Effects of Sand Type and Alkali Equivalent on Drying Shrinkage and Microstructure of Seawater-Sea Sand Alkali-Activated Slag Concrete

The use of seawater and sea sand for the preparation of alkali-activated concretes plays an important role in reducing carbon emissions and alleviating resource scarcity. However, both alkali-activated concretes and products made from seawater and sea sand tend to exhibit significant drying shrinkage. Therefore, this study investigates the effect of the addition of seawater and sea sand on the basic properties and drying shrinkage properties of alkali-activated concretes, and regulates the properties of seawater and sea sand alkali-activated slag (SSAS) concretes with the key parameter of the alkali equivalent. Finally, the mechanism of SSAS drying shrinkage improvement was investigated using XRD, TG, MIP, and SEM. The results show that the addition of seawater and untreated sea sand accelerates the hydration reaction of alkali-excited cement and can significantly reduce its drying shrinkage. A moderate increase in alkali content can improve the compressive strength and reduce the drying shrinkage. However, an excessively high alkali content reduces the flexural strength. Finally, we propose the existence of a quantitative relationship between drying shrinkage, mass loss, and the evaporable water content, which better explains the mechanism of shrinkage variation. These effects are mainly attributed to changes in microstructure and phase composition. This study provides theoretical support for engineering applications of seawater-sea sand alkali-activated materials.