On 27 Dec, 10:52, Paul wrote:
We are in the course of some fairly substantial building works. At the
start of November, we had some drains installed across the lounge
floor. Trenches were dug, drains installed, the trench filled with
shingle, and then concrete added on top of that to a depth of about
100mm below floor level. This was then blackjacked and another layer
of cement added to build it up to floor level.

Not knowing any better, we covered this with carpet, and a week ago
carpet layers were unable to lay a new carpet because it was
(unsurprisingly) too damp. Since then we have, of course, exposed it,
but it has not dried as quickly as expected.

If we have an ordinary fan play across it all night, it appears to
dry. But when the fan is removed, within 60 minutes it is damp again.
Oddly (or perhaps not), it is bone dry at the ends. Last night we
dried it with a hair drier, and the damp doesn't appear to have come
back so fast.

It is as if we can dry the top (say) 1mm, and then the damp comes back
from underneath.

If left to its own devices, how long will it take to dry please, and
how can we speed it up substantially please?

Many thanks,

Paul.

How much water was used in the mix? I would not expect such a long
drying out time especially if the new bit is in contact with old dry
concrete.
You may have a leak in the new drain or perhaps surface water ingress
from outdoors along the run of the trench.
Give it a couple of weeks exposed with normal air movement and
temperature and if the problem remains its time to start opening up
at
the worst part to look for water.