What is this, and how can it burn down your house?
This, below, is the picture of the number one cause of house fires in Massachusetts.
I found a pile of this stuff sitting on the lawn of a house in Belmont. The pile was about two feet in diameter and about a foot high. That was more than enough to start a fire. The substance is fairly soft, a pinkish, whitish, greyish color. (the pink stuff above to the right is mulch.)
Answer in the comments. Once the right answer is posted, I will provide details.