W j d, 7 bytes
The input format is [['a','b',..]]
¡PpǶh#⟰◘Pp½h#û Explanation
Uncompressed:
C';*'#+M M % For every item in the input: C % Convert to a character ';* % Repeat ; ASCII codepoint times '#+ % Append a # Flag:j % Join without newline ```