It happened for the second time that this compost got entombed in dirt. I assume it is fertilizer turned into dirt by temperature since the compost is 126 C and the "melting point" of fertilizer where it turns to dirt is 125 C. Note the compost is made of abyssalite. It was not intentional but the game offered it as default material at the time I was building it and I did not care. I would guess the compost will continue to produce Sand when it gets to high enough temperature.
I believe the temperature of the compost's product should be capped below its melting point. Maybe even the heat generation should be reconsidered for abyssalite buildings, or perhaps availability of abyssalite for buildings that produce "normal operation" heat?
Build a compost out of abyssalite. Set its temperature high enough and watch it produce.