Hello Mahmood,
The material that can be used between 2 walls must be any joint expansion fillers includes low viscosity epoxy resins, silicone gels, and cork and woods such as southern yellow pine, redwood, or western cedar.
Epoxy and silicone sealants also offer excellent moisture, dirt, and plant exclusion protection.
Most liquid or gel sealants are self-leveling to ensure a flat, blemish-free finish and may be painted or colored to match the rest of the structure.