I am going through this exact thing right now.
My hen was attacked by a fox in July. Took her to the vet and got antibiotics and since then she has had several issues (I believe due to the antibiotics killing all the good gut bacteria).
In January I noticed her butt was completely black (she’s is an orange Buff). I brought her in and she tested positive for coccidia and she had the cement stuff all in her vent. It seemed better after antibiotics and coccidia meds as well as vitamins and cider vinegar. I got lazy and stopped the cider vinegar and vitamins and it all came back. I tried to treat myself with Corid. Didn’t work. Had to bring her in again. Shes in her third round of antibiotics and I am waiting to hear back from the vet on the coccidia (this vet is super awesome $35 for visit and I do the meds at home). I noticed her vent looks worse and have been bathing her every day and the poop/cement is the same way, curled up inside her vent. If I stick a q-tip in there I can leverage some out but it’s totally stuck. I called the vet back and they said use a mixture of half white vinegar and half water and spray it in there to break up the nitrates in the poop. It did help me get a bit out gently with tweezers but it’s so irritated it started bleeding just a bit. She didn’t seem happy about the vinegar. I was really trying not to tear or force it out, just break it up a bit. She is a trooper. She has to be miserable. There is still a good bit in there.
The crazy thing is she acts normal, poops look normal, she’s laying eggs, but her vent was almost cemented close to shut and her butt is black from some kind of leakage. I have been soaking/bathing her every day since Monday and applying a silver cream they gave me. For now I bought a used crate at a thrift store and she sleeps in there and hangs out on the back porch till I figure out about the coccidia and this cement/poop/vent issue.