The concrete conundrum - Royal Society of Chemistry

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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

Concrete has a problem. Already concrete is because it is in fact a very In short but also for rather more glamorous
pilloried through its use in countless low impact material,’ says Karen  Concrete production projects. The Burj Dubai skyscraper,
architectural eyesores, from tower Scrivener, head of the construction contributes 5 per cent still under construction but already
blocks to carparks, concrete’s laboratory at the Swiss Federal of annual anthropogenic well over half a kilometre high – the
environmental credentials are also Institute of Technology, Lausanne. global CO2 production, final height remains secret, but the
now coming under scrutiny. The ‘If you replace concrete with any mainly because such vast building is set to dwarf all other
material is used so widely that world other material, it would have a bigger quantities are used man-made structures – relies on a
cement production now contributes carbon footprint. Many people have  Humans have used highly flowable concrete mixture
5 per cent of annual anthropogenic the idea that if you built in steel concrete for millennia – that doesn’t harden before it can
bglooobmalinCgOc2opnrsotdruuccttiioonn,inwdiuthstCryhina’s you’d make things better – but in fact its basic ingredients date be pumped to the top of the tower,
producing 3 per cent alone. And you’d make things worse. The reason back to ancient Egypt yet forms a strong and robust final
the problem looks set to get worse: concrete has a big carbon footprint  CO2 is a product of the product. And Japan’s construction
already produced in over 2 billion as a whole is that there are just such main reaction that makes industry has pioneered ultra-
tonne quantities per year, by 2050, huge quantities used.’ cement – concrete’s key strength varieties from which to
concrete use is predicted to reach ingredient build its earthquake-proof bridges,
four times the 1990 level. Concrete is used in such large  Development of new and the Tokyo apartment blocks that
amounts because it is, simply, a concrete additives could form some of the most expensive
‘The reason there’s so much remarkably good building material: produce a stronger, more real estate in the world.
www.chemistryworld.org not just for basic road construction, workable material whilst
reducing the amount of Humans have been using concrete
cement required and the in their pioneering architectural
resulting CO2 emissions feats for millennia. The basic
ingredients – sand and gravel
Already 605 metres high, (aggregate), a cement-like binder,
the Burj skyscraper in and water – were being mixed at
Dubai will be the world’s least as far back as Egyptian times.
tallest building The Romans are well-documented
masters of the material, using it to
‘The reason create such wonders as the Pantheon
concrete in Rome, topped with its gravity-
has such a defying 43.3-metre-diameter
big carbon concrete dome: now over 2000 years
footprint is old but still the world’s largest non-
because such reinforced concrete dome.
huge quantities
are used’ 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
his son William into a material very
close to the cement used today.

The main reaction occurring in
Aspidin’s kiln was the formation
of calcium silicates, from calcium
carbonate (limestone) and silicates
that make up clay. At temperatures
approaching 1000oC, the two raw
materials break down into their
component oxides – and as the
temperature rises further, then
combine into di- and tri-calcium
silicate. The lesser quantities of
iron and aluminium in the clay also
react with calcium, giving the minor
components of Portland cement.
Finally, this mixture, called clinker,
is ground to a powder, and gypsum
is added.

To convert this powdery mixture
into a concrete, which Aspidin
senior claimed was as beautiful as
Portland stone (hence its name),

Chemistry World | March 2008 | 63

Construction

PASCAL GOETGHELUCK / SCIENCE PHOTO LIBRARY be lowered using additives called
plasticisers.
just add water and aggregate. The strong varieties – so less concrete is Electron micrograph of
resulting calcium silicate hydrates required to do the same job. gypsum crystals (brown) The concrete lorries that rumble
form an extended network of bonds formed in setting along our roads today are likely to
which bind together the solid Achieving strong concrete is a concrete (blue) be carrying a complex mixture of
aggregate. However, the exact role of fine balancing act. Too many pores Karen Scrivener at the chemical additives, in addition to
the lesser components of the cement filled with unreacted water weaken Swiss Federal Institute the basic ingredients of concrete.
remains vague. the final structure, but a certain for Technology, Lausanne But like most aspects of concrete
amount of water is required to keep chemistry, this is not a new
While the chemistry of concrete the mixture workable. However, development – the Romans are
may still not be entirely understood, this threshold of workability can known to have included additives
what has become increasingly clear such as animal blood to improve
ALAIN HERZOG is the material’s environmental concrete performance, and the
impact. ‘The rule of thumb is that Chinese added sticky rice to their
for every tonne of cement you make, mixtures when building the Great
soanyestMonanreioosfSCoOut2siossp, rwohdoucsetudd,’ies Wall during the Ming dynasty.
concrete at the University of
Liverpool, UK. ‘Modern cement ‘People do dispute this, but I
kilns are now more efficient, and would say the water: cement ratio
produce about 800kg of CO2 for complete hydration was 0.32,’
per tonne – but that is still a big said Marios Soutsos. ‘But to get
emission.’ the strength you don’t need to
have complete hydration. We are
Concrete production is going down to a ratio of 0.16, with
breescpaounsesimblaekfionrgsoPomrtulcahndCcOe2ment admixtures, and that gives higher
not only requires significant strength than a completely hydrated
amounts of energy to reach reaction system. The only thing preventing
temperatures of up to 1500oC, us from going below that is the
but also because the key reaction workability of the concrete. More
itself is the breakdown of calcium efficient chemical admixtures may
carbonate into calcium oxide and allow us to.’
5C3O02k.gOifstrheoleseas8e0d0bkygtohfeCliOm2e, astroonuend
decomposition reaction itself. One company hoping to extend
that limit further is BASF. Their
A complex mix construction chemicals business –
Several ways of reducing the greatly expanded by the purchase of
environmental impact of concrete Degussa’s construction business on 1
are now being investigated – one July 2006 – is one of the main drivers
possibility being to produce ultra- of admixture chemistry, according
64 | Chemistry World | March 2008 to Soutsos.

While there have been several
families of concrete additives used
by the construction industry since
animal blood went out of fashion, the
most recently developed, and best
performing, are the polycarboxylate
ethers (PCEs), says Sven Asmus,
head of technical services and
development of admixture systems
at BASF, based in China, where
close to half of the world’s cement
is produced. As a polymer, the
PCE’s structure and properties can
readily be tailored by changing the
monomers used to make it.

‘Many acrylic acid derivatives
[PCE monomers] are manufactured
in large scale, and we’ve been
looking at different combinations
that give good properties,’ says
Asmus. ‘Initially this was by trial
and error, but we have now built
up such expertise that we can use a
directed approach to designing new
mixtures.’

Plasticisers work by preventing
the cement particles from clumping
together. ‘In physical terms, these
are dispersants, and they act through
absorbing onto the surface of the
particle which they are supposed

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to disperse,’ says BASF’s Christian BASF
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,
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 of availability, is finely-ground
cements is also being investigated as concrete bridge potential of clinker replacement is limestone, suggests Scrivener.
an approach to tackling concrete’s ultimately limited. ‘The uptake of ‘Adding up to 5 per cent can have
CsuOc2heams sislasgio(nfrso. Wmabsltaestmfuatrenraicaelss,) ‘It’s almost SCMs has been pretty good – but positive effects, by improving the
and fly ash (from coal-fired power impossible the production of these materials microstructure. And for buildings
stations), are already being used to find out is dwarfed by the demand for such as individual houses, where you
as supplementary cementitious the optimal cement,’ she explains. And while don’t need great strength, there you
materials (SCMs) – and have been material for making cement from a blend of can substitute 20 per cent with good
for some decades. a particular slag and Portland cement is fairly performance.’
structure’ straightforward, entirely replacing Sticking with less cement
‘Replacement of Portland clinker with slag requires alkali to An additional approach to the
cement is key, absolutely, and the be added to the mixture to activate it carbon footprint problem is to
challenge is to address the negative – and that alkali can then go on and reduce the amount of cementitious
effects of this substitution, which attack the aggregate. ‘Alkali-silica material altogether – be it Portland
is mainly related to early strength reaction is becoming more and more cement or an SCM. This is another
development,’ said Hübsch. ‘With 50 of a problem, because as time goes area being researched by BASF, and
per cent clinker replacement with on we’re discovering that more and also by Ravindra Dhir, director of
fly ash, early strength goes down more aggregates are reactive,’ adds the concrete technology group at the
dramatically. We had a discussion Scrivener. ‘For example, here in University of Dundee, UK.
with the big contractors in Germany Switzerland 70 per cent of our power
about this– ideally they would like to comes from hydro, we have 300 ‘We’ve found that you can take
cast concrete in the afternoon, and dams built in the 1950s and 60s, and out at least 20 per cent of the cement
then de-mould the next morning, more and more of them are starting content while retaining durability,’
to go on with the construction. At to show signs of this reaction. So this Dhir says. And it turns out that
colder temperatures this can really is the problem – it can take 60 years reducing the cement levels can
before the problem manifests itself.’

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
cthhaanlleshnogreti-stetormtraCnOs2lastaevtihnign.k‘Tinhge structure he wants to build. We’re improve our understanding of the
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
issue of quality assurance, and they than the job requires – unnecessarily Hübsch also sees a revolution on
mTaiytcroamteitoont_hCeocnonccrleutsieo_nCthWat i5t’/s 2/0u8sing1u4p:m57ore rPaawgme a1terials than the horizon in the field of chemical
not worth the risk, and I would not were really needed. admixtures, with the potential to
dramatically change concrete’s
One Click TitrationTM properties. ‘Admixtures of the
future will actively interfere with
Concrete and Cement ‘In 5–10 years the hydration processes, and ideally
we might be control these processes in terms
testing solutions able to change of reaction rate, and in terms of
the properties the composition and ideally the
Simple, efficient and secure analysis, according to BS EN 196-2 of concrete on morphology of hydration products.
the nanoscale’ This will be the quantum leap
• Alkalinity everybody is trying to achieve. I’d
• Chloride say, on a five to 10 year basis, we
• Metals (Mg, Ca, Fe, Al) might be able to fundamentally
• Sulphate change the properties of concrete on
• Fluoride a nanoscale.’

Download applications now, visit But perhaps the most significant
www.mt.com/uk-concrete reason for optimism is the increasing
engagement of the cement industry
Mettler-Toledo Ltd itself. ‘One of the main things
Tel 0116 234 5005 I’ve been involved in over the
Email [email protected] last five years is putting together
a consortium called Nanocem,
66 | Chemistry World | March 2008 which has brought together, for
the first time, the leading academic
groups throughout Europe with
the industry,’ says Scrivener. ‘For
the first time ever, we have all the
major cement producers signed up
to support fundamental research
in this area, and of course one
of our major preoccupations is
sustainability. It’s very important
that we have all the major producers
involved, because they’re the people,
at the end of the day, who are going
to be able to make a difference.’

Further reading
 J S Damtoft et al, Cem. Concr. Res., 2008, 38,
115 (DOI:10.1016/j.cemconres.2007.09.008)
 K L Scrivener and R J Kirkpatrick, Cem.
Concr. Res., 2008, 38, 128 (DOI:10.1016/
j.cemconres.2007.09.025)

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