looking up the density of HDPE according to the google machine its 0.97g/cm3. so if my maths is correct a 1m2 sheet 10mm thick weighs
HDPE - 0.97 x 10.00 x 1.0 x 1.0 = 9.7kg

Hardox at 3mm is heavy.
Hardox - 7.75 x 3.0 x 1.0 x 1.0 = 23.25

*assuming my calculations are correct does this seem about right?