Microstructural evolution in sulfate solutions of alkali-activated binders synthesized at various calcium contents

Abstract

Alkali-activated binders based on metakaolin, fly ash and slag without curing were exposed directly in air, Na2SO4 and MgSO4 solutions for studying the evolution in mechanical strength and microstructure. A series of binders were synthesized and characterized on 7, 21, 45, 60, 75 and 90 days, respectively. Compressive strength measurement, scanning electron microscope (SEM) mapping, X-ray diffraction (XRD) and unclear magnetic resonance (NMR) were applied in the study. It is found that the exposure in sulfate solutions leaded to the deterioration of the binders by the dissolution of alkalis. In Na2SO4 solution, a higher percentage of zeolite was formed in the binders. While in MgSO4, a precipitate layer (e.g., brucite and gypsum), forming on the binder surface plays an important role in protecting the formation of gel, promoting the compressive strength improvement of alkali-activated slag, and compensating the decrease in compressive strength of alkali-activated metakaolin. These results provide clues for the direct preparation of alkali-activated concrete in aggressive sulfate environment.