Feasibility of storable microcapsules with core-to-shell ratio designs on the setting time and mechanical properties of alkali-activated slag: Toward concrete canvas application environment

Abstract

The rapid dissolution time of solid alkali activator sodium silicate significantly affects the storage performance of alkali-activated slag concrete canvas. In this study, the solid alkali of Na2SiO3 was encapsulated in microcapsules for sustained-releasing activation of alkali-activated slag cementitious material (AASCM) to improve hydration properties. Na2SiO3@EC microcapsules with core-to-shell ratios of 1:1, 3:1, and 5:1 were first prepared with the particle size, morphology and dissolution process studied. Then the microcapsules were added to AASCM to investigate their effects on the setting time, compressive strength, and microstructures. Results showed that microcapsules demonstrated specific storage properties, with a positive correlation between the core-to-shell ratio and loading amount. The presence of ethylcellulose (EC) shell layer retarded the release of alkali activator, i.e., the contact of Na2SiO3 to slag, prolonging AASCM's setting time by 20–30 min when microcapsules core-to-shell ratio was 3:1. The sustained-releasing activation also altered hydration product morphology that transformed sheet-like C–S–H gel into the spherical gel, and successfully slowed down 1 d and 3 d compressive strength development without sacrificing 7 d strength. The research findings demonstrate the feasibility of employing Na2SiO3@EC microcapsules for the storage of alkaline activators, thereby effectively prolonging the setting and hardening time of alkali-activated cementitious materials.

Keywords

• Alkali-activated slag cementitious material
• Solid activator
• Na2SiO3@EC microcapsules
• Storability
• Setting time