Geoscience Reference
In-Depth Information
Surface crazing is a pattern of fine cracking seen on the
outer surface that is caused by drying shrinkage of a wet
mix or imperfect curing techniques. Surface crazing is
liable to become more visible with time as the fine cracks
fill with dark dirt that contrasts with the typically light
colour of the unit. Crazed units will carbonate faster and
increase the risk of corrosion damage in reinforced units.
In unreinforced units, crazing is less of a durability issue
but may cause complaints for aesthetic reasons. By
embedding steel reinforcement into cast stone the
manufacturer builds in a latent defect that would not be
present in natural stone. Carbonation-induced corrosion
of embedded steel reinforcement is a common cause of
cracking and spalling of cast stone during service.
Reinforcement placed within porous facing mix layers,
or at the interface between facing and backing mix layers
is particularly susceptible to corrosion.
×100 magnification. Owing to the porous nature of
aircrete, the matrix would be expected to carbonate fairly
rapidly during service. Figures
243-245
show an aircrete
masonry block comprising aerated Portland cement with
a filler of PFA.
The petrographer may be called upon to investigate
the following problems that are sometimes associated
with aircrete products. Aircrete masonry blocks may
exhibit excessive swelling and shrinkage (unsoundness)
if unstable ingredients are included in the mix. Of
particular concern is PFA filler that can potentially
contain excessively high contents of unburnt coal (BS
EN 450 specifies a loss on ignition of )7%).
Concerns have been raised regarding the in-service
performance of RAAC panels designed before 1980. There
have been many instances where they have exhibited
excessive deflection (sagging), with associated tension
cracking sometimes visible on the soffit face (Building
Research Establishment, 2002b). Excessive voidage
around the reinforcement and/or reinforcement corrosion
may contribute to deflection. Reinforcement used in
RAAC panels must have a protective coating as the
porous aircrete provides insufficient alkaline protection.
If the protective coating is inadequate or fails, the unit is
vulnerable to reinforcement corrosion, especially in
damp environments.
Aircrete, also known as autoclaved aerated concrete
(AAC), is commonly used for the manufacture of mass-
produced lightweight concrete blocks. In addition, it is
produced as panels with steel reinforcement added, to
improve flexural and shear capacity (reinforced
autoclaved aerated concrete, RAAC). Aircrete is an
aggregate-free, aerated mortar with low density and good
insulating properties. It is manufactured by foaming a
slurry of Portland cement (or lime) mixed with siliceous
filler, such as PFA, GGBS, or silica flour. Chemical
admixtures and inert pigments are frequently added. The
aeration is commonly achieved chemically by adding
aluminium powder to the wet slurry. This reacts with the
alkaline lime (calcium hydroxide) within the cement to
produce bubbles of hydrogen gas as shown in the
following equation:
CaOH
2(s)
+ 2Al
(s)
+ 2H
2
O
(l)
→
Calcium silicate units are manufactured from fine
siliceous aggregate (and sometimes silica flour) and 10-
20% ground quicklime or well hydrated lime. Inert
pigments may be included to influence the final brick
colour. The units are shaped into moulds under pressure
and cured by autoclaving at around 170°C for between 4
and 16 hours. During curing some of the lime reacts with
the siliceous aggregate to form calcium silicate hydrate,
which binds the brick together.
The term 'sandlime' is used to describe calcium silicate
bricks in which only natural quartz sand comprises the
siliceous portion of the raw ingredients. 'Flintlime' bricks
include a substantial proportion of crushed flint (Building
Research Establishment, 1992).
In thin section, the appearance of calcium silicate brick
is dominated by the presence of unreacted siliceous
aggregate particles. New bricks exhibit a binder of calcium
silicate hydrates that appear isotropic. Old bricks tend to
be partially or fully carbonated with the binder including,
or composed of, finely crystalline calcium carbonate.
Calcium silicate bricks typically appear fully compacted
with no large entrapped air voids present. The appearance
of a sandlime brick is illustrated in Figures
246-248
, and
CaAl
2
O
4(s)
+ 3H
2(g)
Alternatively, aeration may be achieved by mechanical
means such as entraining air by a whipping process, or
by adding preformed stable foam to the slurry. The slurry
is poured into a steel mould where it sets to form a weak
'cake', which is cured for several hours at elevated
temperature before demoulding, trimming, and final
curing in an autoclave.
In thin section, the texture of aircrete is dominated by
air voids, which may comprise as much as 80% of the
total volume (Building Research Establishment, 2002b).
Air voids are typically spherical and less than 2 mm in
diameter. The materials of the hardened slurry matrix are
finely divided and abundant remnants of unhydrated
cement grains and filler should be visible at greater than
