There are only two ways to join sentences together.
1) a comma and a coordinating conjunction (for, and, nor, but, or, yet, so)
2) a semicolon.
Anything else is wrong!
Coordinating conjunction example: The crowds protested for the ouster of the president, but no one was sure it would really happen.
Semicolon Example: The vice president announced the resignation of the president; the people were elated.