Building with earth - Gernot MINKE (1)

As with concrete, the maximum water
resistance of cement-stabilised soil blocks is
reached after 28 days. These blocks must
cure for at least seven days, and should not
dry out too soon. If not protected against
direct sun and wind, the blocks must be
sprayed by water while curing.
To hasten and enhance the curing process,
20 to 40 g sodium hydroxide (NaOH) can
be added to each litre of water. Similar
effects can be obtained with about 10 g per
litre of water of either NaSO 4 , Na 2 CO 3 and
Na 2 SiO 2 .
Lime
If there is sufficient humidity, then an
exchange of ions takes place in the loam
with lime as stabiliser. The calcium ions of
the lime are exchanged with the metallic
ions of the clay. As a result, stronger
agglomerations of fine particles occur, hindering
the penetration of water. Furthermore,
the lime reacts with the CO 2 in the air
to form limestone.
The optimum lime content for loam differs
and should be tested in advance in each
case. The explanations on p. 43 show that
if only a small amount of lime is added, the
compressive strength may be lower than
that of unstabilised loam.
Bitumen
In Babylon, bitumen was used to stabilise
mud bricks as early as the 5th century AD.
Normally, bitumen is effective for loam with
low clay content. The stabilising effect is
more pronounced if the mixture is compressed.
For that reason the bitumen is
either dissolved in water with an emulsifier
such as naphtha, paraffin oil or petroleum. It
is preferable to use a mixture of 4 to 5 parts
bitumen, 1 part paraffin oil and 1% paraffin,
which is prepared by heating to 100°C. Normally,
3% to 6% of this solution is sufficient
to stabilise the soil. After the solvent and
water evaporate, a film is formed that glues
the particles of loam together, thereby preventing
water ingress.
Soda waterglass
Soda waterglass (Na 2 O · 3-4 SiO 2 ) is a good
stabiliser for sandy loam, but it must be
thinned with water in a 1:1 proportion
before being added. Otherwise, microcracks
will occur which generate strong
water absorption.
Animal products
Animal products like blood, urine, manure,
casein and animal glue have been used
through the centuries to stabilise loam. In
former times, oxblood was commonly used
as a binding and stabilising agent. In Germany,
the surfaces of rammed earth floors
were treated with oxblood, rendering them
abrasion- and wipe-resistant. In many countries,
whey and urine are the most commonly
used stabilisers for loam surfaces. If
manure is used, it should be allowed to
stand for one to four days in order to allow
fermentation; the stabilisation effect is then
considerably enhanced due to the ion
exchange between the clay minerals and
the manure.
In India, traditional loam plaster (gobar plaster)
has a high content of cow dung, which
has been allowed to stand in a moist state
for at least half a day. This technique is still
in use. Investigations carried out at the BRL
showed that a loam plaster sample subjected
to the jet test (referred to in chapter 2,
p. 28) eroded after four minutes, whereas a
sample with 3.5% by weight of cow dung
began showing signs of erosion only after
four hours.
Mineral and animal products
In former times, it was quite common to
enhance stabilisation against water by
adding lime and manure, or lime and whey.
One traditional recipe, for instance, specifies
1 part lime powder mixed with 1 part sandy
loam, which is soaked for 24 hours in horse
urine, after which it can be used for plastering.
Obviously, lime reacts chemically with
certain ingredients of the urine, since one
the appearance of some fine crystals is
observable. The casein in urine and the
manure react with lime to form calcium
41
Improving the earth