Passing Variable To SqlCommand

 Can't seem to pass a variable to the sql statement. I'd appreciate any help.
 
I'm trying to pass pColName to  CommandText = "ALTER TABLE tb_roomInfo ADD @rColName  varchar(50);";
Doesn't seem to work though.
 
CODE: 
 [WebMethod]
    public string addCol(string pColName)
    {
     
     
        SqlConnection cnn = new SqlConnection(connString);
        try
        {
           
            cnn.Open();
            SqlCommand cmd = new SqlCommand();
            cmd.Connection = cnn;


          
            cmd.CommandText = "ALTER TABLE tb_roomInfo ADD @rColName  varchar(50);";

            SqlParameter rColName = new SqlParameter("@rColName", pColName);

            cmd.Parameters.Add(rColName);

            int i = cmd.ExecuteNonQuery();
            cnn.Close();
            return "Insert Successful";
        }
        catch
        {
           
            return "Insert Unsuccessful";
        }
    }