This question has come up before, and we haven't had good answers to date, so I'm glad your question, jjoll: has gotten some attention. To keep it going, lets try to hear more from you regarding the application. How much do you have, what exactly do you have, and why do you need to solidify or make it non-flammable. Do you need to remediate some waste, or clean up an environmental spill? And do you need the substance to be useful afterward. If not, why not? These are at least moderately expensive commodities, destroying them seems wasteful -- both to the loss of the commodity, and to the effort involved.
Consider: a while back, some peanut butter made in the US was inadequately tested, and was contaminated with salmonella. Many years later, some remains of the lot had to be destroyed, because the owner just wanted it gone -- this particular part of the batch was fine, it was just too embarrassing. Trailer trucks of glass jar bottled food had to be land-filled in Mexico. This was an extreme case, but you can see how sometimes, disposal is just excessive.