Building with earth - Gernot MINKE (1)

Additives
In some industrialised countries, expanded
clay is a low-cost and easily available additive.
It has a bulk density of about 300
kg/m 3 , and is produced by burning loam in
rotary ovens at temperatures up to 1200°C
without any other additive for foaming.
Foaming occurs due to the sudden heating,
which causes the water of crystallisation
and the pore water to evaporate, creating
an expansion in the mass (similar to making
pop-corn). The surface of these expanded
clay balls melts and is sintered. Nearly all of
the pores in these expanded clay balls are
closed, and are therefore unsusceptible to
water and frost. The equilibrium moisture
content by volume is only 0.03%.
Foamed glass has characteristics similar to
expanded clay, but has a lower bulk density.
It can be produced by recycling glass with
additional foaming agents.
Expanded perlite is produced from volcanic
rock (found in Europe, on the Greek island
of Milos and in Hungary). It contains 3% to
6% chemically bound water, and when it is
heated up suddenly to 1000°C, this water
evaporates and enlarges the former value
15 to 20-fold. The bulk density may be as
low as 60 kg/m 3 , the k-value is 0.045
W/mK. The vapour diffusion resistance is
about 2.7. The specific heat is 1000 J/kgK.
With a material of bulk density 90 kg/m 3 ,
a k-value of 0.05 W/mK is achieved. The
chemical composition of expanded perlite
is: SiO 2 (60-75%), Al 2 O 3 (12-16%), Na 2 O
(5-10%).
Expanded lava is similar to expanded perlite
of volcanic origin, except that its bulk density
is higher.
Pumice is a naturally porous stone that has
already been “expanded” during its formation
in a volcano. Its bulk density usually
varies from 500 to 750 kg/m 3 .
Mixing
While forced mixers are usually required
to produce loam mixtures (see chapter 3,
p. 37), lightweight mineral loam can be produced
in an ordinary concrete mixer. There,
aggregates can be placed in advance and
the loam slurry poured over it. The mix is
ready in three to five minutes. The slurry
needs to have a rich clay content and binding
force. The production of loam slurry is
described in chapter 3, p. 38.
Grain size distribution
The grain size distribution of mineral aggregates
affects the properties of lightweight
mineral loam. For example, a density as low
as 500 kg/m 3 can be reached with expanded
clay fractions of 8 to 16 mm diameter.
The quantity of loam slurry has to be
designed so that the volumes between
aggregate particles are not completely filled,
that is, the aggregates are only glued
together at points of contact. This density
of 500 kg/m 3 can be reached if 2.5 parts of
loam are added to 12 parts of expanded
clay (8 to 16 mm). However, blocks of this
mixture have a low edge and surface rigidity.
A stronger mixture is obtained with 24
parts expanded clay (8 to 16 mm), 5 parts
expanded clay (1 to 2 mm), and 5 to 7 parts
loam. The density reached by this mixture
will be 640 to 700 kg/m 3 . To achieve higher
density, expanded clay fractions 4 to 8 mm
can be chosen, adding enough loam to fill
all spaces between the aggregates. In this
case, it is advantageous to thin the loam
with coarse sand.
Handling
Lightweight mineral loam, unlike lightweight
straw loam, can be poured or even pumped
if the mix is chosen accordingly. The methods
of preparing and handling this mixture
are explained in greater detail in chapter 10.
Thermal insulation
The thermal insulation properties of lightweight
mineral loam depend mainly on its
density and are equal to that of lightweight
straw loam if the density is higher than 600
kg/m 3 . For mixtures below 600 kg/m 3 , the
thermal insulation properties of lightweight
mineral loams are somewhat better than
those of lightweight straw loams, since
straw has a higher equilibrium moisture
content, and therefore more moisture,
50
Improving the earth