Why is my cat eating concrete and mortar?
That's very strange. Have you got a little food diary over the last few days that you could take to the vet (including how much water he is drinking)? Are his litterbox habits normal?
Has anything changed in the environment or in his diet or medicines? Is there any sort of pesticide or cleaning products that are new on the things he is licking? Could it be the flea treatment? I remember you said that your dog was licking the flea medicine off the cat's neck, so if the cat is rubbing it off on the fireplace (when he is scent-marking or maybe just rubbing to scratch his neck), maybe he is going back to lick it because it tastes good?
Other than that, my senior citizen cat goes through all kinds of weird habits. Sometimes she'll only have breakfast in bed, or dinner has to be on the patio, or maybe the kitchen, but only if you put down her favorite beach towel to sit on while she eats. When I volunteered at a nursing home, the elderly residents were ofter very picky about food, so I think my cat is about the same way.