Frequently Asked Questions
Click on the links below to find answers to the most
frequently-asked questions.
- What is the difference between cement
and concrete?
- How much CO2
is associated with the manufacture of
cement?
- How much carbon dioxide
(CO2) is produced by manufacturing a tonne of
concrete?
- How does the ECO2 of
concrete compare to timber other
construction materials?
- How is Portland cement
made?
- How does concrete perform in a fire?
- What are sustainability
indicators?
- How useful are Carbon Calculators?
Although the terms are often used interchangeably, cement
is an ingredient of concrete. Concrete is a mixture of cement,
water, coarse aggregates, fine aggregates, often using
admixtures, steel reinforcement or pigments.
Cement comprises only 10 to 15 percent of the concrete mix, by
mass. Through a process called hydration, the cement combines with
the water, hardening and binding the aggregates into a rocklike
mass. In principle, this hardening process continues indefinitely,
meaning that concrete gets stronger as it gets older.
Generally, limestone or chalk together with clay, sand
and iron oxide are crushed and heated in a rotating cement kiln at
temperatures of up to 1,450oC. At this temperature
the chemically combined carbon dioxide and any water from the
raw materials are driven off and new calcium silicate
and calcium aluminate compounds are formed. This
cooled material is called clinker and forms marble
sized pellets which are finely ground with a small amount of
gypsum to produce Portland cement. For more information visit the
BCA website.
Concrete provides the best fire resistance of any building
material. It does not burn, it cannot be 'set on fire' like
other materials in a building, it has a slow rate of heat transfer
and it does not emit any toxic fumes, smoke or drip molten
particles when exposed to fire. Concrete and its mineral
constituents enjoy the highest fire resistance classification
(class A1) under EN 13501-1.
This excellent fire performance is due in the main to
concrete's constituent materials (i.e. cement and aggregates)
which, when chemically combined, form a material that is
essentially inert and has poor thermal conductivity. It is
this slow rate of heat transfer that enables concrete to act as an
effective fire shield not only between adjacent spaces but also to
protect itself from fire damage.
The only potential risk to life safety from concrete in fire
occurs in the form of spalling, which principally affects High
Performance and Ultra High Performance Concrete. Even here,
effective measures can be taken to reduce the probability of
spalling.
The concrete industry is seeking to finalise 14 framework
sustainability principles in order to give a clear and complete
picture of the industry's performance. Fewer indicators could be
appropriate in certain applications but would not demonstrate
continuous improvement across the 3 'legs'
of sustainability economic, environmental and social. The
principles also address recognised sustainability reporting formats
such as
Defra and those being developed by key Clients and contractors.
Health and Safety is included within our principles as it forms
part of the social part of the agenda which is under our direct
control. Key performance indicators (KPIs) and reporting will be
voluntary but it is expected that companies will want to
demonstrate progress and competitive benefits
to their clients and customers in a robust
fashion.
A number of "carbon calculators" have been developed for
construction industry. Most of these focus upon embodied carbon
dioxide emissions associated with the supply of construction
products to factory gate. Where the calculators are based on sound
data they provide useful tools for designers, engineers and
specifiers to develop a more sustainable built environment. The
quality of data is therefore an important consideration in the use
or design of these tools. Weaknesses in the data could be caused
by:
- Different "boundary conditions" could include or exclude potentially major impacts;
- The data could include greenhouse gases, other than carbon dioxide;
- Data on biomass materials is generally more difficult to handle and depending on the boundary conditions these figure can appear negative;
This website provides publicly available figures for the
embodied carbon dioxide levels of generic concrete mixes. These
figures are based on the most accurate generic data available from
the UK concrete industry and should provide suitable data for use
in any carbon calculator. Particularly where comparisons are made
to other materials then the quality of data needs to be fully
consistent.
Carbon dioxide is, however, only one factor to be considered
in the design process. Specifying materials with lowest carbon
footprint does not necessarily lead to a sustainable building or
development. According to the Royal Academy of Engineering
emissions in the supply chain tend to be around ten times less than
the operational emissions of carbon dioxide. The carbon calculator
listed for the National
Energy Foundation provides these operational emissions.
References and further reading
- Brunklaus, B, Baumann, H, What does an increase in building with wood materials mean in Sweden for the environment?: Review of comparative LCA studies of frame building materials in houses, The Institution for Environmental Systems Analysis, Chalmers University, Gothenburg, Sweden, 2002
- Battelle, Towards a sustainable cement industry Substudy 8: climate change, report commissioned by World Business Council for Sustainable Development, 2002
- World Resources Institute, page 57, Navigating the Numbers - Greenhouse Gas Data and International Climate Policy, 2005