the best insulation to use for 4 inch underground return

d. nyzankiwsky

what is the best insulation to put around a 6 in steam pipe underground ( houses in stone tunnel) pipe is slightly coroded but will replace pipe in spring as too cold to do work now.(Below zero) Replaced pipe in return pipe (4in wide) due to large hole but this pipe will need to be replaced in spring.
Boiler is large boiler that heats 30 rm hotel/ Underground steam pipe has been there well over 50 years.