Using just cement to plug a narrow channel/hole
Portland cement mixed alone (called &quot;slurry&quot;) will result in a brittle and soft compound. It may fill the void you're seeking to eliminate, but not with ideal results. It may break up and wear away faster than actual concrete. A 6&quot; channel is more than &quot;just a gap&quot;. It's a substantial structure in the end. I wouldn't pour less than 3&quot; thick for strength or you'll have a broken surface in no time.

A typical concrete mix contains 60 to 80 percent sand and gravel, also
known as an &quot;aggregate.&quot; This aggregate compound is more than filler.
It plays an important part in the concrete's composition. The amount
of sand and gravel in a bag of concrete determines the mixture's
strength and texture. In fact, when you remove sand and gravel from
the concrete mixture, it becomes a completely different product.
Sand and gravel in concrete serve several purposes. Because they act
as a filler, they also add more volume to the concrete. More volume
means less air and a stronger product. The size of the gravel also
helps to determine the concrete's strength. Though larger pieces of
gravel produce more friction and make it harder to mix, they also make
a stronger concrete.
source

The bottom line: Find some sand or gravel.  It can't be that rare almost anywhere.