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Kiko Denzer on Art

Cob Cast Earth building- long

crtaylor tms at
Sat Jul 11 17:09:32 CDT 1998

Cross posted to Earth Friendly & COB list

Hello to all, I found this site on CAST EARTH building several months ago,
and thought I'd share some information since earth/fibrous bricks area hot
topic. . It's not a DIY method, but there is good info visivis adding
gypsum and other stength builders to earth blocks etc.  vist the web page
for photos etc, looks like rammed earth without the ramming! (gotta pay big
$$ to train & be a licensed contrator to do cast earth)  But with all those
V8 motors reving up I bet somedoby ( Joanie McNiece) takes a crack at this
method in their backyard.

Cast Earth as a Building Material

Traditional adobe mud can be poured into a wall-sized form and left in
place, but has no
strength when wet and will not dry in the forms. An immense inventory of
forms would be
necessary for construction of multiple buildings. Adding portland cement to
the slurry results
in major strength loss , and cracking as the product dries.

When walls are poured with the Cast Earth mixture, the forms can be removed
on the same
day as the pour, since the wet material is strong enough to support a
complete wall. Because
of the cementitious nature of the gypsum, its slight expansion on setting
and its ability to
inhibit clay shrinkage, cracking is not a problem. Consequently, it is
possible to use a much
wider range of soils than have historically been employed for
earthbuilding. Steel reinforcing
is not used in Cast Earth.

Because calcined gypsum is cementitious, soils for building with Cast Earth
are not restricted
by a criterion of natural cohesiveness; that property is provided when the
gypsum lattice sets.
Thus a much wider range of soils is available than can be used in
traditional adobe or
rammed earth. The principal determinant for soil suitability is fines
content, which
determines water demand.

Historical Uniqueness:

It seems amazing that calcined gypsum has never been incorporated in an
application, since its benefits are profound. Lack of understanding of
retardation and
operating techniques of working with this material may have discouraged earlier

Until about fifty years ago, laying adobe bricks was low cost and there was
no impetus to
develop a poured wall system of earth construction. Inexpensive wood
framing has
dominated the light construction business since that time, but recent
developments are
changing the cost of lumber.

Today, the custom earthbuilding field is dominated by small contractors and
who understand and appreciate the superior structural integrity, mystique
and the energy
and environmental advantages of traditional adobe and rammed earth
construction. These
are people who have decided to build with earth and are willing to pay for
a higher quality
home. Their efforts result in houses which are beautiful and functional
representatives of an
art which has proven its worth for centuries. Many low income families make
adobes and
build their own crude adobe homes. Since they do not place a value on their
labor in this
context, this is for them a low cost housing alternative.

Very little has changed from ancient to modern earth construction. A
limited amount of
obvious mechanization has been introduced in making adobes or in filling
and tamping
rammed earth walls, but these changes are peripheral and only mildly reduce
the great labor
intensiveness of the basic operations of wall building.

Cast Earth is the first breakthrough technology in earthbuilding. A
fundamental change in
wall chemistry is introduced, which permits substitution of machinery for
manpower in the
major labor consuming crafts. Other benefits are described throughout these

How gypsum enables Cast Earth's breakthrough:
  Technical Specifications

Gypsum which has been calcined (gently heated) is a common and inexpensive
mineral. It has cement-like properties, but unlike Portland cement, its
strength is not
destroyed by fine soil particles. This unique binder allows Cast Earth to
set rapidly, have
sufficient wet strength to support itself, and dry to a much higher
strength without cracking
and shrinking. Very importantly, it does these things at an unexpectedly
low content, typically
10% to 15% of the total mass of the Cast Earth.

The strength developed in a Cast Earth mix depends on a number of different
variables. In a
typical mix design, the mix will be targeted to have a wet compressive
strength of at least 50
PSI, and about 600 - 700 PSI when dry. The actual compressive weight at the
base of an 8 ft
wall is only about 10 PSI, so even in the freshly set wet condition, there
is a large margin of

At such a target composition, Cast Earth's dry compressive strength is
about the same as high
quality adobe or rammed earth. However, its tensile strength (Modulus of
Rupture) is
considerably greater than adobe. Cast Earth consistently tests to about 300
PSI for Modulus
of Rupture, or two to three times the usual values obtained with adobe.

A typical house of about 2,000 square feet will require around 10 to 20
tons of calcined
gypsum, depending, primarily, on wall thickness.

Gypsum Properties

    What makes the Cast Earth process work is the unusual chemistry of gypsum
    with heat and water.

Gypsum reactions with heat:

Natural gypsum is a crystal of calcium sulfate in combination with two
molecules of water.
When it is carefully heated in the process of converting it to calcined
gypsum, exactly three
fourths of the water is driven off:

CaSO4x2H2O = CaSO4x1/2H2O + 1 1/2 H2O

The chemical name of the dry product containing one half molecule of water
is calcium
sulfate hemihydrate. In these Web pages, we refer to it as calcined gypsum.
In a very pure and
milled form, it is the well known Plaster of Paris used in casting,
normally at a high
concentration and with little if any aggregate.

Calcined Gypsum's reaction with water:

This seemingly simple and mundane product has an unusual property. When it
is mixed with
water at ambient temperatures, it recombines with the same amount of water
lost during
calcination, and sets to form a strong gypsum crystal lattice:

CaSO4x1/2H2O + 1 1/2 H2O = CaSO4x2H2O

The setting property of calcined gypsum is uncommon and does not occur to
such an extent
with most other crystals when they rehydrate after loss of water. It
imparts a very useful
functionality to an otherwise common and uninteresting material.

Charmaine R. Taylor
 Taylor Publishing & Elk River Press
PO Box 6985 Eureka CA 95502  1-888-307-7650
'Books for people who want to build it themselves'