Geoscience Reference
In-Depth Information
be supplemented by EPM and/or XRD (Eden, 2003).
A comparison of the chemical compositions of ettringite
and thaumasite follows (normalized to 100% and
excluding CO
2
and H
2
O) (
Table 24
).
In terms of optical properties, ettringite and
thaumasite are very much alike, both forming small,
needle-like crystals. The most distinguishing features of
thaumasite are that it often (but not always) has a yellow
colour (light straw yellow to bright yellow) in plane-
polarized light (
206
), rather than being colourless like
ettringite. For this reason and to avoid confusion, it is
best not to impregnate samples for TSA investigation
with fluorescent yellow-dyed resin (colourless or blue-
dyed resins are suitable alternatives). The other
distinguishing optical characteristic is that thaumasite
usually has a significantly higher birefringence than
ettringite. The interference colours of ettringite are rarely
above first-order grey, while thaumasite is typically first-
order yellow to second-order yellow (
207
).
Guidance regarding the investigation and prevention
of TSA is given in:
The Thaumasite Form of Sulfate
Attack: Risks, Diagnosis, Remedial Works and Guidance
on New Construction
(DETR, 1999).
I
NTERNAL SULFATE ATTACK
(
INCLUDING DELAYED
ETTRINGITE FORMATION
)
Internal sulfate attack on concrete is due either to the
inadvertent inclusion of materials containing sulfate, or
aggregates that can oxidize to produce sulfate or the
delayed formation of ettringite (St John
et al
., 1998). The
Table 24
Comparison of the chemical compositions of ettringite and thaumasite
Ettringite
Thaumasite
3CaO.Al
2
O
3
.3CaSO
4
.31H
2
O
CaSiO
3
.CaCO
3
.CaSO
4
.15H
2
O
SiO
2
0
19%
Al
2
O
3
15%
0
CaO
50%
55%
SO
3
36%
26%
206
207
206
Close view of thaumasite in plane-polarized light
showing a mass of needle-like crystals with straw
yellow colour; ×150.
207
Close view of thaumasite in cross-polarized light
showing a mass of needle-like crystals with first- and
second-order interference colours (grey/yellow); ×150.
