Geoscience Reference
In-Depth Information
P
ETROGRAPHIC EXAMINATION AND
COMPLEMENTARY TECHNIQUES
In the absence of a specific standard procedure for
gypsum plaster, petrographic examination would
normally be performed following guidance from ASTM
C1324 (ASTM International, 2005b). Once a sample is
received in the laboratory, an initial visual and low-
power microscopical examination is conducted to
determine the number of material types or layers, layer
thickness, colour, and the relative hardness, relative bond
strength, and coherence of the material. High-power
examination of thin section specimens is then used to
determine the nature of the gypsum plaster matrix, the
quantity of impurities and additions, bond surface
characteristics, and outer surface features, and to screen
the material for evidence of distress or deterioration.
As gypsum plasters are very fine-grained, the level of
confidence in ingredient identification and contaminant
detection is greatly increased if chemical analysis is
performed in addition to optical microscopy. Suitable
techniques include XRD and SEM. The gypsum crystals
that dominate the matrix of gypsum plasters can barely
be resolved by the highest magnifications of optical
microscopy. Therefore, observation using SEM is required
for detailed studies of crystal morphology.
I
NTRODUCTION
Gypsum-based plasters are produced by heating gypsum
at relatively low temperatures (130-170°C), driving off
three-quarters of its water to form hemihydrate as
follows:
2(CaSO
4
.2H
2
O)
+
heat
2(CaSO
4
.½H
2
O) + 3H
2
O
→
Gypsum
Hemihydrate
Hemihydrate is also known as bassanite or plaster of
Paris and it is suitable for a use in a wide range of plaster
products. Heating gypsum at temperatures in excess of
400°C causes all of the water to be driven off to form
anhydrite as follows:
CaSO
4
.2H
2
O
+
heat
CaSO
4
+2H
2
O
→
Gypsum
Anhydrite
Anhydrite is restricted to use in wet plastering
applications only. Alternatively, crushing natural
anhydrite rock will produce plaster directly without
heating.
In use, water is added to the hemihydrate and/or
anhydrite, which then rehydrates to a mass of set gypsum
crystals (the reverse of the above reactions). Calcium
sulfate plasters normally set very rapidly and chemical
additives are often added to retard the setting time.
For internal plastering, a number of different bagged
gypsum products is available to cope with range of
internal surfacing applications. Premixed lightweight
plaster comprises retarded hemihydrate with a
lightweight aggregate. Retarded hemihydrate plaster is
used as finishing plaster or, with the addition of sand, as
a heavy undercoat plaster. A great deal of hemihydrate
production is consumed by production of plasterboard
(wallboard) for dry lining internal surfaces, providing a
smooth and rigid background upon which a plaster finish
can be applied. Plasterboard is hydrated plaster
compressed between two layers of strong paper.
Typical applications of petrographic examination to
investigation of gypsum plaster in service include:
• Determining the number of layers and their
thickness.
• Identifying the type and source of the plaster.
• Assessing the effectiveness of the manufacturing
process.
• Identifying the presence/type of impurities and
intentional additions.
• Identifying the causes of failures.
• Diagnosing decay mechanisms and assessing the
level of deterioration.
P
LASTER PRODUCED FROM NATURAL GYPSUM
SOURCES
Pure gypsum is a white to transparent mineral, but
impurities can cause it to have a grey, brown, or pink
tint. Occurring naturally as an evaporite mineral, gypsum
is often associated with 'red-bed' sediments and may be
contaminated by haematitic clays. In Britain, gypsum
from natural sources yields a plaster that is pink in
appearance, due to the persistence of finely disseminated
haematite. There are few constraints on the purity of
gypsum required for plaster products and standards
specify a hemihydrate content as low as 85% in the
plaster building product (although they usually contain
over 95% hemihydrate). The remainder may comprise
impurities including calcium carbonate, clay, and, more
rarely, organic matter.
