[DanielDudley]

So...

Cement is a powder that when mixed with water forms a supersaturated solution that turns into limestone crystals. Limestone is not super strong. Concrete is cement mixed with small rocks known as aggregate. The cement forms limestone crystals around the hard stone aggregate. concrete is very strong, for a variety of reasons, mostly having to do with the aggregate.

Epoxy is a catalyzed plastic. JB Weld is epoxy with steel filings as aggregate. Fiberglass is epoxy with glass fibers. Epoxy isn't inherently strong, but it is a good binder. Carbon fiber, glass fiber, and steel filings are all inherently strong. Epoxy binds them, and gives them form. JB Weld isn't epoxy. It is steel, with epoxy as a binding agent.