Orthodontic brackets attached directly to the tooth are retained by a:
Bonding of resins (bracket-adhesive) to etched enamel is based on micromechanical interlocking produced by the resin tags that are formed by the penetration of the resin into micro-porosities created in the surface of the enamel after acid etching.
The essential components of composite resin are a resin matrix, filler particles, a coupling agent and a curing system.

​Key comment:
The chemical method of bonding tried using GIC failed to withstand the compressive, shear stress in orthodontic bracket bonding. 
Textbook of ORTHODONTICS Sridhar Premkumar
Compressive force
Hydrostatic force
Mechanical bond
Chemical bond