Bond between alkali-activated steel slag/fly ash lightweight mortar and concrete substrates: Strength and microscopic interactions

The fire-retardant lightweight mortar spalling from the tunnel lining substrates can threaten the safety of vehicles. The use of steel slag (SS) in alkali-activated cement can reduce the carbon emission and cost of lightweight mortar. The effects of SS on the bond strength and the microstructure of the interface between alkali-activated SS/fly ash (FA) lightweight mortar (ASFm) and concrete substrates was studied. The SS contents, namely, the mass ratio of SS to the summation of FA and SS, were 0 %, 20 %, 40 %, 60 %, and 80 %. The properties of ASFm, including compressive strength, fire resistance and bond strength, were studied. Results demonstrated that the bond strength of ASFm increases and then decreases with the increase in SS content. The highest bond strength of 0.65 MPa was obtained at 40 % SS content. Bond strength was determined by the pore structure and micromechanical properties of the matrix. When the SS content was increased from 0 % to 40 %, the content of C-A-S-H gel and C-N-A-S-H gel contents increased, and the porosity was reduced; hence, the bond strength of ASFm became larger. When the SS content increased from 40 % to 80 %, SS acted as an inhibitor, which reduced the micromechanical properties and increased the porosity of ASFm; therefore, the bond strength of ASFm decreased. This achievement provides an alternative way for the application of SS in tunnels. •The effects of steel slag (SS) on the bond strength between alkali-activated SS/fly ash (FA) lightweight mortar (ASFm) and concrete substrate was studied.•The bond strength between the ASFm and concrete substrate increased and then decreased with the increase of SS content.•The overlay transition zone (OTZ) no longer constituted a weak zone following the incorporation of SS.