Building with Earth: Design and Technology of a Sustainable Architecture

blocks lie exposed to direct sun and wind,

so that they dry out sooner, then their final

strength may be reduced by 20% com-

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

final strength (Houben, Guillaud, 1984). If

pozzolana is added together with lime, an

additional stabilisation effect is achieved and

the quantity of lime can be reduced. Certain

volcanic ashes exhibit pozzolanic properties,

as do fly ash and ash of rice husk. Brick dust

from low-temperature baked bricks also

exhibits slight pozzolanic properties, but

dust of high-temperature baked bricks from

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 three-

dimensional 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-

ing force can be increased approximately

10 to 20 times in this way.

Addition of fibres

Fibres are usually added to reduce shrink-

age. The oft-mentioned assumption that

fibres always increase compressive strength

is false. When fine fibres or hair are added in

small amounts, tensile strength – and there-

fore compressive strength – is increased

slightly. The addition of cut straw, however,

has the opposite effect, as shown by investi-

gations carried out at the BRL (see table

4.20).

Strength against abrasion

Experiments conducted at the BRL intended

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-petrole-

um, floor wax, and double-boiled linseed

oil, showed that an addition of 10% water-

glass produced the most resistant surface.

However, several hairline cracks occurred,

allowing water to penetrate. (It may have

been possible to avoid this had the water-

glass been mixed beforehand with water

in a proportion of 1:1.)

The second highest strength was achieved

by adding 5% linseed oil, whereby the sur-

face was smoothened with a trowel during

curing, closing hairline cracks in such a man-

ner that the surface remained glossy. The

third-best solution was achieved by adding

5% low-fat white cheese and 5% lime.

Strength against abrasion

increased with coatings. Here, it must be

kept in mind that the coatings must pene-

trate deep into the material and must be

renewed periodically. Experiments show

that coatings and additional application of

floor wax increase abrasion resistance con-

siderably.

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

frequently used in former times.

Increasing thermal insulation

The thermal insulation of loam can be

increased by adding porous substances

such as straw, reeds, seaweed, cork and

other light plant matter. Naturally or artificial-

ly foamed mineral particles like pumice, lava,

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

Lime added (%)

Clayey

loam

Silty

loam

Sandy

loam

Compressive strength (N/mm

2 )

4 .13Change in compres-

sive strength of loams

with the addition of

cement

4 .14Change in compres-

sive strength of loams

with the addition of lime

4 .15Change in tensile

bending strength and

compressive strength of

loam mortars and sand

with the addition of

cement

4 .16Change in compres-

sive strength of adobes

(clay 11%, silt 14%, sand

75%) with the addition

of cement

4 .17Compressive

strengths of loams and

sand with the addition

of 6% cement

4 .17

4 .14

Sand 0 - 4 Bentonite : Sand = 1 : 9Kaolinite : Sand = 1 : 9Silty loam : Sand = 6 : 4

Compressive strength (N/mm

2 )