Answered: NEC ECC: Material added by the Contractor to ease construction not in the Works Information

From what you have said I am assuming that the Contractor has no design responsibility and that you are also under one of the 'cost reimbursable' options, C (with target), D (with target) or E.

The application of an interlocking, or clutch, sealant is frequently used as a water retention measure, to either minimise water from entering the excavation or for ground stabilisation.  If this wasn't specified then it is unclear why the Contractor elected to use it.

If you instructed the removal of the sealant then it wouldn't be a compensation event as it isn't one of the compensation events stated in the contract (last bullet point of clause 61.4).

This matter would also be a Disallowed Cost as it is 'Plant or Material not used to Provide the Works', noting that 'Provide the Works' is in accordance with the Scope / Works Information, which doesn't specify the sealant.

It sounds like the Contractor needs to approach the contract quite differently and I would suggest giving an early warning by notifying this matter as one of the appropriate bullet points to also discuss the operation of the contract procedures.  If one party is operating the contract as intended and the other one isn't, it nearly always creates problems for both.