Taking the heat out of bricks - unfired clay building materials

Materials World magazine
,
2 Sep 2011
"London" bricks

Dr Jonathan Oti from the University of Glamorgan
looks at the development of unfired clay building
materials for sustainable building construction.

Fired clay materials, such as
fired bricks and blocks, are
conventionally used for
mainstream building construction.
But they suffer from the rising price
of fuel as well as other related
environmental problems – high energy
usage and CO2 emissions, for instance.
The development of unfired clay building
materials for sustainable building
construction may solve these problems.
It has been proven economically viable
when environmental factors are
considered and when the unfired clay
building materials meet appropriate
performance specifications and
standards. Therefore, to meet the
increasing demand for low-cost and
environmentally responsible construction
materials while strengthening economic
growth and competitiveness, researchers
are attempting to design and develop
sustainable alternative materials,
techniques, services and processes
while maintaining the material
requirements affirmed in the standards.

Concept development

Unfired clay building material has already
been known for centuries, starting as
sun-baked brick. However, the main
deficiency of the sun-baked clay brick
is its susceptibility to water damage. This
problem is overcome by stabilising clay
soil with the addition of a small amount
of lime or Portland cement (PC), thereby
enhancing many of the engineering
properties of the soil and producing an
improved construction material. The
drawback in using stabilised lime or
PC-stabilised soil alone results in
durability problems that have been
overcome by the addition of ground
granulated blastfurnace slag (GGBS)
to a lime or PC-stabilised soil system.
This has improved many engineering
properties of the soil, including
robustness and durability. In order to
make the unfired clay building
materials more sustainable, various
waste types and by-product materials
are incorporated into the manufacturing
process. This has become necessary to
maximise the use of surplus regional
and national industrial wastes worldwide
and minimise the environmental
impact of their disposal.

It is widely acknowledged that CO2
emissions from firing clay bricks in kilns
have a direct effect on climate change, and
more changes in weather effects are
expected as climate change progresses.
Summer heatwaves make homes
uncomfortable, while more frequent
flooding and drought may expose them
and their occupants to greater risks.
To mitigate the impact of climate change,
researchers within the faculty of advanced
technology at the University of Glamorgan,
UK, have done extensive research work
on the development of unfired clay
masonry bricks, using 1.5% lime as an
activator to an industrial by-product –
GGBS – to stabilise Lower Oxford clay.
This is a very low level use of lime and is
not sufficient for most road construction
applications, where far less strength values
are needed and where three to eight
per cent lime is required for effective soil
stabilisation. Hence, the final pricing of
the unfired clay brick is expected to be
relatively low.
The project was funded by the Welsh
Assembly Government via the Sustainable
Development Branch, Department for
Environment, Sustainability and Housing
and the Collaborative Industrial Research
Programme, and the Knowledge
Exploitation Funding initiatives. The
industrial-scale brick specimens were
produced during two separate industrial
trials, the first at Hanson Brick Company
Ltd, Bedfordshire, UK, and the second
at PD Edenhall Ltd, Bridgend, South
Wales, UK. The pictures above right show freshly
produced unfired clay masonry bricks
during both trials.

Thermal properties

The table below shows the design values
for thermal conductivity (λU) and thermal
resistance (RU) for the unfired clay brick
relative to that of fired clay bricks:

 

It can
be seen that the design values for thermal
conductivity and thermal resistance of
the unfired clay masonry bricks are lower
than that observed for fired clay brick.
It is preferable that the optimum design
values for λU and RU of masonry material
is as low as possible because the thermal
values adopted for a particular material
can only comply with current building
regulations if the thermal transmittance
values are low, provided that minimum
strength and durability parameters are
also met. The design values for λU of
unfired clay-based systems is, therefore,
of great interest, and the output of the
research works could be exploited by
engineers interested in energy efficiency
design, especially when making
specifications for new buildings and
eco-friendly technologies.

Analysis for balance

A striking contrast between unfired clay
building materials and conventional fired
materials is the energy consumed during
the production process and carbon
emissions. This illustration shows energy inputs and emissions
outputs for the production process of
one tonne of unfired brick, fired brick
and sun-baked brick:

 

The total energy
usage for the unfired brick was estimated
at around 657.1MJ/tonne and the CO2
emission for the unfired brick was
estimated at 40.9 kgCO2/tonne. For
common fired bricks, the total energy
usage (input) is estimated at 4186.8
MJ/tonne with equivalent output
emissions of 202 kgCO2/tonne.

This large difference in energy usage
between the common fired brick and
the unfired brick may be attributed to
the heating to high-temperatures (900-
1,200°C) used in kilns during the firing
of conventional bricks, to give the final product the strength and durability it
requires to perform in service.

Furthermore, firing clay-based material
to such high temperatures generally
results in the release of several gases
other than carbon dioxide that further
exacerbate pollution.

Traditional sun-baked bricks tend to have
the least energy usage (525.6 MJ/tonne),
with emissions of 25.1 kgCO2/tonne. In
the past, there have been several reported
cases on the environmental performance
of new products, using various wastes and
by-product materials. However, there has
been no attempt to combine energy use
and emission to come up with one
parameter rating. This is the earliest
attempt to combine energy use and
emission for unfired clay bricks relative
to those bricks used in mainstream
construction, as steps towards the
formulation of one combined
parameter rating.

Author’s details

Dr Jonathan Oti, B.Eng, MSc, PhD, CEng, MICE, FNSE, MACE, Civil Engineering Research
Unit, Department of Engineering, Faculty of Advanced Technology, University of Glamorgan,
Pontypridd, CF37 1DL, UK Tel: +44 1443 483452 Fax: +44 1443 482169
Email: joti@glam.ac.uk