Shotcrete is a placement method for concrete so most admixture or supplemental cementitious materials that can be used in cast concrete will work with shotcrete placement. In fact, shotcrete contractors have been some of the most innovative adopters of new concrete technologies. Silica fume (microsilica) is a ultrafine particle 100 times smaller than cement that enjoys early and wide use in shotcrete due to its ability to make concrete stickier and more cohesive. This facilitates overhead placements and can provide thicker layers. Shotcrete has also used other ultrafine and nanoparticles, like colloidal silica, clay-based particles and the carbon nanotubes you mentioned.
The ultrafine and nanoparticles can provide many benefits to fresh concrete, as well as hardened properties. This may include:
• Improve the pumpability of wet-mix concrete;
• Reduce rebound and dust due to increased “stickiness”;
• Ease the finishing process;
• Reduced permeability by filling pores between cement in the paste;
• Enhanced corrosion resistance for embedded reinforcement;
• Enhanced resistance to chemical attack.