Borax is added to the papercrete recipe to prevent fire, mold, bacteria, and bugs.
According to 'slurryguy' from the papercreters yahoo group:
do not know of any scientific testing data to offer as a difinitive
Common practice among people using borax in papercrete has been to
add borax at a ratio of about 3% to 5% compared to the amount of dry
paper being used (by weight).