What is the difference between 43 and 53 grades of cement?

The OPC was classified into three grades, namely 33 grade, 43 grade and53 grade depending upon the strength of the cement at 28 days when tested as per IS 4031-1988. If the 28 days strength is not less than 33N/mm2, it is called 33 grade cement, if thestrength is not less than 43N/mm2, it is called 43 grade cement, and if the strength is not lessthen 53 N/mm2, it is called 53 grade cement.