String are immutable: controls special characters are not getting replaced

string mar = "00600cam  006400276260005300340300002500393BK0000954071"
mar.Replace("", "@@@@@@");

Output: mar would be same as that of original value:
00600cam  006400276260005300340300002500393BK0000954071

string mar = "00600cam  006400276260005300340300002500393BK0000954071"
mar = mar.Replace("", "@@@@@@");

Output:
00600cam  006400276260005300340300002500393@@@@@@BK0000954071@@@@@@