Bonding / cement mix?

Hi

I hope this is the right forum, apologies if not.

I'm having a new fireplace / gas fire fitted. The new hearth is lower then the old one so the fire box base is too high. The gas fitter recommended knocking out the base of the old fire box, which I did. Yesterday he came to run in the gas pipe prior to me fitting the hearth and fireplace.

He recommended building up / sealing the base of the fire box with a bonding / cement mix. I should have asked him more about this at the time and now he's not available for a couple of days.

Anyone heard of a bonding / cement mix? What are the mix proportions? Does it contain sand?

Thanks in anticipation.

Stuart