gst rate on cenosphere

18% GST over HSN code '2621'

There is a discrepancy.  HSN Code 2621 is for fly ash also.  In this code GST is 5%.  Hence there is no definite rate as yet.  However, I feel that the rate should be 5% obly as Cenosphere is the floating part of Fly Ash.  Since Fly Ash is calculated as 5%, a part of the same item should be the same rate. This still needs clarification from the competent authority.