Chris Johnston
Omono
I haven't found our local haydite to have any tendency to break down. However, haydite is actually a term for lightweight aggregate used for pan decks on raised concrete decks (second floor and up). Locally, it's steam-expanded shale. Brussel Martin uses something he calls haydite that is simply a fired clay product. Other things probably serve the same purpose in other areas.
The important thing to know is that each kind of haydite has differing characteristics, with wildly varying cation exchange capacities. To continue the examples, our local haydite is less porous, taking the place of pumice in bonsai soil. Brussel's haydite is more porous, perhaps replacing lava (except that it's more porous than pumice but not porous or jagged enough to replace lava).
As to breaking down, the commonly used akadama product will give you about two good years, at which time you will need to repot with fresh soil. There is a harder version (red stripe) which will last longer.
The important thing to know is that each kind of haydite has differing characteristics, with wildly varying cation exchange capacities. To continue the examples, our local haydite is less porous, taking the place of pumice in bonsai soil. Brussel's haydite is more porous, perhaps replacing lava (except that it's more porous than pumice but not porous or jagged enough to replace lava).
As to breaking down, the commonly used akadama product will give you about two good years, at which time you will need to repot with fresh soil. There is a harder version (red stripe) which will last longer.