Importance of understanding the development and significance of sulphates in the London Clay

Although the differences in the engineering properties of the brown and grey London Clay are generally appreciated, less is known about the chemical changes in the unsaturated/seasonally aerated (mottled) zone. With pyrite and calcite present, calcium sulphates accumulate at this horizon.
The paper describes the formation of pyrite, its various forms and the significance of its oxidation in the mottled zone. It comments on the acid soluble sulphate results measured at 0.2 m vertical intervals which show a pronounced SO4 peak at the brown/grey junction. However, two chemical profiles only 35 m apart illustrate a pronounced variability in chemistry. The mobilization of sulphates associated with the heat of hydration is discussed and the engineering implications of this, the sulphate-rich zones and the associated
low pH are considered with particular reference to the significance for any concrete/steel emplaced at this depth.