Building with earth - Gernot MINKE (1)

blocks lie exposed to direct sun and wind,
Strength against abrasion
so that they dry out sooner, then their final
strength may be reduced by 20% com-
Experiments conducted at the BRL intended
Compressive strength (N/mm 2 )
pared with blocks kept covered with moist
stacking. If this moist cover is not possible,
the blocks should at least be protected from
direct sun and sprinkled with water several
times a day. When 10% cement is added,
this protection is of less relevance to the
to increase the strength of a rammed earth
sample containing 14% clay, 41% silt and
45% sand, and involving the addition of
soda waterglass, animal glue, low-fat white
cheese and lime, paraffin, paraffin-petroleum,
floor wax, and double-boiled linseed
final strength (Houben, Guillaud, 1984). If
oil, showed that an addition of 10% water-
pozzolana is added together with lime, an
glass produced the most resistant surface.
additional stabilisation effect is achieved and
However, several hairline cracks occurred,
Clayey
loam
Lime added (%)
Silty
loam
Sandy
loam
the quantity of lime can be reduced. Certain
volcanic ashes exhibit pozzolanic properties,
as do fly ash and ash of rice husk. Brick dust
allowing water to penetrate. (It may have
been possible to avoid this had the waterglass
been mixed beforehand with water
4.14
from low-temperature baked bricks also
exhibits slight pozzolanic properties, but
in a proportion of 1:1.)
The second highest strength was achieved
dust of high-temperature baked bricks from
by adding 5% linseed oil, whereby the sur-
4.13 Change in compressive
strength of loams
with the addition of
cement
4.14 Change in compressive
strength of loams
with the addition of lime
4.15 Change in tensile
bending strength and
compressive strength of
loam mortars and sand
with the addition of
cement
4.16 Change in compressive
strength of adobes
(clay 11%, silt 14%, sand
75%) with the addition
of cement
4.17 Compressive
strengths of loams and
sand with the addition
of 6% cement
industrial brick plants do not. An interesting
stabilisation effect is observed when clay,
chalk and quartz powder are mixed with
waterglass. This product, called geopolymer,
is derived from poly-condensation: a threedimensional
network, which occurs in an
alkaline state with the release of water. This
product may be extruded, pressed or
foamed with hydrogen peroxide (H 2 O 2 ).
Organic additives
The compressive and binding strengths of
Kaolinite can be significantly increased by
adding urea and ammonium acetate (Weiss,
1963). Weiss also suggests that the high
strength of porcelain comes from Kaolinite
soaked in putrid urine (which contains urea
and ammonium acetate). The tensile bend-
face was smoothened with a trowel during
curing, closing hairline cracks in such a manner
that the surface remained glossy. The
third-best solution was achieved by adding
5% low-fat white cheese and 5% lime.
Strength against abrasion can also be
increased with coatings. Here, it must be
kept in mind that the coatings must penetrate
deep into the material and must be
renewed periodically. Experiments show
that coatings and additional application of
floor wax increase abrasion resistance considerably.
A traditional German recipe that produces a
hard-wearing, strong surface is a coating of
oxblood sprinkled with Fe 3 O 4 , which is then
hammered into the loam surface. Coatings
of cow’s blood, cow’s bile and tar were also
ing force can be increased approximately
frequently used in former times.
4.17
10 to 20 times in this way.
Compressive strength (N/mm 2 )
Addition of fibres
Fibres are usually added to reduce shrinkage.
The oft-mentioned assumption that
fibres always increase compressive strength
Increasing thermal insulation
The thermal insulation of loam can be
increased by adding porous substances
is false. When fine fibres or hair are added in
such as straw, reeds, seaweed, cork and
small amounts, tensile strength – and there-
other light plant matter. Naturally or artificial-
fore compressive strength – is increased
ly foamed mineral particles like pumice, lava,
Sand 0- 4
Bentonite : Sand = 1 : 9
Kaolinite : Sand = 1 : 9
Silty loam : Sand = 6 : 4
slightly. The addition of cut straw, however,
has the opposite effect, as shown by investigations
carried out at the BRL (see table
4.20).
expanded clay, foamed glass, expanded
perlite and foamed plant matter like
expanded cork can also be added. Waste
products like sawdust, wood shavings, husk
47
Improving the earth