Adam Jonathan : Perhaps future Newby Fellow Members may fall into error from the mention in your title of Cement and a Rocket Mass Heater RMH, I Am Sure
That at some time in the past we have mentioned that All forms of Portland Based Cement Will fail if in close proximity to the Firebrick of the RMHs Combustion
Zone like for instance as a poor substitute for mortar to hold those bricks together or a poor binder for the insulating materials surrounding the Firebrick !
This mistake, and attempting to use iron / steel in place of firebrick or a refectory cast combustion Chamber is a mistake that must be watched for and stopped !
I am not trying to Highjack your post just protect your future fellow members ! Thanks

Big AL