Thanks again to Harry for this perspective. It has been done for hundreds of years, and seems to be a Good Thing, where it is available. You cook the cactus yourself. This is an interesting possibility that uses an industrial waste product that already exists, all over the country, and is possibly the best quality lime you can get for plaster. Explain the use you have for the lime.
Most people are happy to help.
A few states such as California have outlawed the process of making backyard acetylene because of the leftover caustic waste situation. By developing a beneficial use for the carbide lime residue, we coud solve an environmental problem that would suit everybody.