After a few hours failing to get the electric water heater shell to work as an outer chimney we decided to pick up an 18″ diameter skacrete cast.
Next, mix up some refractory cement to a cookie dough consistency and fill the void.
Perlite Refractory Cement Mix Recipe:
- 1.5 parts Portland Cement
- 2 parts sand
- 1.5 parts Perlite
- 2 parts fireclay
source: Homemade Refractories