A MX-80 sodium bentonite crushed granite rock mixture is being saturated in the Äspö Hard Rock Laboratory managed by SKB (the Swedish Company of nuclear waste management) as part of the “Backfill and Plug Test Project”. The Äspö Hard Rock Laboratory is placed in the Swedish Island of Äspö, an underground full-scale laboratory where different testing construction procedures and handling techniques are being studied. The groundwater of Äspö has a variable salt concentration according to the zone considered. The added water during the mixing process of both materials had an average salt concentration of 6 g/L. However, backfill is being saturated with salt water containing higher salt content (up to 16 g/L, 50 / 50 of NaCl and CaCl2 by mass) to speed up the saturation process. The mixture swelling capacity is small due to the low backfill bentonite content, but its activity is still large if compared with natural clayey soils. A coupled hydro-chemical approach was used to simulate the backfill hydration process, comparing the results with in situ measurements. An intrinsic permeability law, depending on salt concentration in the liquid phase, was incorporated into the model. The retention curve was also determined taking into account the effect of the chemical species on backfill behaviour. The simulation of the saturation process shows the importance of studying these problems with a HC (hydro-chemical) formulation, especially if long term behaviour of such mixtures is going to be reproduced.