Calcium Silicate Bricks

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16 Structural Survey
Introduction
The calcium silicate brick was
invented and patented in Britain in
1866 but, like other British inventions,
it was first developed commercially
overseas, in this case Germany,
towards the end of the nineteenth
century and not until the beginning of
the present century was it set going in
the UK.
In Germany, Holland and the former
USSR, where sand is widely
distributed, this class of brick became a
major structural building material, so
much so that in Holland there were
laws enacted to protect the clay brick.
The limited availability of suitable fine
silica sand and competition of cheaply
produced types of clay brick may have
restricted the size of the industry in
Britain which, nevertheless, soon
managed to exhibit a marked increase
over the years. Even today, the calcium
silicate brick is dominant in Holland
and Germany and used extensively
throughout Europe.
In 1905 in the UK for example,
there was one producer thought to have
been able to manufacture perhaps
4,000,000 bricks per annum and by
1932 there was an increase to some 15
works making in the region of
100,000,000 bricks per annum. By
1964, 25 works turned out in the region
of 350,000,000 bricks. Present
production capacity is in excess of
150,000,000 per annum, with sales
given at around 50,000,000 a year.
Today the industry has rationalized
and reorganized. Instead of numerous
small manufacturers there are four
major producers situated in Kent,
Dorset, Cumbria and Nottinghamshire.
Process of Manufacture
The process of manufacture consists
essentially of an intimate bonding
together of an inherently strong
aggregate (sand or crushed siliceous, or
flintstone, or a mixture of both) by
hydrated calcium silicates, which are
synthesized as part of the process by
the action of high-pressure steam. The
component raw materials, aggregate,
lime and water, together with stable
and inert pigments added as required,
are mixed and accurately pressed under
very high mechanical pressures to give
the required degree of compaction into
the shape and size of brick required.
The bricks are then subjected to
high-pressure steam in an autoclave,
during which the lime combines with
part of the sand to form calcium
silicate. As soon as the bricks are
cooled they are ready for immediate
use.
Durability and Precision
The durability of calcium silicate
bricks of the appropriate quality for the
job under all normal conditions is
good, as any trip around the
countryside will show, and, although
adequate experience is available only
over a period of some 80-100 years, the
records of behaviour when combined
with extensive test results are more
than adequate to indicate the quality
requirement necessary for a
satisfactory life under most conditions
of use.
The technical properties of a
calcium silicate brick must depend on
the chemical and physical nature of the
bonding agent, the proportion of
bonding agent, the properties of the
aggregate (sand or crushed siliceous
stone) and the physical structure of the
hardened body – with the structure for
the most part largely dependent on
particle size distribution and particle
shape of the aggregate, together with
the effectiveness with which particles
have been packed together and pushed
into contact.
Since the bricks are pressed in
accurately-shaped moulds and there is
no measurable change in dimension
during the hardening process, the
shape, strength and size can be
maintained very accurately to the
permitted small tolerances.
Most building materials are apt to
change slightly in dimension and a
great many (excepting metal and glass)
change between wet and dry
conditions. Although changes may be
slight, this can cause cracking and
deterioration so that it is necessary to
provide for this in the design of the
building and the techniques of use,
with care taken for movement
provision, particularly between
materials with different characteristics.
Attributes
Knowledge over the years, together
with modification in order to obtain
results and quality, has been helped by
research and development in the
industry, encouraged and sponsored by
its own Association and aided,
originally, by the Building Research
Station and, now, by British Ceramic
Research Ltd whose leading authority
is Dr G.J. Edgell. This has meant that
much better quality and more reliable
bricks are now produced than in the
early years of the industry.
Calcium Silicate
Bricks
Bryan Bowley
Vol. 12 No. 6, 1993/4, pp. 16-18, MCB University Press, 0263-080X
Volume 12 Number 6, 1993/4 17
The durability of calcium silicate
bricks shows their resistance to
damage. Cycles of wetting and drying
carried out alternately do not have an
adverse effect, and repeated freezing
and thawing have demonstrated that
the bricks are highly resistant to
damage, added to which the
efflorescence caused by the
crystallization of soluble salts does not
normally occur with the bricks unless
contaminated from external sources.
Interestingly, at the present time
when pollution is much discussed, it
has been found that atmospheric
carbon dioxide acts slowly on calcium
silicate bricks and their strength and
hardness have been found to increase
as a result of the changes.
There are some areas in which it is
advisable not to use them, for instance
exposure to strong acid fumes, but they
are considered unaffected by chlorine
atmospheres and in such places as
swimming-pool buildings. However,
where brickwork is continually
splashed or in constant contact with
water, the application of a silicone
solution will avoid discoloration and
advice on this can easily be obtained;
however, use in contact with pure
acids, sea and salt water or continually
chlorinated water is not recommended;
so, in unusual circumstances, the
manufacturer should always be
consulted.
The texture of the bricks is normally
dependent on the grading of the
aggregate. A fine sand used alone gives
a fine textured smooth brick, while
crushed stone or gravel with some
deficiency of “fines” gives a coarse
texture with rather rougher arrises.
Mottled texture may be produced by
using a proportion of dark coarse
material, such as crushed stone and
slag, in company with light-coloured
sand.
Manufacture
Essentially the original bricks were
white, or rather near white, and, if one
travelled around the south of England
during the 1930s, it was possible to see
houses and bungalows sprouting up all
over the place, with their white walls
built by this inexpensive brick, a great
many of which were then produced in
the Sussex area by Midhurst Whites – a
firm which started in 1913 based at
Midhurst in Sussex, having its own
lime works nearby on the Sussex
Downs but sadly no longer producing
bricks. Nonetheless, in spite of
closures and after rationalization of the
industry, still busy producing are
Ryarsh Brick, Mansfield Brick, Beacon
Hill Brick and Esk Manufacturing.
The natural calcium silicate bricks in
the near white have a high diffuse light
reflection coefficient which makes
them very advantageous to use for
some internal facing work, but a
considerable range of coloured bricks
are available and used for facing work,
particularly in Britain, the latter being
obtained by the incorporation of
pigments of natural and artificial
mineral type. These include ochres
(buff or cream colours), iron oxides
(pink, red, brown or black), chrome
oxide (green), which can be expensive,
to name but a few; however, the
pigment has to be controlled carefully
to give acceptable and consistent
results.
Thermal Conductivity
Thermal conductivity of calcium
silicate bricks is not significantly
different from that of other building
bricks or blocks of comparable density.
But fire resistance of the bricks is good
as demonstrated by standard tests in
various countriesand by practical
experience up to the present time. This
resistance to fire was particularly noted
in Europe during the Second World
War.
Thermal expansion of calcium
silicate bricks is small but can be
somewhat more than the average clay
brick and therefore more attention has
to be paid to this in the UK than in
countries with a less variable climate;
calcium silicate bricks are more
inclined to shrinkage than expansion.
However, this having been said,
attention ought to be drawn to the fact
that in no way do the occurrences
mentioned above inhibit the continual
extensive use of the calcium silicate
brick throughout divers European
countries.
General
Adhesion of mortars and gypsum
plasters is satisfactory, with the surface
suction moderate rather than high, and
the absence of soluble salts eliminates
risk of expansion trouble with Portland
cement base mixes and also risk of
efflorescence of salts from brickwork
penetrating the surface of the plaster.
Any type of paint finish may be
applied, the limitation in this respect
being the mortar of the joints which
can affect some types of paint, so, as
with clay bricks, alkali-resistant-type
paints should be used. The smooth face
and accurate shape make it easy to
apply a painted surface. For dwelling
and larger housing projects calcium
silicate bricks provide the architect and
builder with excellent opportunities to
use, at an economical cost, a material
of controlled strength with an
extremely pleasing appearance and
proven durability.
From time to time, the question has
been asked, why has the calcium
silicate brick in the UK not reached the
usage level in relation to clay bricks
found on the Continent?
It may, of course, be just a question
of dynamic marketing or perhaps a
native architectural prejudice, but then
again it might be the resultant hangover
from the early days when the clay brick
producers naturally resisted the
emergence of a cheaper competitive
product; or was it something inherent
in the early bricks which has long since
been eliminated?
Decline in production during the
1930s was very apparent as a result
perhaps of the 1929-32 slump and the
subsequent development in the
concrete block industry; moreover it
should not be dismissed from one’s
mind that much of the plant used in the
production of calcium silicate bricks
was easily convertible to the
manufacture of concrete blocks and in
the USSR, who were one of the biggest
producers, a decline took place in
favour of large industrial units of
autoclaved calcium silicate material.
Then again closure of production
units is not always an aid to popularity.
It has been said that investing funds
do not favour buildings constructed
with calcium silicate bricks because of
the high coefficient of expansion
leading to cracking, but is this a valid
criticism of the brick? Alternatively,
after proper evaluation, might a finger
conceivably be pointed at the builders,
and questions asked concerning the
way materials are, or are not, used in
the erection process?
The code of practice recommended
for calcium silicate bricks should
always be followed; in particular, BS
5628 part 3 should be consulted and the
mortar mix recommended by the
manufacturers followed. The correct
specification is essential.
Developing Technology
It is an industry which progressively
looks to the future and utilizes
continuous research, being well geared
to take advantage of any upsurge in the
economy.
Members of the Calcium Silicate
Brick Association (CSBA) conduct
continuous programmes of product
development on an individual
company basis and collectively
maintain programmes of research into
structural and environmental
performance of calcium silicate bricks
at the British Ceramic Research
Laboratories from which up-to-date
information should be obtained. Added
to this, membership of the Calcium
Silicate Products Association
encourages an exchange of technical
information to meet the challenge of
the European single market.
Conclusion
Here is a brick that is load-bearing
appropriate to the strength for use both
externally and internally, above and
below ground, with a range of textures
produced to accurate UK brick size but
also in some Continental sizes
supported by the British Board of
Agrément. Many special shapes can be
obtained with the choice of some
several dozen or more different colours
in various textures ranging from white
and buff to lilac and green having
satisfactory resistance to the influence
of chlorides, organic growths, coastal
environments, sulphates and pollutive
elements with the assertion that they
are colour-fast. One particular
manufacturer advertises its colour co-
ordinated design service as having the
ability to offer “virtually limitless
variety of coloured bricks going
beyond its 40 standard colours”.
Further Reading
Churchill, W.M., “Site Surveys of
Movement in Calcium Silicate
Brickwork”, BCRA Technical Note 327.
Edgell, G.J., “The Use of Bed Joint
Reinforcement to Enhance the Lateral
Load Resistance of a Calcium Silicate
Brickwork Wall”, BCRL Special
Publication 117.
Edgell, G.J., Goodwin, J.F. and Saunders,
J.D., “The Design and Use of Dowels to
Transmit Bending Movements across
Movement Joints in Calcium Silicate
Brickwork”, BCRL Research Paper
784.
Goodwin, J.F., “The Performance of
Calcium Silicate Brickwork in High
Sulphate Environment”, BCRA
Technical Note 368.
Goodwin, J.F. and Saunders, J.D.,
“Investigation of Bond between
Calcium Silicate Bricks and Mortar”,
BCRL Special Publication 126.
Hodgkinson, H.R., Goodwin, J.F., West,
H.W.H. and Haseltine, B.A., “The
Lateral Performance of Calcium Silicate
Brick Walls with Varying Amounts of
Reinforcement”, BCRA Technical Note
344.
Tellet, J. and Edgell, G.J., “The Shear
Strength of Reinforced Calcium Silicate
Brickwork Beams”, BCRA Technical
Note 332.
West, H.W.H., Hodgkinson, H.R., Beech,
D.G. and Goodwin, J.F., “The
Compressive Strength of Calcium
Silicate Brick Walls under Axial
Loading”, BCRA Technical Note 262.
West, H.W.H., Hodgkinson, H.R.,
Goodwin, J.F. and Haseltine, B.A., “The
Resistance to Lateral Walls Built of
Calcium Silicate Bricks”, BCRA
Technical Note 288.
Above obtainable from: British Ceramic
Research Ltd, Queens Road, Pekhull,
Stoke-on-Trent ST7 4LQ.
Further information and technical data
may be obtained from: Calcium Silicate
Brick Association, 24 Fearnley Road,
Welwyn Garden City, Hertfordshire AL8
6HW.
Appendix: Classifications of Calcium
Silicate Bricks
Calcium silicate bricks are designated by
BS 187: 1978, according to their
compressive strength and appearance, into
the classes shown in Table AI. Use and
appearance are categorized as follows:
l Use: brickwork that will be heavily
loaded, repeatedly exposed to frost
while in a saturated condition or
continuously wet (e.g. below dpc).
l Appearance: bricks specified for
facing purposes to be of agreed colour
and texture, with the arrises of one
stretcher face and one header face
reasonably free from damage. The use
of reference panels is recommended
for all significant projects.
Where a manufacturer operates a quality
control scheme to meet the “Special
Category” requirements of BS 5628 part 1:
1978, the predicted lower limits of samples
drawn from production will be consistently
above the minimum mean strength
specified for the class of brick concerned.
The minimum classification of calcium
silicate bricks manufactured by members of
the CSBA is Class 3.
18 Structural Survey
Minimum predicted
Crushing lower limit of
Strength crushing strengtha
Class (N/mm2) (N/mm2)
7 48.5 40.5
6 41.5 34.5
5 34.5 28.0
4 27.5 21.5
3 20.5 15.5
Source: CSBA technical data
a If a further sample of ten bricks was
taken, the probability that the mean
would be below this limit is
approximately1 in 40
Table AI. BS 187: 1978 Classes of Calcium
Silicate Bricks
Bryan Bowley is a retired Managing
Director of George Jackson & Sons Ltd,
and is currently involved in restoration and
allied subjects and writing on various
aspects of same.
19 Structural Survey