Probing the micromechanical properties of seawater-mixed alkali-activated slag: Insights from nano-indentation and EDX

Abstract

Limited research has been conducted to elucidate the micro-scale strength degradation of seawater-mixed alkali-activated slag (AAS). In this study, the effects of the primary salts (e.g., NaCl, MgCl₂, CaCl₂ and Na₂SO₄) in seawater on the micro-mechanical properties of the AAS were evaluated by a combination of nanoindentation and quantitative BSE-EDS image analysis. The results showed that the sea salts could react with silicates from the activator, decreasing the content of silicates, which is necessary for strength development. In a low alkali system, the various salts in seawater caused severe strength degradation on the AAS, and the Mg ion was identified as a primary factor contributing to the deterioration of the modulus and hardness properties of the C-(A)-S-H gels. When a high alkali content was used, the micro-mechanical properties of the AAS were less vulnerable. Additionally, chloride in seawater preferentially reacted with dissolved Al to form Friedel's salt rather than hydrotalcite, as observed in the AAS prepared with deionized water. The formation of additional hydrotalcite and Friedel's salt in seawater-mixed AAS resulted in the improvement of the micro-mechanics of the gels around a dark rim of reaction products surrounding the slag particles observed in BSE-SEM images. This investigation provided guidance for using seawater as mixing water in AAS.