Building with Earth: Design and Technology of a Sustainable Architecture

Even after several months, small increases in
strength may be observed. A certain
amount of humidity is essential to this cur-
ing process, so the loam or earth elements
have to be sheltered against direct sun and
wind.

4. The optimum water content is raised
   with the addition of lime, while the density
   at this new optimum level is less than that
   without lime (4 .12).
   Results of experiments performed at the
   BRL (4 .13) show that the compressive
   strength of a highly silty loam containing
   12% clay, 74% silt and 14% sand, and hav-
   ing a compressive strength of 50 kg/cm^2
   without cement, decreases with the addi-
   tion of small quantities of cement. The origi-
   nal compressive strength is reached again
   with the addition of 2% cement.
   As can be seen in 4 .14, this original strength
   is reached only at 4% when adding lime.
   In this case, it decreases again after 6% of
   lime stabilisation.
   Even more significant is the reduction of
   compressive strength while stabilising lean
   mud mortars, as shown in 4 .15on the right.
   The left side of the same figure shows the
   corresponding changes in tensile bending
   strength. The values of the dry and the wet
   compressive strengths of handmade adobes
   with varying percentages of cement content
   are shown in 4 .16.
   Investigations at the ENTPE show that test-
   ing pure Kaolinite with 4% cement increases
   compressive strength, while with Mont-
   morillonite, the same amount of cement
   shows a decrease in strength. With the
   addition of 4% lime and 2% cement, the
   compressive strength of both types of clay is
   increased by nearly 100% (Oliver, Mesbah,
   1985). It should be noted that these tests
   were done with optimum water content
   and with pure clay. However, in actual prac-
   tice this increase may not be so high, as
   loam used in construction usually has a clay
   content of 5% to 15% and may not be used
   with optimum water content.
   Results of tests conducted at the BRL with
   handmade adobes are shown in 4 .17and
   4 .18. Here, four different mixtures of sand

and clay were tested with the addition of

6% cement and lime, respectively. It is inter-

esting to note that the results were nearly

the same in the case of sand for plastering

and sand with Bentonite. By adding lime to

these mixes, the compressive strength of

Kaolinite loam is even lower than that con-

taining sand (4 .18).

From these investigations, we derive the

following guidelines:

1. Loam with high Kaolinite content should
   be stabilised with cement (and not with
   lime).


2. Loam with high Montmorillonite content
   should be stabilised with lime or with a
   mixture of lime and cement in the ratio 2:1
   (and not with cement).


3. Strong compaction increases the com-
   pressive strength of Montmorillonite signifi-
   cantly. This effect is significant in Kaolinite.
   CRATerre suggests appropriate stabilisers

on the basis of liquid limit, plastic limit

and plasticity index (4 .19), not taking into

account the type of clay minerals (CRATerre,

1979).

When adding cement to loam, the mixture

should be used immediately, since the

setting of cement starts at once. If the mix is

allowed to stand for several hours before

being pressed into soil blocks, the compres-

sive strength of these blocks may be

reduced by as much as 50%. However, if

lime is added, this time lag has no negative

influence on the final strength. If less than

5% cement is added, the drying process

affects the compressive strength. If the

46 Improving the earth

Clayey loam plaster

Silty loam plaster

Sand

Tensile bending strength (N/mm^2 )

Cement added (%)

Dry strength

Wet strength

Compressive strength (N/mm^2 )

Clayey loam plaster

Silty loam plaster

Sand

Compressive strength (N/mm^2 )

Cement added (%)

Cement added (%)

Clayey

loam

Silty

loam

Sandy

loam

4 .16

4 .13

4 .15

5.0

4.0

3.0

2.0

1.0

0.0