Adhesion performance of alkali-activated material for 3-dimensional printing of tunnel linings at different temperatures

Robotic-based technologies such as automated spraying or extrusion-based 3-dimensional (3D) concrete printing can be used to build tunnel linings, aiming at reducing labor and mitigating the associated safety issues, especially in the high-geothermal environment. Extrusion-based 3D concrete printing (3DCP) has additional advantages over automated sprayings, such as improved surface quality and no rebound. However, the effect of different temperatures on the adhesion performance of 3D-printed materials for tunnel linings has not been investigated. This study developed several alkali-activated slag mixtures with different activator modulus ratios to avoid the excessive use of Portland cement and enhance sustainability of 3D printable materials. The thermal responses of the mixtures at different temperatures of 20 and 40 °C were studied. The adhesion strength of the alkali-activated material was evaluated for both early and later ages. Furthermore, the structural evolution of the material exposed to different temperatures was measured. This was followed by microstructure characterization. Results indicate that elevated temperatures accelerate material reactions, resulting in improved early-age adhesion performance. Moreover, higher temperatures contribute to the development of a denser microstructure and enhanced mechanical strength in the hardened stage, particularly in mixtures with higher silicate content.