Geoscience Reference
In-Depth Information
hydroxide (portlandite) is attacked most readily. Limestone
aggregate will also be attacked by acid. The attacking acid
can usually be identified by the presence of the salt of the
acid deposited in the concrete. For example, sulfuric acid
reacts with calcium hydroxide in the cement matrix to
produce gypsum (calcium sulfate) as follows:
H
2
SO
4
+ Ca(OH)
2
→
For sulfuric acid attack the following zones can be
recognized in thin section:
•
Zone 1
(outer) - exposed cement paste completely
disintegrated together with any calcareous
aggregate.
•
Zone 2
(outer middle) - portlandite depleted and
gypsum deposited.
•
Zone 3
(inner middle) - ettringite formed and sulfate
attack.
•
Zone 4
(inner) - unaffected.
CaSO
4
.H
2
O
As the attack proceeds all of the cement compounds are
eventually broken down and leached away together with
any calcareous aggregate. With sulfuric acid attack there
is an additional issue with the gypsum formed by the
initial reaction then reacting with the aluminate phases
in the cement to form ettringite (calcium sulfoaluminate),
which on crystallization can cause further expansive
disruption of the concrete. Generally, for acid attack of
concrete surfaces, the following characteristic zones can
be recognized in thin section:
•
Zone 1
(outer) - exposed cement paste completely
disintegrated together with any calcareous
aggregate.
•
Zone 2
(middle) - portlandite depleted and acid salts
deposited.
•
Zone 3
(inner) - unaffected.
Another form of sulfuric acid attack is associated with
the problem of hydrogen sulfide gas in sewers. Sewage
effluent is normally alkaline and does not directly attack
concrete sewer linings. However, sewage contains sulfur
compounds that can be decomposed by anaerobic
bacteria present in sewers to release hydrogen sulfide gas
(Pomeroy, 1992). This gas is slightly heavier than air and
is absorbed by the moisture coating the sewer walls
which contain aerobic bacteria (Thiobacilli). The aerobic
bacteria oxidize the hydrogen sulfide to sulfuric and
sulfurous acids, which corrode vulnerable materials
including concrete. The acid dissolves the cement matrix
at concrete surfaces and reacts with it to produce
expansive gypsum (calcium sulfate). In hand specimen,
concrete surfaces exhibit exposed aggregate with the
remaining outer 10-20 mm of cement paste being
noticeably softened (like putty). Figure
215
shows the
microscopical appearance of sewer concrete that has
been subjected to this form of acid attack. The cement
Figure
214
shows an example of acetic acid attack of a
suspended reinforced concrete floor slab at a factory
involved in the production of cellulose acetate flake. The
photograph shows the appearance of zone 2 following
acid attack.
214
215
214
Acetic acid attack of concrete with deterioration
of the cement matrix (brown, with yellow fine cracks).
Quartz fine aggregate particles are shown white and a
dolomite coarse aggregate particle appears lower left;
PPT, ×35.
215
Sulfuric acid (from hydrogen sulfide gas) attack of
precast concrete sewer lining, showing fine cracks
filled with gypsum (white) and deteriorated cement
matrix (light brown). Quartz fine aggregate particles
appear white; PPT, ×35.
