Regarding olive waste, as you say, it can generate heat while oxidizing and, similarly, other organic materials when used as compost will reach a level of heat (as explained in Wikipedia regarding compost, that it can reach a temperature of 71 degrees), and the next part of the Mishnah indeed refers to compost.

Your question is relevant regarding salt, lime, and sand which do not oxidize. Regarding salt, there are many kinds of salt, some of which include chemicals like magnesium. This would explain why the Gemara on Daf 41 theorized that salt is "easily cooked," whereas our salt requires 801 degrees Celsius to be cooked. Therefore, it can be assumed that the salt refered to in Chazal is something like sodium acetate, or salt with high magnesium. Sodium acetate is used for making hand warmers which are [mostly disposable] packets which are held in the hand and produce heat on demand to warm cold hands.

The lime referred to here may be calcium oxide or quicklime which, when coming into contact with water, reacts vigorously and produces slaked lime releasing a large amount of heat.

As for sand, on very hot days sand can reach a heat which can almost cook inside it, which will explain the Mishnah in the previous chapter about placing an egg inside sand "so that it should be roasted." Therefore, despite being inorganic, sand can be so hot that it causes other items to heat up.

Yoel Domb