Microstructure evolution of interfacial transition zone between alkali-activated fly ash/slag matrix and aggregate

Interfacial transition zone (ITZ) between matrix and aggregates is always the focus of concrete materials and it would determine mechanical properties and durability of alkali-activated concrete as well. To understand its formation mechanism and microstructure evolution, alkali-activated fly ash (AAF)/slag (AAS) concrete was prepared by using two types of aggregates (basalt and limestone), various ratios of water to solid, different moduli and concentrations of activator in this study. The experimental results found that excessively high concentration of alkali activator yielded a certain inhibitory effect on alkali-activated reaction, but did not adversely affect the microstructure evolution of ITZ, and the increase in the alkali concentration can promote the chemical reaction between aggregate surface and matrix so as to densify the interface microstructure. Basalt aggregate would mainly release more Si4+ and Al3+, while limestone aggregate can release Ca2+. These released ions can participate in alkali-activated reaction to form N-A-S-H and C-A-S-H gels, which can thus adhere to the aggregate surface and fill the pores in the ITZ. For AAF concrete, the chemical reaction on the aggregate surface was significant in the early stage, and the increase in the resistivity rho of the ITZ from 5 h to 1 day was even greater than that of AAS concrete. However, the resistivity rho in the ITZ of AAS increased significantly from 1 to 7 days, indicating that the chemical reaction between the aggregate and the matrix was only obvious during this period.