Most capping compounds are mixtures of sulphur and fly ash or additional materials such as microsilica.
The sulphur is the bad actor. If it catches on fire, the fumes are noxious and dangerous. It burns with an almost clear blue flame that is not always detectable in daylight so if you are breaking caps off and storing the debris outside, watch out.
When in contact with water, a weak acid develops (sulphurous acid (not sulphuric acid)) which can corrode and polute.
If you are careful, the incidents are minimal and the material is safe as well as proper for the application. Use good ventilation with the molten sulphur mix and make sure your people are trained to deal with problems as they occur. Develop and post a hazard warning for the material and maintain a fire extinguisher nearby.
