BENEFICIATION
C
oncrete is one of the most used
materials in the world, second
only to water. At the same time,
the cement used in concrete
is one of the largest producers of carbon
dioxide, contributing an estimated five
to eight per cent of all human-generated
atmospheric carbon each year. Last year
alone, 4.2 billion tonnes of cement were
produced (Statista 2017). With the increasing
focus on environmental sustainability and
climate change, these two aspects need to
somehow be reconciled. One of the areas
that is helping with this is the use of alkali-
activated and geopolymer concrete.
The ultimate goal of research into
these areas is to obtain optimal strength
from ‘green’ concrete by producing a
hybrid material of superior strength with
a significantly lower carbon footprint,
reduced water consumption, and a
lower cost. One of South Africa’s biggest
proponents of low- and no-cement concrete
is professional chemist Cyril Attwell
from ARC Innovations. Until earlier this
year, Attwell was the group concrete and
research manager at Murray & Roberts,
where he worked on Transnet’s City Deep
Container Terminal upgrade, as well as the
Loeriesfontein and Khobab wind farms and
Cape Town’s Portside Tower skyscraper.
“People tend to think that concrete is
simple, but this isn’t the case. Concrete is the
most complex crystalline structure known to
man,” Attwell explains. “There are over 500
families of crystals in normal concrete, and
when you start looking at geopolymer and
alkali-activated concrete, for example, this
number increases to over 800. It’s extremely
complex. I got into concrete research more
by accident than by design, but once I got
into it and started looking at the complexity
of it, it captivated me. Everyone sees
concrete as being a simple, highly ordered
product, when in reality it is highly complex
with a huge chaotic streak, and I like trying
to order chaos.”
A self-described ‘bunny hugger’, Attwell
has been looking at ways to reduce the
carbon footprint of concrete since he
first started working with it over 20 years
ago. But creating green cement is easier
said than done. “For every cubic metre of
concrete used, the cement required releases
about 0.9kg of carbon dioxide, with an
average of 300–350kg of cement. Now with
alkali-activated, low-cement concrete,
every cubic metre used comprises 20–30%
— or 200–300ℓ — of a commercial activator,
usually a combination of a hydroxide with
a silicate.
“The problem with this is that for every
kilogram of silicate used, you produce around
0.66–0.75kg of carbon dioxide (CO 2 ), because
sodium carbonate is used to get to the sodium
silicate, driving off CO 2 . So while this amount
is lower than that produced by cement, it is
not substantially so,” he explains.
“Then there is the fact that with most
alkali activation, the 200–300ℓ of activator
are unreactive at ambient temperatures,
so you need to heat them up, increasing
the CO 2 . And this is one of the problems I
have with this whole green mentality when
it comes to the silicates, especially when
you've got 200–300ℓ of commercialised
material. And with most alkali activation,
the commercial activator is unreactive at
ambient temperatures, so they need to be
heated up, which uses more energy and
creates more CO 2 .”
South African projects
Attwell explains that he was involved in
the Gautrain project. “Murray & Roberts
brought me in to design the concrete and
do the production on the Gautrain. On that
project, we were actually able to reduce the
amount of cement used from the original
estimates by 144 000 tonnes, from the
original 354 000 tonnes estimate to 210 000,
and we replaced that 144 000 tonnes with
140 000 tonnes of ash from the Lethabo
Power Station. Generally, people don’t
use fly ash for precast, especially
not for such a big project,
but by applying advanced
re-crystallisation (ARC)
technology, we were able to
utilise a 32% substitution for
the precast, and we didn’t
use any steam at all.”
ARC is the process
of optimising the
On the Gautrain project, Murray & Roberts was able
crystallography of a
to replace 144 000 tonnes of cement with 140 000
hydration, polymer, or
tonnes of fly ash from the Lethabo Power Station.
QUARRY SA | SEPTEMBER 2017 _ 25