Thermalite and joists

Hi
I am putting in floor joists. The problem I have is my walls in the garage have been rendered, and it is thermalite behind the render. How can I fix a wall plate (to hang the joists off) to the wall. I usually use thunderbolts but thermalite is obviously soft and surely it would crumble? I was thinking maybe chemfixing?
What would be best?
Thanks