Orthodontic brackets attached directly to the tooth are retained by a:
Compressive force
Hydrostatic force
Mechanical bond
Chemical bond
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