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The concrete
conundrum
Concrete is the single most widely used material in the world – and it has a carbon
footprint to match. James Mitchell Crow looks at some of the approaches being
used to ease the material’s environmental impact
62 | Chemistry World | March 2008 www.chemistryworld.org
but also for rather more glamorous
In short
projects. The Burj Dubai skyscraper,
Concrete production still under construction but already
contributes 5 per cent well over half a kilometre high – the
of annual anthropogenic final height remains secret, but the
global CO2 production, building is set to dwarf all other
mainly because such vast man-made structures – relies on a
quantities are used highly flowable concrete mixture
Humans have used that doesn’t harden before it can
concrete for millennia – be pumped to the top of the tower,
its basic ingredients date yet forms a strong and robust final
back to ancient Egypt product. And Japan’s construction
CO2 is a product of the industry has pioneered ultra-
main reaction that makes strength varieties from which to
cement – concrete’s key build its earthquake-proof bridges,
ingredient and the Tokyo apartment blocks that
Development of new form some of the most expensive
concrete additives could real estate in the world.
produce a stronger, more Humans have been using concrete
workable material whilst in their pioneering architectural
reducing the amount of feats for millennia. The basic
cement required and the ingredients – sand and gravel
resulting CO2 emissions (aggregate), a cement-like binder,
and water – were being mixed at
least as far back as Egyptian times.
The Romans are well-documented
masters of the material, using it to
create such wonders as the Pantheon
in Rome, topped with its gravity-
defying 43.3-metre-diameter
concrete dome: now over 2000 years
old but still the world’s largest non-
reinforced concrete dome.
With the loss of Roman concrete
expertise as the Empire fell into
decline, concrete’s secrets didn’t
re-emerge until just 200 years ago.
Modern concrete was born in the
early nineteenth century, with the
discovery of Portland cement, the
key ingredient used in concretes
today. The process of roasting, and
then grinding to a powder, limestone
and clay to make ‘artificial stone’ was
patented in 1824 by Joseph Aspidin
of Leeds, UK, and later refined by
Already 605 metres high, his son William into a material very
the Burj skyscraper in close to the cement used today.
Dubai will be the world’s The main reaction occurring in
tallest building Aspidin’s kiln was the formation
of calcium silicates, from calcium
Concrete has a problem. Already concrete is because it is in fact a very carbonate (limestone) and silicates
pilloried through its use in countless low impact material,’ says Karen that make up clay. At temperatures
architectural eyesores, from tower Scrivener, head of the construction approaching 1000oC, the two raw
blocks to carparks, concrete’s laboratory at the Swiss Federal materials break down into their
environmental credentials are also Institute of Technology, Lausanne. component oxides – and as the
now coming under scrutiny. The ‘If you replace concrete with any temperature rises further, then
material is used so widely that world other material, it would have a bigger combine into di- and tri-calcium
cement production now contributes carbon footprint. Many people have
‘The reason silicate. The lesser quantities of
5 per cent of annual anthropogenic the idea that if you built in steel concrete iron and aluminium in the clay also
global CO2 production, with China’s you’d make things better – but in fact react with calcium, giving the minor
booming construction industry you’d make things worse. The reason
has such a components of Portland cement.
producing 3 per cent alone. And concrete has a big carbon footprint big carbon Finally, this mixture, called clinker,
the problem looks set to get worse: as a whole is that there are just such is ground to a powder, and gypsum
already produced in over 2 billion huge quantities used.’
footprint is is added.
tonne quantities per year, by 2050, Concrete is used in such large because such To convert this powdery mixture
concrete use is predicted to reach amounts because it is, simply, a into a concrete, which Aspidin
four times the 1990 level. remarkably good building material:
huge quantities senior claimed was as beautiful as
‘The reason there’s so much not just for basic road construction, are used’ Portland stone (hence its name),
www.chemistryworld.org Chemistry World | March 2008 | 63
Construction
be lowered using additives called
PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY
plasticisers.
The concrete lorries that rumble
along our roads today are likely to
be carrying a complex mixture of
chemical additives, in addition to
the basic ingredients of concrete.
But like most aspects of concrete
chemistry, this is not a new
development – the Romans are
known to have included additives
such as animal blood to improve
concrete performance, and the
Chinese added sticky rice to their
mixtures when building the Great
Wall during the Ming dynasty.
‘People do dispute this, but I
would say the water: cement ratio
for complete hydration was 0.32,’
said Marios Soutsos. ‘But to get
the strength you don’t need to
have complete hydration. We are
going down to a ratio of 0.16, with
admixtures, and that gives higher
strength than a completely hydrated
system. The only thing preventing
us from going below that is the
workability of the concrete. More
efficient chemical admixtures may
allow us to.’
just add water and aggregate. The strong varieties – so less concrete is Electron micrograph of One company hoping to extend
resulting calcium silicate hydrates required to do the same job. gypsum crystals (brown) that limit further is BASF. Their
form an extended network of bonds Achieving strong concrete is a formed in setting construction chemicals business –
which bind together the solid fine balancing act. Too many pores concrete (blue) greatly expanded by the purchase of
aggregate. However, the exact role of filled with unreacted water weaken Degussa’s construction business on 1
the lesser components of the cement the final structure, but a certain July 2006 – is one of the main drivers
remains vague. amount of water is required to keep Karen Scrivener at the of admixture chemistry, according
While the chemistry of concrete the mixture workable. However, Swiss Federal Institute to Soutsos.
may still not be entirely understood, this threshold of workability can for Technology, Lausanne While there have been several
what has become increasingly clear families of concrete additives used
is the material’s environmental by the construction industry since
ALAIN HERZOG
impact. ‘The rule of thumb is that animal blood went out of fashion, the
for every tonne of cement you make, most recently developed, and best
one tonne of CO2 is produced,’ performing, are the polycarboxylate
says Marios Soutsos, who studies ethers (PCEs), says Sven Asmus,
concrete at the University of head of technical services and
Liverpool, UK. ‘Modern cement development of admixture systems
kilns are now more efficient, and at BASF, based in China, where
produce about 800kg of CO2 close to half of the world’s cement
per tonne – but that is still a big is produced. As a polymer, the
emission.’ PCE’s structure and properties can
Concrete production is readily be tailored by changing the
responsible for so much CO2 monomers used to make it.
because making Portland cement ‘Many acrylic acid derivatives
not only requires significant [PCE monomers] are manufactured
amounts of energy to reach reaction in large scale, and we’ve been
temperatures of up to 1500oC, looking at different combinations
but also because the key reaction that give good properties,’ says
itself is the breakdown of calcium Asmus. ‘Initially this was by trial
carbonate into calcium oxide and and error, but we have now built
CO2. Of those 800kg of CO2, around up such expertise that we can use a
530kg is released by the limestone directed approach to designing new
decomposition reaction itself. mixtures.’
Plasticisers work by preventing
A complex mix the cement particles from clumping
Several ways of reducing the together. ‘In physical terms, these
environmental impact of concrete are dispersants, and they act through
are now being investigated – one absorbing onto the surface of the
possibility being to produce ultra- particle which they are supposed
64 | Chemistry World | March 2008 www.chemistryworld.org
to disperse,’ says BASF’s Christian
Hübsch, marketing support,
branches and industries, Europe.
‘Polycarboxylates act by a steric
repulsion, so in simple terms they act
as a spacer between two particles.’
To make ultra-high strength
concrete, you need very strong
plasticisers. ‘Final strength is
achieved by maximum water
reduction, which needs ultra-strong
dispersant molecules,’ adds Hübsch.
‘These are the molecules with the
longest side chains, because they
provide the strongest dispersing
forces. Due to their sheer size, they
provide the longest-range repulsion
forces.’
Ultra-high strength concrete was
pioneered in Japan, where the added
cost of the material was offset by
the need for earthquake-resistance,
and the fact that property in Tokyo
area costs up to ¤160 000 per square
metre, encouraging the construction
of buildings with the thinnest
possible concrete superstructure.
‘But we also see huge potential in
Europe, and cost is a major issue
here,’ adds Hübsch. ‘The solution is
not at hand yet; nevertheless we see
higher and higher strength classes
coming up with almost normal
mix designs, applying specially
designed concretes which have a
lower cement content. Cement is
the most expensive component of
ultra-high strength concrete, and if
you can substitute some of this with
alternatives such as slag or fly ash,
BASF
this is, economically, beneficial.’
Cut the clinker The Tatara bridge in become a problem.’ A more viable long-term clinker
Replacing Portland clinker, either Japan is the world’s However, Scrivener says that the substitute, certainly in terms
partially or entirely, with alternative longest cable-stayed potential of clinker replacement is of availability, is finely-ground
cements is also being investigated as concrete bridge ultimately limited. ‘The uptake of limestone, suggests Scrivener.
an approach to tackling concrete’s SCMs has been pretty good – but ‘Adding up to 5 per cent can have
CO2 emissions. Waste materials, the production of these materials positive effects, by improving the
such as slag (from blast furnaces) is dwarfed by the demand for microstructure. And for buildings
and fly ash (from coal-fired power cement,’ she explains. And while such as individual houses, where you
stations), are already being used making cement from a blend of don’t need great strength, there you
as supplementary cementitious slag and Portland cement is fairly can substitute 20 per cent with good
materials (SCMs) – and have been straightforward, entirely replacing performance.’
for some decades. clinker with slag requires alkali to
‘Replacement of Portland be added to the mixture to activate it Sticking with less cement
cement is key, absolutely, and the – and that alkali can then go on and An additional approach to the
challenge is to address the negative attack the aggregate. ‘Alkali-silica carbon footprint problem is to
effects of this substitution, which reaction is becoming more and more reduce the amount of cementitious
is mainly related to early strength of a problem, because as time goes material altogether – be it Portland
development,’ said Hübsch. ‘With 50 ‘It’s almost on we’re discovering that more and cement or an SCM. This is another
per cent clinker replacement with more aggregates are reactive,’ adds area being researched by BASF, and
fly ash, early strength goes down
impossible Scrivener. ‘For example, here in also by Ravindra Dhir, director of
dramatically. We had a discussion to find out Switzerland 70 per cent of our power the concrete technology group at the
with the big contractors in Germany comes from hydro, we have 300 University of Dundee, UK.
about this– ideally they would like to
the optimal dams built in the 1950s and 60s, and ‘We’ve found that you can take
cast concrete in the afternoon, and material for more and more of them are starting out at least 20 per cent of the cement
then de-mould the next morning, to show signs of this reaction. So this content while retaining durability,’
to go on with the construction. At
a particular is the problem – it can take 60 years Dhir says. And it turns out that
colder temperatures this can really structure’ before the problem manifests itself.’ reducing the cement levels can
www.chemistryworld.org Chemistry World | March 2008 | 65
Construction
actually improve the durability argue with them. This ability to predict
of the final concrete. ‘If you think ‘I think we’re really moving performance depends on our
about concrete in terms of cement towards a breakthrough – and I ability to understand the complex
paste and aggregate, it is the cement think it’s high time we did – of chemical reactions involved in
paste that is more porous, so it is designing concrete intelligently for concrete formation. ‘Historically,
the cement that provides a route performance, both the engineering because we have very complex
by which elements of exposure can requirements and the exposure materials, and it hasn’t been
go in and out. So in theory, the less requirements.’ possible to precisely understand the
you use, the better the concrete Scrivener agrees that a key chemistry, people have fallen back
should be.’ Pores in the material obstacle to using concrete on a kind of empirical approach,’
allow corrosive materials such as efficiently is our current inability says Scrivener. ‘Now, because of the
chlorides and sulfates to penetrate to easily predict the performance way characterisation techniques
the structure and attack the metal of a particular mixture. ‘Under the have advanced – we have atomic
reinforcement – the cause of well current European standards, there force microscopy, scanning and
over 90 per cent of problems of are something like 170 different transmission electron microscopy,
concrete durability, adds Scrivener. cement types available, and if a x-ray diffraction, NMR – this enables
However, Dhir points out that the person wants to build a structure us to have a real understanding of the
ultimate strength of the concrete it’s an almost impossible task to chemistry, which we need to be able
is equally if not more important decide the optimal material for the to work on a less empirical basis.’ To
than short-term CO2 saving. ‘The structure he wants to build. We’re improve our understanding of the
challenge is to translate thinking starting to work towards good performance of different concrete
and laboratory findings into the real prediction of performance, which mixtures, it’s simply a matter of
world – and in the world of concrete is one of the first things you need to applying these techniques, Scrivener
that’s not easy, and it will always be start working towards.’ This current adds, which just comes down to time
a slow pace. You’ve got to be sure lack of knowledge means that often and effort.
about variability of materials, the a concrete is used that is stronger Hübsch also sees a revolution on
issue of quality assurance, and they than the job requires – unnecessarily the horizon in the field of chemical
Titration_Concrete_CW 5/2/08 up more Page 1
may come to the conclusion that it’s using14:57 raw materials than admixtures, with the potential to
not worth the risk, and I would not were really needed. dramatically change concrete’s
properties. ‘Admixtures of the
future will actively interfere with
the hydration processes, and ideally
One Click TitrationTM control these processes in terms
of reaction rate, and in terms of
the composition and ideally the
morphology of hydration products.
This will be the quantum leap
everybody is trying to achieve. I’d
say, on a five to 10 year basis, we
might be able to fundamentally
change the properties of concrete on
a nanoscale.’
But perhaps the most significant
reason for optimism is the increasing
engagement of the cement industry
itself. ‘One of the main things
I’ve been involved in over the
last five years is putting together
a consortium called Nanocem,
Concrete and Cement which has brought together, for
the first time, the leading academic
testing solutions groups throughout Europe with
the industry,’ says Scrivener. ‘For
the first time ever, we have all the
Simple, efficient and secure analysis, according to BS EN 196-2 major cement producers signed up
• Alkalinity to support fundamental research
in this area, and of course one
• Chloride
of our major preoccupations is
• Metals (Mg, Ca, Fe, Al) sustainability. It’s very important
• Sulphate that we have all the major producers
involved, because they’re the people,
• Fluoride ‘In 5–10 years at the end of the day, who are going
Download applications now, visit we might be to be able to make a difference.’
www.mt.com/uk-concrete able to change Further reading
Mettler-Toledo Ltd
the properties J S Damtoft et al, Cem. Concr. Res., 2008, 38,
115 (DOI:10.1016/j.cemconres.2007.09.008)
Tel 0116 234 5005 of concrete on K L Scrivener and R J Kirkpatrick, Cem.
Email enquire.mtuk@mt.com Concr. Res., 2008, 38, 128 (DOI:10.1016/
the nanoscale’ j.cemconres.2007.09.025)
66 | Chemistry World | March 2008 www.chemistryworld.org
