First, you never initialize the variable newtext.
Second, you're misusing strcat. From the strcat man page:
char *strcat(char *dest, const char *src);
The second parameter (src) is a const char * -- not a char. You can't change how strcat works by typecasting one of its arguments.
EDIT: And incidentally, your typecast doesn't do anything because text[i] is already
a char. I'm guessing you got a warning from the compiler and you thought that an explicit typecast might make the warning go away,but you didn't take the time to figure out what the warning was trying to tell you.