When you tighten the nuts onto the closet bolts that hold the base of a toilet secure, avoid over tightening or you can easily crack the porcelain toilet. It might not seem as bad as a crack to another toilet part, such as the tank, but it still can lead to leaking. If the toilet base is not too damaged, glue it back together.
One correct adhesive choice for the cracked toilet base is a two-part epoxy. Epoxy is a thermosetting resin that, mixed with a hardener, forms a tough bond that will keep the break sealed up. It will also be resistant to water, which is an obvious concern around a toilet. To ensure the epoxy that bonds with porcelain, follow the specific instructions included by the manufacturer. Wipe the toilet base clean of all debris and water, using a clean, dry rag. Thoroughly stir and mix the epoxy, according to the instructions, and don't vary from the ratio that the instructions give for mixing the two parts.
Polyurethane glue is an additional choice that will create a strong bond for the broken toilet base. The difference between polyurethane glue and epoxy is that the toilet surface should be a little wet when you apply the polyurethane glue. Also follow the manufacturer's instructions for applying the glue properly. Another difference with polyurethane glue, compared to epoxy, is that you have to clamp the toilet parts together after applying the glue, as the polyurethane glue expands when it dries.
No Caulk or Silicone
Although caulk and silicone might be initial considerations to seal the broken toilet base, they won't provide the same long-term success that using epoxy and polyurethane glue provides. The advantage of the epoxy is the bonding strength it provides. Yes, silicone and caulk might both provide an adhesive seal against water leaking through the crack, but there will be no strong bond for the broken base. With the amount of heavy use that toilets endure, the crack will reoccur, and possibly lead to a leak from underneath the fixture.
If the broken toilet base is not properly glued back together, when the base shifts under a user's weight, or when the toilet is otherwise moved due to the broken base, the seal created by the wax ring can easily be broken also. This allows both water and sewer gases into the room. The water can damage the flooring around the toilet, and the sewer gases can cause physical ailments, such as dizziness. The only repair is to stop using the toilet, lift it up and install a new wax ring.