ingress of aggressive solutions to the pore structure [12,14,49,50,51].…”

Investigation of the Effects of Magnesium-Sulfate as Slag Activator

“…The compressive strength values of alkaline activated slag cement using the same mixed activation of 5% NaOH + 5% Na 2 SiO 3 and 10% NaOH + 10% Na 2 SiO 3 were measured higher than MS [28]. Both 5% and 10% activators showed higher strength values than MS, and 10% activator showed better compressive strength value than 5%.…”

“…The cause of this result was reported that excessive gypsum caused expansion inside the specimen, increasing porosity and decreasing compressive strength. A study by Kim et al [28] using 5% NaOH + 5% Na 2 SiO 3 and 10% NaOH + 10% Na 2 SiO 3 showed a sharp decrease in the total porosity of the 10% activator than 5%. Increasing the concentration of the activator promotes the formation of the hydration reactant and creates a dense structure, reducing the porosity.…”

“…Therefore, gypsum affects the production of ettringite, but unhydrated gypsum is present between the hydration reactants, causing expansion and thus reducing compressive strength [22]. In contrast, in the study using sodium hydroxide or sodium silicate as individual admixtures or blending, the main hydration reactant was C-S-H gel [27,28]. In their XRD analysis, gypsum and ettringite were not observed.…”

“…This may be due to an expansion crack or a large pore between the relatively large re-precipitated gypsum crystals. Figure 3b shows the total porosity of Park et al [22] and Kim et al [28] at 28 days. Park et al [22] noted that the 10% gypsum sample had the lowest total porosity and the highest compressive strength.…”