You come upon a network of holes and you want to fill them in, but it is a tight network that you can't go into, and you don't want to waste your new Fortune III pick digging into that spot just so you can neaten it up.
So what are you going to do?
Why mix up a batch of cement of course!
Go to your crafting table and mix together:
1 bucket of water
1 item I haven't figured out yet. (slimeball? bonemeal?)
That will give you 1 bucket of cement.
Using the cement (the bucket will be returned) will create a cement source block. This source block will create a flow of cement moving at roughly the same rate as lava, but after 30 seconds the source block and all blocks of flowing cement will harden into concrete blocks.
Your hole is now filled in without you having to dig out all those odd blocks that were in your way.
Concrete is not as strong as regular stone, and returns nothing when mined, unless silk touch is used, but it can be a useful item for building or filling in inconvenient holes.
Oh yeah. It also finally creates a use for all that useless gravel that has been piling up in your chests.
I'm pretty sure this has been suggested before... Volcanic Ash would be ideal for the last ingredient to use as the bonding agent, however, I agree with Syeonyx, that clay would be a reasonable substitute that is already available in the game.
There would have to be a limit as to how many blocks 1 use would 'fill' (maybe a 13 block limit from the flow? up to 2 in each direction + the 1 block center and diagonals on a flat surface)... or just a flat 3x3 square surface area (9 blocks).
As to using flint... gravel acts just fine for the aggregates IMO, I don't think requiring flint as part of the recipe is necessary.
Maybe offer an alternate recipe to add a steel ingot to make reinforced concrete which is harder to break with anything less than a steel pickaxe and is a bit more resistant than stone when mining.