This compound is a mix between calcium and silicon. Even though it is nowadays used as an alternative to asbestos fireproof insulation, for a stint of several decades, the product contained asbestos fibers. This was done in order to make it more durable and more resistant to extreme heat.
It is very likely for old buildings to contain this type of material, mostly surrounding pipes but also found in sheets placed in walls. Despite the claims of their prolonged durability, these asbestos-containing products would break down over time and expose the workers that would have made direct contact with them. You could have been exposed if: