3103
Importance of understanding the development and significance of sulphates in the
London Clay
L'importance de comprendre le développement et la signification des sulfates dans l'Argile de
Londres.
Hawkins A.B., St John T.W.
University of Bristol
ABSTRACT: 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 SO
4
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.
RÉSUMÉ : Bien que les différences dans les propriétés techniques de l'argile marron et gris de Londres soient généralement
appréciées, moins est connu au sujet des changements chimiques dans la zone non saturée/aérée saisonnièrement (tachetée). Quand la
pyrite et la calcite sont présentes, les sulfates de calcium s'accumulent à cet horizon.
Le papier décrit la formation de pyrite, ses diverses formes et l'importance de son oxydation dans la zone tachetée. Il commente les
résultats de sulfates solubles acides mesurés à intervalles verticaux de 0,2 m qui montrent une crête prononcée de SO
4
à la jonction du
marron/gris. Cependant, deux profils chimiques séparés de seulement 35m montrent une variabilité chimique importante. La
mobilisation des sulfates associées à la chaleur d'hydratation est discutée et on considère ses implications techniques, ainsi que ceux
des zones riches en sulfate et le niveau faible de pH y associé. Une référence particulière est faite à la signification pour tout béton
ou acier mis en place à cette profondeur.
KEYWORDS: sulphate, pyrite, London Clay, gypsum, sulphate attack.
1 INTRODUCTION
The London Clay Formation (LCF) is a Tertiary marine deposit
up to 150 m in thickness. The sequence is predominately
composed of silty clays with thin silt bands and is typically
sand-rich towards the base. Selenite, the euhedral form of
gypsum (calcium sulphate) is particularly common in the upper
weathered zones, whilst pyrite, present as nodules, replacement
of organic matter and framboids, is abundant in the unweathered
Clay. During deposition, the climate was warm and rivers
draining the adjacent land mass would have carried a high
proportion of organic matter. The organic content would vary
laterally and may have changed with time during deposition. As
a result of variations in sea bed terrain and sea level, the
deposits often accumulated in anoxic conditions.
2 SULPHIDES IN THE LONDON CLAY
Many dark grey/black sediments have high organic contents, the
organic matter decaying the burial process. Oxygen is
consumed by bacteria such that the sulphates are reduced to
sulphides. When ferric iron is present in these oxygen-depleted
environments, it too is reduced and reacts with the H
2
S
produced by the sulphate-reducing bacteria, ultimately to form
pyrite (iron sulphide) (Figure 1).
Pyrite is typically found in well-bedded fine grained
sediments which are dark in colour due to the high organic
(carbonaceous) content. Whilst a pyrite content of some 2-4% is
present throughout the grey LCF, it is often abundant in the
silt/sand-rich horizons near the base and where similar horizons
occur elsewhere.
Figure 1. Sedimentary pyrite formation (after Berner, 1984).
Four major forms of pyrite are present in the LCF:
a) Large crystals with varying habits (cubic, octahedral etc)
b) Groups of microcrystals which have clustered together to
form framboids up to 10-15 µm in diameter (eg Figure 2)
c) Irregular, often greenish-coloured pyrite nodules
d) Replacement of organic matter
Figure 2. Pyrite framboid and octahedra in the LCF.
3 SULPHATES IN THE LONDON CLAY
As the LCF is a randomly fissured, jointed material, its
secondary permeability is such that during past climates or de-
watering phases, the weathering extended to considerably
