Aging effects in Boom Clay

A quantitative account is given on the effect of air exposure to the Boom clay. The process responsible for this alteration is pyrite oxidation and the resulting effects are in every respect similar to results in acid sulphate soils. Complete oxidation of FeS2 (+-1.5%) generates about 4.5 meq acidity per gram, the lower pH limit of the clay being about 3-3.3. This results in a dissolution of CaCO3 (.05 - .1 mMole/g) and a progressive aluminization of the clay. In the final stage, aluminium ions take about 60-80% of the exchange capacity, displacing ions of lower valence in the process. In turn, the high sulphate contents, part of which is precipitated as jarosite, lead to the in situ formation of gypsum. The process is furthermore characterized by an increase in Mg++ content (+-.1 meq/g clay), originating from the smectite lattice displacement by ferric ions, and a decrease in exchangeable potassium, precipitated as jarosite. The overall result is an increase in ionic strength which may, under in situ conditions, amount to 400 meq/L. The degradation is also demonstrated by redox potential measurements. The overall redox capacity of unaltered samples is about 2 meq/g and the Esub(h) value at in situ conditions is around -280 mV (H2 scale). Esub(h) values rise by some 400 mV within a few hours of air exposure. Using redox (iodometric) titrations, it is shown that an oxidation corresponding to 1% of the redox capacity leads to a 200 mV increase in Esub(h).