Paul Kirby wrote:

> Creating concrete requires the procesing of limestone. This is energy
> intensive. Carbon dioxide is also liberated  directly by the chemical
> process. (making areated concrete is more energy intensive even than
> "ordinary concrete" though this might be traded of through its improved
> insulating value)

In the ananlysis that I have read on Autoclaved Aerated Concrete the amount
of materials that are used in comparison to conventional concrete is one
fifth. In AAC the main ingredient on a volume basis is air. The process
works because it requires a small amount of aluminum paste, approximately
0.8%. This aluminum paste reacts with the lime (calcium carbonate) to
produced hydrogen gas. The liberation of the hydrogen gas creates a 'cake'
with millions of tiny air bubbles.

In a modern home with a basement, there is more concrete used than in an
entire home made from AAC. The blocks of AAC can be cut with conventional
carpentry tools. This means that it is relatively easy to construct the
home. In fact the just about any one can build the modern AAC home, even if
they were to only work on the weekends and eveninngs. The labour cost of
constructing the AAC home is 50 % less than the modern wooden home.

Actually studies being conducted by the US DOE indicate that the energy
inputs of a AAC home compared to a conventional wooden home are 5 % less.
This finding is consistent with other studies in Europe where AAC has been
used for over 50 years. In Germany the use of AAC is common.

There are more energy savings in addition here as well. The AAC home is
infinitely 're-useable' and can easily last for 100's of years. The wooden
house on average here in Canada has a life time of 60 years. Some wooden
houses last as long as 100 years, or even more, but the average is only 60
years. The other thing that must be considered is the impact of forest
destruction in the primary rainforests, and other forests of the world on
species, climate, soil and water quality.

Forest clearing and destruction causes up to 40 % of the additional sources
of Green House Gases that are emitted by humans. This contribution is far
less than any C02 that is released by AAC. In fact the only way that AAC can
be released into the atmosphere by the AAC is if an acid water source comes
in contact with it. Limestone that is currently in use would not increase
substantially with AAC because it only uses 20 % of that normal concrete
uses. In fact all kinds of things can be made with AAC: swimming pools,
patios, etc., so that net benefit of AAC goes beyond what is currently
extracted. Today there are many uses of ceramics in the world as well:
ceramic tiles on roofs in the South American countries, ceramic tiles in
floors, ceramic panels on the outsides of houses in Japan.

One other source of sand that is used in AAC is 'flyash' from coal fired
electric plants. In the US there is an industry devoted solely to disposing
of this waste that costs the consumer of electricity in the US about $1
billion per year. There is enough fly ash to build over one million homes
per year in the US. This amount would meet all the new homes constructed
this year, and probably more.

>
> In the UK some of our limestone reserves are in, or adjacent, to National
> Parks! The aggregates used in concrete also need to be quarried and on our
> small island we have difficulty in deciding which landscape to sacrifice.

If  the UK was required to produce all of its' demand for domestic wood
consumption for building materials, then the forests of the UK would simply
vanish in about 5 years. There is no way that the ecological footprint would
be sustainable if that was the case. I am sure that the amount of lime in
AAC would actually be reduced in terms of the ecological footprint required
if the country stopped using wood as the primary building material in homes.

At the current rates of consumption worldwide the demand for wood goes up by
70 million cubic meters per year (at least). That means that a forest
roughly the size of the British Columbia forest (commercial forest) would
have have to be cut down to meet that demand. British Columbia is the
largest softwood producer and exporter in the world. Now multiple that
figure by 10 years, and we can easily see that in about 25 years that there
will be no primary forests left in the world. The tropical forests of the
world are vanishing, and most of the boreal forests are already committed to
'sustained yield'. The forests of Europe are in dreadful decline. The oak
and deciduous  forests of England are now 5% of their original size.
Flooding and soil erosion is common all over Europe because of deforestation
and forests that lack soils with sufficeint moisture retention capacity.
Species extinction and habitat loss in the forests of Europe are
dissappearing at rates of the 'eigenblinkt' and the we sit by and watch as
it happens.

The most wasteful use of wood is in paper products that are turned into
newsprint. The bulk of any newspaper is advertising. This adverstising is
never read by anyone, and the paper often ends up in landfills, or is
burned.

> Timber is seen to have advantages because it is a renewable resource and
in
> the long term is carbon neutral. (From growth to use to decay). (and trees
> look prettier than a mechanised hole in the ground (even the rigid rows of
> conifer plantations))

It is partially renewable. The common experience with forestry is after
about 3 rotations the productivity of the forest goes into a decline.

I believe that there are 'sustainable' models of forest management that have
longer rotations of 250-350 years and do not utilize clearcutting. There is
a body of literature on this (cf. Chris Maser, Soule, etc.) Recent studies
in the Pacfic Northwest indicate that natural selection forestry carried out
in the Cascades demonstrates that longer rotations of 250 years result in
productivity rates of up to 6 cubic meters. This means simply that over a
period of 250 years or so, that the amount of biomass wood would be removed.
Each year in principle, about 1/250th of the forest would be cut. This is
sustainable, not a rotation of 80 years.


> I am sure that John could tell us that sustainable forestry is problematic
> (lack of bio-diversity in plantations is one) but  the alternatives are
not
> cost free either. While individuals involved in design might want to make
> ethical decisions (re sustainability)  it is diifficult to do so in tha
> face of this complexity.
>
> Regards Paul K