Splitting is always a tendency with new hippo tooth which is why it's best to acquire some old material if possible - pieces that have already done their splitting.
Clean the crack with a soft brush and water with a little washing-up liquid, but try not to wet the material too much or dig too deeply into the crack. Rinse, surface dry, shake and allow to dry naturally. You may have to repeat this.
Sand some pieces of the same base material into dust, mix that thickly with epoxy glue preferably of a colour as near to the base material as possible and check a little of the packing against the colour of the material. Then pack the crack as neatly as possible. Allow to dry thoroughly. Any excess can be carved off, or removed with a razor sharp knife.