Article found in "New Energy News"
VOLUME 9, NUMBER 8 ISSN 1075-0045
Editor: Patrick Bailey
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Submitted by: Tony Smith

There is a way to make our city streets as green as the Amazon rainforest.
Almost every aspect of the built environment, from bridges to factories to
tower blocks, and from roads to sea walls, could be turned into structures that
soak up carbon dioxide, the main greenhouse gas behind global warming. All we
need to do is change the way we make cement. John Harrison, a technologist
from Hobart, Tasmania, reckons his alternative cement based on magnesium
carbonate rather than calcium carbonate, could reduce climate change without
sacrificing modern living. It's a big claim, and Harrison has set about trying to
convince the building industry to adopt his ideas.
"The Kyoto Protocol was a good effort", says Harrison " but it got things
wrong when it assumed that trees were the only things that could absorb carbon
from the air." Instead, he wants to replace the ubiquitous Portland cement with a
substance that he calls 'eco-cement'. "This magnesium based material," he says.
"could be cheaper to manufacture than Portland cement, more durable and soaks
up CO2 as well," and, cIaims Harrison, "if the building industry listens, cities and
their suburbs could turn into sinks for C02, as effective as, for example, the
naturaI grass and woodland they replaced". Our modern world is largely built of
Portland cement, invented almost 180 years ago by a Yorksbire stonemason
called Joseph Aspdin. In 1824, he obtained a patent for "an improvement in the
modes of producing artificial stone" that involved roasting chalk and clay in a
kiln, grinding the resulting 'clinker' into a fine powder containing mainly calcium
silicates and mixing it with water. This starts a complex chemical reaction that
forms crystals of calcium silicate hydrate, for example, which hardens the mix.
The 19th century was a time when the great cities of Britain were under
construction, and many other inventors were working on artificial stone. But
Aspdin cracked the problem by subjecting the ingredients to the ultra-high
temperatures of a glassmaker's kiln in his home town of Hunslet. He called the
product Portland cement because of its resemblance to the most popular natural
stone of the day, from the Isle of Portland in Dorset.
Portland cement proved cheap to make and immensely versatile, and
soon became the basic ingredient of both cement and mortar, the building blocks
of every city on the planet. Every year, some 1.7 billion tonnes of Portland
cement are now produced worldwide, a staggering quarter of a tonne for every
6 person on Earth. But there's a problem. The manufacture of Portland cement
produces massive amounts of CO2. This is partly because of the huge amounts of
energy required to raise temperatures inside cement kilns to the 1450 0 C needed
to roast the calcium carbonate (from chalk or limestone. And it's also because the
process of conversion itself creates CO2.
For every tonne of Portland cement emerging from the kilns, roughly a
tonne of CO2 gas escapes into the atmosphere. Cement manufacture is
responsible for around 7 percent of total man-made CO2 emissions worldwide, a
figure that rises above 10 percent in fast-developing countries such as China,
which currently manufactures one in every three tonnes of cement made around
the world.
If we mean to control global warming this situation can't go on. And,
says Harrison, it need not. His solution, being brought onto the market by his
small company 'TecEco' is to replace the calcium carbonate in the kilns with
magnesium carbonate -a rock that occurs widely on its own, as the mineral
magnesite, or in mixtures with calcium calbonate, such as dolomite.
Magnesium based cements aren't new. They were first developed in 1867,
by French man Stanislas Sorel, who made cement from a combination of
magnesium oxide and magnesium chloride. However, his mixtures couldn't stand
long exposure to water without losing their strength. Put a tower block of the
cement in a Manchester or Seattle drizzle and it would eventually crumble away.
Harrison's magnesium carbonate based 'eco-cements', on the other hand,
are chemically quite similar to calcium carbonate based Portland cement, are far
more robust than Sorel's material. And according to Harrison, his material has a
number of major environmental advantages. For a start, the kilns don't need to be
run so hot. Magnesium carbonate converts readily to magnesium oxide at around
650 C. This means that emissions of CO2 from the energy used to fire kilns are
roughly halved.
The roasting process for the manufacture of eco-cements produces more
CO2, but during setting and hardening, a process called carbonation reabsorbs
most of this from the air.
When conventional concrete made from Portland cement is fresh, water
in the mix also slowly absorbs CO2 from the atmosphere. This solution then
reacts with the alkaline calcium containing components in the concrete matrix to
deposit calcium carbonate crystals, which reduce the strength of the concrete.
But carbonation is quicker and more efficient in Harrison's ecorement.
Magnesium carbonate crystals are stronger than those of calcium
carbonate, so they add to the material's strengtb.
If eco-cement is used to make porous materials like masonry blocks
virtually all the material will eventually carbonate says Harrison. A tonne of
concrete can end up absorbing up to 0.4 tonnes of CO2" he says, "equalling about
100 kilograms of carbon. The opportunities to use carbonation processes to
sequester carbon from the air are just huge", says Harrison "It can take
conventional cements centuries or even millennia to absorb as much as ecocements
can absorb in just a few months".
This means that eco-cements quietly carbonating in a tower block
could be performing much the same atmospheric function as a growing tree. And
if eco-cements gained a foothold in our cities, they could immediately reduce the
cement industry's contribution to global warming, reabsorbing much of what was
emitted in their creation. By directly replacing Portland cement with his eco-
cement, Harrison estimates that we could eliminate over a billion tonnes of CO2
each year.
The idea is "a world first", says Fred Glasser of the University of
Aberdeen's chemistry department, one of the leading authorities on cement
technologies. And eco-concrete has another way of proving its greenhousefriendly
status. The material has huge potential for incorporating all sorts of waste
matter, including carbon-based organic wastes that would otherwise rot or burn
and release CO2 into the air.
Adding inert waste such as fly ash to cement is routine in the industry. But
for Portland cement there are strlct Iimits. Because the cement is alkaline,
mixtures can react with aggregate to crack the concrete or make it brittle,
sometimes causing failure. "Magnesium cements are much less alkaline, and the
potential problems are far less," says Glasser, who believes this could be one key
to their eventual widespread use.
"Organic waste from rice husts to sawdust, plastics and rubbers can all be
incorporated as a bulking material in magnesium cement without it losing
significant strength," says Harrison, "thus turning the cement in buildings,
bridges and so on, into significant stores of carbon. We have made bricks that are
over 90% ash," he says, "We can probably get three to four times more waste into
our cement than Portland cement and would also massively reduce the amount of
cement needed in the first place."
"His magnesium based cements may not meet every requirement," he
admits, "You might not want to replace Portland cement entirely for, say, bridge
beams. But Harrison reckons magnesium cements could eventually replace 80%
of cement. This move wouldn't come cheap. Prime sources of magnesium
carbonate, such as magnesite and dolomite, cost more to mine than calcium
carbonate. But the price should fall with economies of scale, says Glasser.
Harrison has already gone into manufacture. He sold his first ecocement
bricks for a commercial building project in May this year. But he fears
the cost of maintaining his patents could force him out of business before it really
gets going.
"The main problem," says Glasser, "is that the building materials
industry is intensely conservative, It prefers what it knows- Portland cement.
Engineers are familiar with its mechanical properties. And of course, Portland
cement is cheap. It may guzzle energy like there is no tomorrow, but a couple of
dollars will buy you as much of the stuff as you can carry away from a hardware
store. The market for Portland cement is so vast that it is difficult to see
magnesium cements making much of an in road in the next 10 years." says
Glasser, "But perhaps, as the world tries to think up new ways of cutting back on
its emissions of CO2, eco-cement may have its day. Our burning of fossil fuels is
force-feeding Earth's atmosphere with CO2 at a rate that vegetation can no longer
absorb. The logical way out is to accelerate the formation of carbonate with our
own man-made rocks. What better way" says Harrison, "than in cement? ".