134 Damage to concrete structures
will not be affected by acids or alkalis. Unfortunately, silicon dioxide can
also be present in a disordered, amorphous form. These poorly crystallised
silicas are more prone to chemical reaction. To a certain degree, silica can
be replaced by water, yielding an amorphous hydrous silica which can be
highly reactive in an alkali rich environment. Although this principle is
clear and well accepted, the detailed reaction process is probably not yet
fully understood because of its extreme complexity. Some basic principles
of the reaction process are given hereafter.
The chemistry of silica dissolution, which results in the formation of
alkali silicate solutions, is very complex (Helmuth et al. 1993). However, it
is only by understanding the dissolution of soluble silica and the formation
of swelling alkali silica gels that a good understanding of the ASR damage mechanism can be reached. This not only involves the role of alkalis
(Na and K), but also the role of calcium ions.
Probably the first comprehensive attempt to fundamentally explain the
mechanism of ASR damage was made by Powers and Steinour (1955, Parts
1 and 2). The strength of their approach was the combination of chemical
aspects with expansion studies, enabling conclusions in terms of ‘safe’ and
‘unsafe’ reactions. Based on their studies of opal rock as aggregate particles, Powers and Steinour concluded that the relative amounts of calcium
and alkalis in the reaction product determine whether swelling damage will
occur. High calcium content alkali silica gels will not cause swelling damage. In case of a lack of calcium ions, low calcium content gels are formed,
which produce considerable swelling and possible damage to the concrete.
In order to have a ‘safe’ gel formation, Powers and Steinour also concluded
that silica must be able to diffuse out from the gel, while water, calcium,
and alkalis must be able to diffuse into the gel. Figure 5.24 shows the principles of the model developed by Powers and Steinour (1955). It is redrawn
here for historical reasons, while it is clear nowadays that this model does
not duly take into consideration some important aspects, e.g. the fact that
in the presence of alkali, very little calcium will be present in the pore solution because the high pH value will substantially decrease the solubility of
Ca(OH) 2 . Nevertheless, the pioneering research by Powers and Steinour
was certainly remarkable.
A closer look at the pore solution of concrete reveals that, because of the
presence of alkalis in cement or in other constituent materials, it contains
alkali ions (Na + and K + ) and hydroxyl ions (OH − ). In principle, the Ca(OH) 2
resulting from cement hydration could also dissolve in the pore solution.
However, due to the high alkali content, the calcium is nearly insoluble.
The pore solution can thus be considered as a fluid with high concentrations of hydroxyl ions balanced by sodium and potassium ions. It is now
accepted that the hydroxyl ions initiate the chemical reaction of the silica,
while the alkali ions are only relevant when they are incorporated into the
gel (Swamy 1992, Bazant and Steffens 2000).
will not be affected by acids or alkalis. Unfortunately, silicon dioxide can
also be present in a disordered, amorphous form. These poorly crystallised
silicas are more prone to chemical reaction. To a certain degree, silica can
be replaced by water, yielding an amorphous hydrous silica which can be
highly reactive in an alkali rich environment. Although this principle is
clear and well accepted, the detailed reaction process is probably not yet
fully understood because of its extreme complexity. Some basic principles
of the reaction process are given hereafter.
The chemistry of silica dissolution, which results in the formation of
alkali silicate solutions, is very complex (Helmuth et al. 1993). However, it
is only by understanding the dissolution of soluble silica and the formation
of swelling alkali silica gels that a good understanding of the ASR damage mechanism can be reached. This not only involves the role of alkalis
(Na and K), but also the role of calcium ions.
Probably the first comprehensive attempt to fundamentally explain the
mechanism of ASR damage was made by Powers and Steinour (1955, Parts
1 and 2). The strength of their approach was the combination of chemical
aspects with expansion studies, enabling conclusions in terms of ‘safe’ and
‘unsafe’ reactions. Based on their studies of opal rock as aggregate particles, Powers and Steinour concluded that the relative amounts of calcium
and alkalis in the reaction product determine whether swelling damage will
occur. High calcium content alkali silica gels will not cause swelling damage. In case of a lack of calcium ions, low calcium content gels are formed,
which produce considerable swelling and possible damage to the concrete.
In order to have a ‘safe’ gel formation, Powers and Steinour also concluded
that silica must be able to diffuse out from the gel, while water, calcium,
and alkalis must be able to diffuse into the gel. Figure 5.24 shows the principles of the model developed by Powers and Steinour (1955). It is redrawn
here for historical reasons, while it is clear nowadays that this model does
not duly take into consideration some important aspects, e.g. the fact that
in the presence of alkali, very little calcium will be present in the pore solution because the high pH value will substantially decrease the solubility of
Ca(OH) 2 . Nevertheless, the pioneering research by Powers and Steinour
was certainly remarkable.
A closer look at the pore solution of concrete reveals that, because of the
presence of alkalis in cement or in other constituent materials, it contains
alkali ions (Na + and K + ) and hydroxyl ions (OH − ). In principle, the Ca(OH) 2
resulting from cement hydration could also dissolve in the pore solution.
However, due to the high alkali content, the calcium is nearly insoluble.
The pore solution can thus be considered as a fluid with high concentrations of hydroxyl ions balanced by sodium and potassium ions. It is now
accepted that the hydroxyl ions initiate the chemical reaction of the silica,
while the alkali ions are only relevant when they are incorporated into the
gel (Swamy 1992, Bazant and Steffens 2000).