What happens with a pipe bomb is that the explosion, being confined to a sealed container, produces a large volume of gas that actually causes the walls of the pipe to balloon and stretch until broken pieces of shrapnel fly out. Gas compression bombs of this nature have a strong shock wave effect, to the order of 7000 mph. More serious damage is done by the blast effect than the fragmentation effect. It's like having a hurricane in a box. Low explosive materials are only lethal when they are confined in a container.
Ammonium nitrate is a common fertilizer.
When mixed with fuel oil, it's called ANFO, and this was the kind of explosive used on New York's World Trade Center in 1993. TNT is also unaffected by moisture, and is the stuff inside of WW II-era bombs, shells, and grenades. It's pure nitroglycerin, sealed in wax. TNT is rarely encountered in bombings today. It has a very plastic, dough-like consistency and charges can be shaped for special detonation effects.