Geoscience Reference
In-Depth Information
In good-quality concrete, the brucite precipitates in the
pores at the surface forming a protective surface layer
that impedes further reaction. Some precipitated calcium
carbonate, CaCO
3
(as aragonite), arising from the reaction
of calcium hydroxide and carbon dioxide (CO
2
), may also
be present blocking the surface pores (Neville, 1995). In
addition, secondary deposits of ettringite (calcium
sulfoaluminate) may be generated by sulfate attack of
the C-S-H of the cement matrix (sulfate attack is
discussed on p. 102).
Sea water causes the development of a zone of alteration
adjacent to the outer surface, which will widen with time
provided that the surface is not removed by abrasion.
Petrographically, the zone of alteration and the
occurrence of sea water attack can be detected by the
presence of disruption and deposition of secondary
minerals (brucite, aragonite, gypsum, ettringite) within
the cement matrix and air voids. In thin section, brucite
is difficult to differentiate from calcium carbonate
minerals (calcite and aragonite) as it often occurs as
small crystallites with similar refractive indices and
birefringence (St John
et al
., 1998). Consequently, the
petrographer may only report the presence of carbonate
minerals unless the presence of brucite has been
confirmed by chemical analysis (XRD or SEM
microanalaysis). Figure
212
shows concrete that has been
exposed to sea water, having secondary deposits of
carbonate minerals in air voids. Where present, gypsum
is easily identified in thin section by its low relief and
weak birefringence (up to first-order pale grey) and
fibrous or lath-shaped crystals (
213
). It should be borne
in mind that secondary minerals such as gypsum and
ettringite are relatively soluble in sea water and are liable
to be leached away, while brucite is almost insoluble and
will remain
in situ
.
Although sea water contains alkali metals (sodium and
potassium) they are thought unlikely to cause deleterious
ASR as they are in the form of salts, rather than the alkali
hydroxides known to be involved in ASR (ASR is
discussed on p. 109).
A
TTACK BY ACIDS AND ALKALIS
Concrete may be subjected to chemical attack in
industrial and other aggressive environments (Plum &
Hammersley, 1984). Petrographic examination is able to
determine the nature and depth of damage to the
concrete and, in favourable circumstances, may provide
an approximate estimation of the rate of attack.
Acidic solutions are among the most aggressive to
Portland cement concrete and include both mineral acids,
such as sulfuric, hydrochloric, hydrofluoric, nitric,
phosphoric, and carbonic, and organic acids, such as lactic,
acetic, citric, formic, humic, and tannic. All of the cement
matrix hydrates are susceptible to acid attack but calcium
212
213
212
Concrete exposed to sea water with secondary
deposits of carbonate minerals, probably brucite and
aragonite (light brown/yellow) in air voids. Quartz fine
aggregate particles appear grey/white and the cement
matrix is dark brown; XPT, ×150.
213
Concrete suffering from sea water attack showing
extensive replacement of the cement matrix by
gypsum (grey). Quartz fine aggregate particles appear
white/grey/black and air voids are shown dark green;
XPT, ×150.
