durability in a non-aggressive environment. However, concrete can be attacked by sulphates
and sulphuric acid occurring naturally in soils, by corrosive chemicals which may be present
in industrial waste in fill materials, and by organic acids and carbon dioxide present in
groundwater as a result of decaying vegetable matter(10.6). Attack by sulphates is a disruptive
process whereas the action of organic acids or dissolved carbon dioxide is one of leaching.
Attack by sulphuric acid combines features of both processes.
The naturally occurring sulphates in soils are those of calcium, magnesium, sodium and
potassium. The basic mechanism of attack by sulphates in the ground is a reaction with
hydrated calcium aluminate in the cement paste to form calcium sulphoaluminate. The
reaction is accompanied by an increase in molecular volume of the minerals, resulting in the
expansion and finally the disintegration of the hardened concrete. Other reactions can also
occur, and in the case of magnesium sulphate, which is one of the most aggressive of the
naturally occurring sulphates, the magnesium ions attack the silicate minerals in the cement
in addition to the sulphate reaction. Ammonium sulphate, which attacks Portland cement
very severely, does not occur naturally. However, it is used as a fertilizer and may enter
the ground in quite significant concentrations, particularly in storage areas on farms or in
the factories producing the fertilizer. Ammonium sulphate is also a by-product of coal-gas
production and it can be found on sites of abandoned gasworks. Because calcium sulphate is
relatively insoluble in water, it cannot be present in sufficiently high concentrations to cause
severe attack. However, other soluble sulphates can exist in concentrations that are much
higher than that possible with calcium sulphate. This is particularly the case where there is a
fluctuating water table or flow of groundwater across a sloping site. The flow of groundwater
brings fresh sulphates to continue and accelerate the chemical reaction. High concentrations
of sulphates can occur in some peats and within the root mass of well-grown trees and
hedgerows due to the movement and subsequent evaporation of sulphate-bearing ground-
water drawn from the surrounding ground by root-action. The severity of attack by soluble
sulphates must be assessed by determining the soluble sulphate content and the proportions
of the various cations present in an aqueous extract of the soil. These determinations must be
made in all cases where the concentration of sulphate in a soil sample exceeds 0.5%.
The thaumasite form of sulphate attack which consumes the binding calcium silicate
hydrates in Portland cement, thereby weakening the concrete, has been investigated exten-
sively in recent years(10.7). The reaction requires the presence of sulphates, calcium silicate,
carbonate, and water, and appears to be more vigorous at temperatures below 15 C.
Carbonation of concrete due to atmospheric carbon dioxide acting on the calcium hydroxide
in the concrete matrix causes a reduction in the pH rendering the concrete susceptible to
sulphate reactions forming thaumasite.
Free sulphuric acid may be formed in natural soil or groundwater as a result of the
oxidation of pyrites in some peats, or in ironstone or alum shales. Sulphuric acid can also
be present in industrial waste materials which have been contaminated by leakages from
copper and zinc smelting works, and from dyeing processes. The acid has an effect on the
cement in hardened concrete that is similar to that of sulphate attack, but the degradation
may not result in significant expansion. Figure 10.3 shows the disintegration of the concrete
in the shaft of a bored and cast-in-place pile caused by the seepage of sulphuric acid into
porous fill material.
In the UK, sulphates occurring naturally in soils are generally confined to the Keuper
Marl (Mercia Mudstone), and to the Lias, London, Oxford, Kimmeridge and Weald Clays.
They are also found in glacial drift associated with these formations. Sulphates may be
present in the form of gypsum plaster in brick rubble fill.
The durability of piled foundations 487