T
Sounds like pu glueWhen I used NMP I followed instructions by putting a bead of their tubed adhesive in every 2mm joint between boards. Once it had set I then ran a stanley blade across to remove any excess. The adhesive is like an expanding foam, although not on that scale and is actually very strong.
It’s water “resistant” so won’t go mouldy or break downIsn't it cement board? So not waterproof? Never used it but I have seen it years ago. Looked a lot like cement board to me.