Bureau of Indian Standards (BIS) has classified Ordinary Portland Cement (OPC) into 3 different grades namely, OPC 33 Grade, OPC 43 Grade and OPC 53 Grade cements. The grade number indicates the minimum compressive strength that the cement is required to attain at the end of 28 days eg., the minimum compressive strength of 53 Grade OPC attained on the 28th day shall not be less than 53 MPa or 530 kg/sqcm. It may be noted that OPC 33, OPC 43 and OPC 53 grades do not differ in chemical content. The only difference is that the higher-grade cements are ground much finer during the final grinding process, creating a product that is much stronger and more durable than the less finely ground cement. Hence only Grade 63 NOT a grade of cement according to the classification of Ordinary Portland Cement (OPC) by Bureau of Indian Standards (BIS).