You face a series of threats that come in waves.
Immediately, there's the problem of making the mold properly. Just pouring concrete over someone won't work. They move. They breathe. This presents issues. If the mold sets improperly, your victim cannot breathe, as others have pointed out.
Wet concrete is an alkali, so your victims are being burned as the concrete cures as others have pointed out.
Concrete contains sand and often small rock particles. These will cut your victim as they sit there, entombed for days.
Concrete is exothermic while curing. This paper points out that wet concrete poured at 20c (68f) will rise to 35c (95f) at it's peak.
So your victim is suffering from burns, abrasions, and temperature differentials while the cement cures.
Your villain must spend a great deal of time setting up the victim properly. An IV must be introduced to provide fluids and nutrients. Some kind of exit path must exist for fluids and solids.
Air tubes must be provided. Or better yet, a mask, to prevent cement from entering the mouth or nose.
If the face isn't protected, you now have cement blocking your ears and destroying your eyes. Without a full head shield, your victim is probably blind and possibly deaf after the ordeal.
Now you face hypothermia once the cement cures. (Ever sit on a cold concrete floor? Yeah, that, but now it's your whole body.)
And your victim will be put through high levels of pain as their muscles spasm and joints suffer under the physical strain.
And that's just the first day or two.
You also face high risk of infection from all of the cuts and burns and "bed" sores suffered up to this point.
Then there are the mental issues. You've just put someone in a sensory deprivation tank that they cannot escape and cannot even move inside of. They are at higher risk of hallucinations. The link also describes someone who:
For 1,307 days, Padilla was kept in a 9' x 7' cell with no natural
light, clock or calendar. When Padilla left his cell, he was shackled
and fitted with heavy goggles and headphones. His counsel argues that
while he was being interrogated Padilla was subjected to harsh lights
and pounding sounds. While meeting with his counsel, they reported
Padilla exhibiting facial tics, random eye movements and unusual
contortions of his body. According to them, Padilla had become so
"shattered" that he became convinced his lawyers were part of a
continuing interrogation program and saw his captors as protectors.
So your victim is being mentally and physically destroyed.
Muscle atrophy can begin to set in within days.
Disuse of the muscles, such as when muscle tissue is immobilized for even a few days of unuse – when the patient has a primary injury such as an immobilized broken bone (set in a cast or immobilized in traction), for example – will also lead rapidly to disuse atrophy.1
And your victim is at higher risk of blood clots.