We are pursuing an alternative approach, which seems to
us to be a more ecologically efficient solution, since it is closer to the
earth, and seems to offer greater advantages and less cost. The basic idea is to
use a preparation of hemp hurds and lime to create a kind of cement
("Agstone") which may be used to construct floors and walls which are
stronger and more durable than cement or concrete, yet less brittle and much
lighter. It also exhibits superior properties of insulation, waterproofing, and
fireproofing, and can effectively replace not only the structural elements of a
wall, but it can stand alone without any additional exterior or interior wall
covering. In addition to offering all of these benefits, the material is easy to
work with, and can end up saving considerable expense when compared with
traditional building methods.
Drums made of HEMPSTONE by
Our research with this approach began with the use of
hemp hurds and lime, but since we don't have enough hemp hurds here to work
with, we have adapted the idea to the use of other materials: rice straw (a good
choice because of the high silica content like hemp hurds) or just about any
available chipped up weeds, brush, straw, or other agricultural waste. In the
last couple of years we have been implementing these ideas through experimental
While there is a wide variation of possible recipes
that will all work, it is useful to have a basic understanding of the chemistry
involved with the use of lime. When lime (hydrated lime; calcium hydroxide) is
used for an exterior coat, it hardens up by simply combining with the carbon
dioxide in the air to change back to the limestone from which it was derived (by
burning). However, interior lime hardens up by a different chemical reaction: it
combines with materials high in silica to harden up into stone. The classic way
to do this is to combine lime with clay. This is "Roman cement," and
it is really the origin of the whole idea.
Djembe made of HEMPSTONE by
Other sources of silica are also used, but there seems
to be no agreement about the chemistry involved. Sand is very high in silica,
but because of the particle size, it is less significant than clay. However, I
have experimented with mixes higher and lower in sand, and my personal
experience is that sand is an essential ingredient in the mix. Mixes with little
or no sand are just not as hard or durable as mixes made with plenty of sand.
Then there is the question of the silica component in the aggregate, the
agricultural waste, weeds, chips, or straw. Hemp hurds are high in silica, and
that is probably one of the reasons why the hemp hurds plus lime formula has
been such a success in France. My personal experience confirms that hemp hurds
perform better than just about any other similar aggregate material. Rice straw
is also very high in silica, and, while I have not had personal experience using
rice straw, I am confident that it would turn out to be an excellent material
for this purpose.
Then there is the sand and clay. This ideally should
come altogether if you can find a good subsoil that has a high percentage of
clay mixed with sand and silt (no organic matter).
Speaker backing made from
hempstone by Zellform GmbH
Finally, there is Portland cement. Many purists
recommend eschewing the use of Portland cement altogether for technical and
religious reasons. I will avoid this controversy here except to mention that the
technical arguments suggest that too much Portland cement creates problems, but
small amounts of cement mixed with lime avoid those problems. We use about 10%
cement in our mix because it works: it helps it to set up quickly, and it
assists the chemical reaction with the lime. This small a concentration avoids
all of the objections to its use other than its supposedly high environmental
cost of manufacture, in terms of fossil fuel use, and the release of CO2
into the atmosphere. Anyway, I have experimented with and without cement, and I
have found that using only lime for a binder will usually set up hard
eventually, but it may take quite a long time to do so. We haven't yet found a
really good local source of clay; perhaps if we did we could effectively
eliminate cement from our mix altogether.
My latest addition to the recipe is wood ash. The main
reason I want to use it is because there is only so much wood ash I can add to
my compost, and I want to use it up. Also, lime is highly alkaline and this high
pH is required for the chemical reaction to take place, so it can't hurt to use
Digeridos made from
hempstone by Zellform GmbH
In addition to all of the above, we also use rocks and
gravel, especially in the lower courses of our structures. It just makes good
sense to use up these rocks where they can do no harm. The more rocks you use
up, the less Agstone you will need to use.
Our process allows the use of a much greater quantity
of agricultural waste (chips, weeds, or straw) than other methods. We keep
changing our recipe, experimenting with new ideas, but the following should give
you a very sturdy mix:
For purposes of working out formulas, I always start
with one bucket of chips/weeds/straw and call that 10 parts, by volume (we find
that formulas measured by volume are much easier to reproduce than formulas by
weight). Our mix is as much as 50% chips, weeds, or straw. We use this material
because it is freely available and because all those pieces strengthen the mix.
We use a random mix, striving for some variation in the size of the pieces. If
broken glass is used, that would be part of the 50%.
We used to begin by adding the lime to the chips or
weeds, but we have found that it is better to mix up the lime, clay, sand and
cement with water and then add the chips or weeds. We recommend three parts of
lime (15%), a total sand/clay portion of about five parts (25%), and about two
parts Portland cement (10%).
There you have it -- this mix works very well for us,
but you can modify it in many directions. You can add chipped up paper and
cardboard to extend the mix, but it makes it softer and weaker. You could add
more sand and clay, more lime, less cement, broken bottles, floor sweepings, old
clothes (shredded), broken toys . . . We pour on site in wooden forms,
which are removed the following morning, but you could also press bricks and
mortar them together with the above mix, leaving out the chips, weeds, and/or
You can apply a top coat of lime plaster, which is just
lime water with fine sand, to smooth out the surface. For a really smooth inner
surface, you could add gypsum to the plaster, along with any natural
HEMP SHOWS OFF
ALL ITS UPRIGHT QUALITIES:
Traditionally Linked In The Public Mind To
Cannabis, Hemp In Its Industrial Form Is A Valuable Building
Material. Sally Smith Considers Its Surprising Virtues:
The rain is relentless and an insidious
easterly penetrates every joint. We're standing in Ralph
Carpenter's unheated workshop.
Yet neither chill nor damp seeps from the
quarry-tiled floor. Our feet are perfectly warm. Touch the
bare walls and instead of a moist clamminess there is a
"Hemp," says Carpenter, with
confident satisfaction. "It's under the floor tiles, it
lines the roof, the walls are made of it. It's simply a superb
insulator; warm in winter, cool in summer."
An architect practising from Hartest, a
Suffolk village south of Bury St
Edmunds, for the past 15 years, he has been pioneering the use
of hemp as a building material: the core of the stem a
replacement for brick and concrete, the fibres as insulation.
He has put it into the fabric of a listed medieval building
and built an extension to his own house with it.
Now it is being studied by the Building
Research Establishment (BRE) in a comparative housing project.
For Carpenter working with hemp has involved seeking out
specialist suppliers, relearning old building techniques
and devising new ones, understanding its nature and the
materials that are employed with it, and even finding the one
brand of cement-mixer capable of getting it to the correct
He lists hemp's benefits: environmentally
friendly (no toxic by- products and can be fully recycled);
thermally efficient; lower fuel costs; no condensation;
absorbs sound; non-flammable.
"Above all it's infinitely renewable
because you just grow
it. To build our extension took five tonnes and you can
crop five tonnes on one hectare in three months. It doesn't
need fertiliser. It outgrows weeds and is not prone to pest
"At the moment at Pounds 550 a square
metre, it is between 5 and 10 per cent more expensive than
brick; but once the carbon taxes that will have to be paid on
conventional building materials clock in, hemp will
become more cost-effective," Carpenter says.
For building, the core of the hemp is
chopped and treated in a secret
petrification process developed in France and mixed on site.
Construction begins with a timber frame of
wood of any quality. Uprights are tied in to a brick plinth
set in shallow footings. Plywood panels are lightly attached
between the uprights to form a mould into which the hemp is
A day later the panels come off and between is
a solid infill, strong enough to hold the timbers firm without
bracing. The next lot of panels is attached and the process
The result is so flexible, strong and durable
that the footings under the
plinth can be shallow, usually 18in deep, half the depth of
footings. The trench is lined with sand, stone, brick rubble
or any waste building materials. There is no concrete.
"We eliminate the need for cement," he explains.
Carpenter's first hemp building, the
conversion of a derelict garden shed, was for former Beirut
hostage Terry Waite who lives nearby.
The extension to his own house and other small
schemes further perfected the processes. Then came the
renovation of a wattle and daub building for Bury Town Trust
and the discovery that hemp was a more than adequate
substitute for traditional daub made with cattle manure.
English Heritage, at first sceptical, was soon
convinced as was the
specialist who had been collecting her materials from the
market. Some panels have been repaired, others entirely
replaced, but it is impossible to say which from the texture
Hugh Belsey, curator of the Gainsborough
Museum in Sudbury, lives there. Initially concerned by wetness
in the walls, he says they soon dried out and the ancient
structure is warm and dry and not in any way prone to
condensation, which can be a problem with renovated old
"And I have been pleasantly surprised by
the heating bills, far lower than I had expected," he
says. St Edmundsbury Borough Council, which grant-aided the
trust in saving the building, is continuing to develop the use
of hemp with Suffolk Housing Society.
A pair of semi-detached houses are being built
of hemp alongside another pair, identical in size and design
but in conventional brick and block. On the ground floors hemp
will also be laid directly over the ground without a membrane
and will be used elsewhere for sound and thermal insulation.
Research - monitored by the BRE - will cover
financial and environmental costs in construction and in use
over a minimum two-year period. By then hemp may have
succeeded in winning over the doubters.
Hope for unloved cousin of cannabis
Hemp is in your tea-bags, disposable nappies,
jeans and "paper" handkerchieves. Plumbers put it
into radiator joints. The oil from the seed is said to be the
most nutritious of all oils.
In eastern Europe it is still widely grown for
sail-cloth and ropes. But it
was banned in the US because of its threat to the paper
Britain followed suit.
The convenient argument against it was its
narcotic properties, although it has next to none unlike its
cousin cannabis. Nonetheless, to grow it today farmers must
have a Home Office licence.
Pubdate: Sat, 31 Mar
2001 Source: Financial Times (UK) Copyright: The Financial
Times Limited 2001
Author: Sally Smith
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