麻豆传媒

Winnipeg, Canada

ROGER JONES wants to take us back to the Stone Age, though what he has in
mind has nothing to do with primitive living. In his sights Jones has cars,
computers and even circuit boards made out stone鈥檚 artificial counterpart,
concrete. Jones, who works for Materials Technology in Reno, Nevada, thinks his
new concretes and cements could soon be replacing wood, metals, plastics and
ceramics in everything from toothbrushes to the panels that shield spacecraft
from the heat of re-entry. He has even made a flexible form of concrete that
would be ideal for bendy traffic bollards.

Jones says his treatments could be applied to existing concrete structures,
allowing him to toughen up and waterproof houses, churches, statues and dams.
And although a world filled with concrete sounds like anathema to the
environmentally minded, Jones claims green credentials for his material. He can
even make it almost entirely from waste, he says.

Conventional concrete is used by the construction industry everywhere, and it
pours out several billion tonnes of the stuff every year. The most common recipe
blends sand or crushed rock, water and portland cement鈥攁 mixture of
various calcium silicates produced from limestone. These react with the water to
produce gels which then set into a firm, rock-like mass.

But that鈥檚 not the end of the story. Concrete continues to harden after it
has set, as calcium compounds in cement slowly react with carbon dioxide in the
atmosphere. This process, known as carbonation, turns these compounds back into
limestone, which is a much tougher material than the original cement. Normally
this happens very slowly indeed: a large slab of concrete could take thirty
thousand years to carbonate fully.

The carbonation reaction proceeds at this snail鈥檚 pace because it generates
water, which plugs the pores of the concrete and stops more CO₂ getting
in. Engineers have tried to speed up this toughening by methods such as locking
slabs of concrete in a sealed pressurised chamber containing CO₂, in
the hope of forcing the gas in. This approach has been partially successful, but
it still runs up against the problem that the water it generates slows the
reaction down.

Perfect solution

In 1994 however, Jones had a brainwave. He was pondering the problem of
carbonation hardening when he came across an article on new uses for
supercritical carbon dioxide (SCCO₂). This is a special form of carbon
dioxide which dissolves compounds as effectively as normal liquids, but diffuses
easily through materials just like a gas (see 鈥淪olvents get the big squeeze鈥�,
麻豆传媒, 6 August 1994, p 32). Normal CO₂ becomes
SCCO₂ when compressed to 73 times atmospheric pressure at temperatures
above a modest 31 掳C.

鈥淚 had a flash of recognition,鈥� says Jones, 鈥渁nd literally jumped off the
couch.鈥� SCCO₂, he realised, would be ideal for carbonating the cement
in concrete. It is so aggressive that it can penetrate a solid block of plastic.
Cement is much less tightly packed than plastic so, reasoned Jones, SCCO
₂ should just rip through even a fully hardened concrete matrix. 鈥淎nd
that鈥檚 exactly what it did,鈥� he says.

Working on his own and with Frank Baglin of the department of chemistry at
the University of Nevada in Reno, Jones established that pumping SCCO₂
through a block of concrete changes the cement back to limestone in a matter of
minutes rather than millennia. It is a simple technique that works, and it works
amazingly well. Jones discovered that SCCO₂ even drives out the water
generated in the reaction. 鈥淭here鈥檚 so much energy in [SCCO₂] that it
surrounds the water molecules like an escort,鈥� says Jones. 鈥淚t grabs the water
droplets and drags them out of the matrix.鈥�

The compressive strength of the treated concrete, which is a measure of how
much weight it can support, doubles as the carbonates fill the pores and make
the material denser. And the concrete鈥檚 tensile strength鈥攊ts strength when
pulled鈥攊ncreases by about 75 per cent.

The process has environmental benefits too. Cement is made from limestone in
a process that releases a large amount of CO₂into the atmosphere as
calcium carbonate is transformed into calcium oxide. Cement kilns are generally
fired by fossil fuels, which produces more CO₂. Changing the cement
back into limestone traps large amounts of CO₂, compensating for much
of the gas released during the cement production.

One possible application of the new concretes is in disposing of nuclear
waste. Jones is working on this with chemist Craig Taylor and materials engineer
James Rubin at the Los Alamos National Laboratory in New Mexico. 鈥淐ement is
currently used as an immobilisation matrix for radioactive hazardous waste,鈥�
explains Taylor. The US has several bunkers filled with 55-gallon drums
containing radioactive wastes set in concrete. The wastes include solid residues
from plutonium pressing as well as cleaning fabrics and rags containing fine
particles of radioactive metals.

Before these drums can be disposed of or even moved they have to pass tests
for safety. 鈥淭he headspace at the top of the drum is tested for the emission of
volatile compounds,鈥� says Taylor, 鈥渁nd heat evolution is also checked.鈥� A high
level of volatile compounds means that solvents in the waste are escaping, while
excess heat shows that water in the cement is being broken down by radiation
from the waste. This can be a serious problem, says Taylor, because it generates
hydrogen gas, which is highly explosive.

Treatment with SCCO₂ seems a perfect solution. It pulls water from
the set concrete, and at the same time toughens it and makes it less porous.
Taylor is quick to point out that the radioactive portion of the waste stays
safely embedded in the concrete. 鈥淪upercritical carbon dioxide will not dissolve
the radioactive metals,鈥� he says. The supercritical treatment certainly looks
promising. 鈥淭he next stage is leachability studies,鈥� says Taylor, which will
look at how the radioactive metals move in the new cement matrix.

Taylor and Rubin have other ideas, too, for the new concretes. Take railway
sleepers. The concrete sleepers used for Japan鈥檚 high-speed lines last only for
three years or so. Taylor estimates that concrete sleepers treated with their
method would last at least twice as long.

Another potential application for the SCCO₂process is in making a
protective sealing for new and existing concrete structures. Flushing the outer
few centimetres of a structure with SCCO₂ would give it a hard,
impermeable surface that would withstand acid rain and the elements. 鈥淲e could
treat pre-existing high value items such as historic churches and statues,鈥� says
Taylor. The process could also be applied to dams, bridges and roads. In many of
the world鈥檚 major cities, buildings, pavements, curbs and bridges made of
concrete can begin to show signs of degradation just a few months after they are
built, says Jones. Sealing the concrete using SCCO₂ should protect
these structures, and could even repair some of the damage, he says.

Rubin also foresees a kind of coloured, waterproof concrete. Normally,
painting concrete does not protect it very well, because paint does not adhere
well to the inner surfaces of the concrete pores. Treating painted concrete with
SCCO₂ could dissolve the paint, and carry it into the pores, creating a
waterproof seal. Taylor admits that there may be problems. What, for example,
will this do to the steel reinforcing bars commonly used to strengthen concrete
structures? 鈥淏ut we do know that removing water will stop the bars rusting,鈥� he
says.

It is not just paint that can be dissolved in SCCO₂. There are a few
metals, polymers and other chemicals that can also be dissolved in the fluid.
Last year Joseph DeSimone, a chemist at the University of North Carolina in
Chapel Hill, developed a polymeric soap which allows SCCO₂ to dissolve
large hydrocarbons. This could be useful for making materials containing
pharmaceuticals or exotic plastics, but the soap is still very expensive.

Materials Technology has managed to dissolve lighter hydrocarbons such as
styrene directly in SCCO₂, and have used it to infuse cement. This
material can then be infused with SCCO₂ containing benzoyl peroxide,
which kick-starts the reaction in which styrene molecules link up to form
polystyrene. The result is a cement that Jones calls a memory material鈥攊t
will spring back into its original shape after being bent. This would be ideal
for traffic bollards, car bumpers, or other materials that need to be tough and
retain their shape .

鈥淣ot a day goes by where we do not find a new opportunity,鈥� says Jones. He is
making cements containing polymers that can then be extracted with SCCO
₂, to leave empty pores of controlled dimensions. These materials could act
as biofilters, desalinators or catalytic surfaces. Alternatively, says Jones,
metals could be introduced into the pores to produce concretes that conduct
electricity. You could even make materials that are radar-invisible, says
Jones, by adding ferrite, which is known to absorb radio waves.

Jones鈥檚 SCCO₂ technique is not the only game in town, however. Other
researchers have produced similar results simply by adding polymers and metals
to the cement slurry before it hardens. James Beaudoin of the Institute for
Research in Construction in Ottawa, Canada, has blended materials such as
graphite and steel fibres with pre-poured cement to produce concrete that
conducts electricity and will also heat up when a current passes through it.
Preparing these concretes is very simple, and they are both light and tough.
Applications include heated building floors and electrically heated driveways
that clear themselves of ice and snow.

Jones considers this research complementary to his own, but thinks that
SCCO₂ wins out when it comes to treating existing roads and buildings.
Rather than pouring a fresh layer of conductive concrete, the surface of
existing concrete could be treated by putting down a layer of solid CO₂,
better known as dry ice, mixed with metals or other reagents. Running over
this with a steam roller would force SCCO₂ and whatever was dissolved
in it into the surface.

With such a versatile process, Jones foresees a long list of future
possibilities. And in addition to implanting plastics and metals, Jones is also
using SCCO₂ to make cement from waste materials (see 鈥淔ly-away
success鈥�). 鈥淚f we can just get materials to stick together we can probably treat
them supercritically,鈥� he says. By further modifying these materials using
SCCO₂ to pump in polymers and metals, he believes you could make
materials to meet a huge range of demands. And whereas a steel or
aluminium-making plant consumes vast amounts of energy, reinforced concretes
would be much less demanding and significantly cheaper to make.

On the drawing board there are already plans for weird and wonderful future
uses, including toothbrush handles, body armour, semiconductors and other
electronic components, and even the tail rotor blades of helicopters. Materials
Technology has already been approached by a firm that manufactures car
components.

Jones hopes that similar approaches will come flooding in. 鈥淲e will wait for
word to spread and for people to come to us with their problems,鈥� he says. And
he is confident that when they do, he will have the solutions they are looking
for. 鈥淚 truly believe that in a hundred years these materials will have
completely replaced steel, paper, wood and other conventional materials.鈥�

* * *

Fly-away success

Industrial waste isn鈥檛 an obvious source of construction materials. But Roger
Jones of Materials Technology in Reno, together with Craig Taylor and James
Rubin of the Los Alamos National Laboratory in New Mexico, are developing a
concrete substitute made from a mixture of fly ash from coal-fired power plants,
lime or cement kiln dust and alum sludge from water treatment plants. This
material is loosely based on an ancient formula for pozzuolanic volcanic ash
cement. 鈥淧eople think this stuff is new,鈥� says Taylor, 鈥渂ut the basic concept
has been around for thousands of years.鈥� The Romans used it鈥攖he dome of
the Pantheon was made from pozzuolanic cement. 鈥淲hen we treat this material with
supercritical carbon dioxide,鈥� Jones explains, 鈥渨e end up with a product
superior to Portland cement鈥攎ade out of waste.鈥�

Jones admits that they are not quite sure what the supercritical CO₂
(SCCO₂) is doing to strengthen this material, as it does not contain
much calcium. His best guess is that rigidly structured silicates, known as
zeolites, may be formed. Initial spectrographic tests seem to support this
theory.

Materials Technology, in partnership with Sierra Pacific Power Company and
Custom Building Products of California, is developing a wallboard made entirely
from fly ash. Before treatment with SCCO₂, the board is not much good,
mainly because it breaks up when wet. But after treatment, the board is strong
enough to be used for building houses. And, as Taylor points out, they are now
waterproof, unlike conventional drywall.

The team plans to market cement wallboard in North America and Europe, while
in developing countries the new technology would be ideal for making fly-ash
cement blocks, roofing and floor tiles. It is a manufacturing process in which
everyone gains. Power companies are more than happy to supply the waste material
that they would otherwise have to pay to dispose of, and the supercritical
SCCO₂ can be taken straight from high-pressure precipitators within
power station flues. The building company gets a better, more profitable
product. And less CO₂ ends up in the atmosphere.