plastic to cast iron stack cement type

I have a cast iron shared stack runs through my flat, from this there is a cast iron branch tee with a plastic 45 bend cemented into the cast iron. I need to turn this around 180deg for my new bathroom plan, but I've been chiselling away all morning with very little effect. Does anyone know what type of cement would have been used circa 1965 I'm guessing. The plastic is a fairly tight fit into the cast iron spigot, and its very hard getting a chisel screwdriver in the gap, but what is in there would appear to be the hardest material known to man It goes dusty and flakey when it eventually comes out, a little asbestoesque I'm wondering if some acid might help soften it down a bit.. Or just keep chiselling for the rest of the week

its called prompt cement,sticks like sh1te to a blanket cut the plastic bend off then cut the plastic at 90d to the soil and prise it out,you can now chisel the cement out of the branch.you need to be very careful or the collar will split and you will be forked have a nice day sorry forgot to add it may be pc4 a very dodgey substance keep it wet and wear a proper rubber mask with particle filters NOT a cheap paper one.

all sorted cheers