Biopolymers: Cement Replacement: Industrialized Natural Resources for Architecture and Construction

Full text

AGRICULTURE
110
Cement production has a significant impact on the
environment. With an annual production exceeding
3 billion tonnes,1 cement consumption reaches an
average 500 kilograms per person per year or more
than 1,000 kg per person and year in urban areas,
which is more than the average person eats. The
production of cement requires significant amounts
of resources (limestone, clay, sand) and energy
(mostly coal or lignite) and results in significant
amounts of carbon dioxide emissions (about 1 ton
CO2 per ton of cement).2
Biopolymers are explored in search of sustainable
alternatives to replace cement as a binder in con-
struction materials. Produced by living organisms,
biopolymers are composed of small molecules
(monomers), which are bonded together by cova-
lent bonds to form larger molecules. Biopolymers
are classified in three groups based on their mono-
mer molecules:3 polynucleotides, like human DNA,
consist of 13 or more different small molecules;
polypeptidesare, short polymers of amino acids, are
the basic components of proteins; and polysaccha-
Biopolymers: Cement Replacement
Leon van Paassen and Yask Kulshreshtha
▲ CoRncrete samples based on various
aggregates.

1 Sand
2 Corn flour
3 Water
4 Mould
5 Thermal treatment
6 Cultivated building element
7 Biological decomposition
1
2
3
4
76
5
111
▲ CoRncrete is a hardened solid
material produced by mixing corn
starch with sand and water and
heating the mixture in a microwave or
oven.
Biopolymers: Cement Replacement
1 Sand
2 Corn our
3 Water
4 Mould
5 ermal treatment
6 Cultivated building element
7 Biological decomposition
▲

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112
rides, like cellulose, starch or alginate, are long
linear chains of carbohydrate molecules, such as
glucose or fructose.
Biopolymers show a large variety of properties and
consequently lend themselves to different applica-
tions. Cellulose for example, which is the most
common organic compound found on Earth, forms
a hard and strong, solid material that provides, as
one example, for the structure and strength of
wood. Alginate, on the other hand, which is being
considered a potential alternative for cement, is
water-soluble. The properties of biopolymers de-
pend on their monomers and the amount and type
of bonds between these.4
In nature, there are many examples where biopoly-
mers act as cementing agents, dating as far back as
4 billion years, to the earliest moments of Earth’s
existence. In those times, micro-organisms lived in
the shallow seas along the old continents. By ex-
creting biopolymers, they attached themselves
onto rocky surfaces. This process created thin ad-
hesive films, which trapped suspended sediments
in order to form dome-shaped, thin-layered rock
structures known as stromatolites. Still today,
many organisms use excreted biopolymers to in-
crease their adhesion to surfaces, such as molluscs
and oysters in the seas or termites and ants on
land.
While the main application for natural biopolymers
is in food production, their renewability and bio-
degradability make them an interesting material for
industrial applications as well. The current industri-
al exploitation in this field focuses mainly on their
use in biodegradable plastics as a replacement for
polystyrene- or polyethylene-based plastics. Appli-
cations of biopolymers in the building and construc-
tion industry include their use as a binder in ther-
mally insulating composites, an admixture for
viscosity modification in concrete, a modifier in
asphalt, or a retarder in the cement hydration pro-
cess.
▲ At this shore, micro-organisms
created thin adhesive films which
trapped suspended sediments to form
dome-shaped, thin-layered rock
structures known as stromatolites.

113
Biopolymers: Cement Replacement
Biopolymers as cement replacement in
construction
The potential performance of biopolymers as a
cementing agent can be demonstrated using corn
starch.5 Mixing corn starch with sand and water
and heating the mixture in a microwave or oven
results in a hardened solid material, named CoRn-
crete, with unconfined compressive strengths
reaching values up to 20 mega Pascal. Optimum
strength is obtained at a starch-to-sand ratio of 1
to 5 and a water content of 16 per cent.6 At room
temperature, the starch is poorly soluble in water,
however, when mixed with sand and water at the
optimum ratio, the mixture forms a viscous liquid
with self-compacting behaviour. The resulting
densely-packed aggregates are one of reasons for
the material’s high compressive strength after bak-
ing. During heating, the starch molecules dissolve
partially and form a gel, which glues the sand
grains together and hardens when dried out. Next
to the mixing ratio of sand, starch, and water, the
strength of the hardened CoRncrete depends on
the grain size distribution of the aggregate sand, as
well as the heating procedure and time.
Alginate is another biopolymer which can be used
as cementing agent. It is found in a wide variety of
seaweeds inhabiting temperate as well as cold
oceans.7 The advantage of using alginate is that the
seaweed from which it is extracted grows in the
sea and does not require land surface to be culti-
vated. Alginate also differs from starch in that it is
soluble in water. However, when mixed with dis-
solved calcium ions, it will form a gel as the multi-
valent ions form electrostatic bonds between the
long-chained alginate polymers. In trial experi-
ments, dry sand was mixed with 1 per cent alginate
powder, percolated with seawater, and simply
dried in open air. The resulting sand samples tested
at more than 800 kilo Pascal.
Biopolymers can also be grown within a material.
Percolating a solution of soluble substrates through
permeable granular aggregates activates indige-
nous bacteria to grow and produce inorganic min-
erals or extracellular polysaccharides (EPS).8 The
application of bio-mineralization in ground engi-
neering applications has been demonstrated at
large scale, using microbially-induced calcite pre-
cipitation (MICP).9 EPS has not yet been shown to
result in sufficient strength improvement to be
used as a construction material, particularly in wet
environments. However, it has been shown that
biofilms can sufficiently strengthen sandy soils to
reduce coastal erosion or suppress windblown
dust.10 Besides acting as adhesive component
themselves, biopolymers can improve cementation
indirectly by controlling the precipitation process
of inorganic minerals.
Materials cemented with biopolymers such as algi-
nate or corn starch are strong in compression as
long as they are dry. However, once exposed to
water, they weaken and disintegrate easily. Further
development is required to enhance the durability
of biopolymer-cemented materials. Another chal-
lenge is the scaling-up to construction applications.
Although the temperatures required to heat and
dry biopolymer-cemented materials are much low-
er than for the production of ordinary cement or
the fabrication of clay bricks, the heating procedure
so far cannot be applied at the scale of a building.
◀ Scanning electron microscopic
(SEM) image of biopolymers and
calcite crystals grown by feeding
nitrate-reducing bacteria within sand.

AGRICULTURE
114
▲ Large-scale biocementation experi-
ment.
▼ Biofilms can sufficiently strengthen
sandy soils to reduce coastal erosion
or suppress windblown dust.

1 Nutrient solution is pumped to the injectors
2 Injectors release nutrients into soil
3 Nutrients pass through soil
4 Depleted nutrient solution is pumped out
5 Nutrient solution circulates until specified values are reached
5
1
2
3
4
115
▲ Test setup of bio-mineralization
experiment.
1 Nutrient solution is pumped to the
injectors
2 Injectors release nutrients into soil
3 Nutrients pass through soil
4 Depleted nutrient solution is
pumped out
5 Nutrient solution circulates until
specied values are reached
▲
Biopolymers: Cement Replacement

AGRICULTURE
116
The sustainability of biopolymers generated from
agricultural resources such as corn starch is debat-
ed, as their cultivation requires a significant
amount of land, nutrients, and fertilizers. In that
respect, the exploitation of biopolymers from sea-
water organisms such as alginate seems especially
promising. Still, the industrial production of bio-
polymers for non-food applications competes with
the food market and may contribute to rising glob-
al food prices. However, recent advances in bio-
technology enable the extraction of biopolymers
from waste materials, such as the stems and roots
of plants, or composted sludge from wastewater
treatment plants.11 Even when combined, the quan-
tities of biopolymers generated from the sea-based
cultivation of alginate and an optimal exploitation
of land-based organic waste streams will not be
sufficient to cover the current consumption of ce-
ment. There is no doubt, however, that in combina-
tion with other alternative resources and a shift
towards a circular building industry, the cultivation
of biopolymers is one step towards a sustainable
society.
▲ Bio-mineralization in ground engi-
neering applications has been demon-
strated at a full scale, using microbi-
ally-induced calcite precipitation
(MICP).
◀ A setup of nozzles and pipes perco-
lates soluble substrates through the
ground to activate indigenous bacte-
ria in the ground.
▶ Full-scale field test installation.

117
ENDNOTES
1 United States Geological Survey
(2011), “USGS Mineral Program
Cement Report” (Jan 2011)
2 EIA – Emissions of Greenhouse
Gases in the U.S. 2006-Carbon
Dioxide Emissions. US Department
of Energy.
3 Mohanty, A. K., Misra, M. and Drza,l
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http://www.cybercolloids.net/
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November 2016
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Biopolymers: Cement Replacement