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GSBN:re: Which type of sand?



Hello Andre & all,

The many opinions, experiences, lost knowledge etc. make this and many
other rendering issues very confusing however, please find our
experiences "re: sand for rendering" for what it is worth as follows

We agree that River sand (round sand) is not well suitable for rendering
strawbale walls (with either lime or clay). This is especially the case
when applying the base (brow, main, second) coat as the render mix gets
very heavy with the River sand and does not 'interlock" as well as with
Brickies (crushed) sand. The heavy (River sand) mix may slide down the
wall, to avoid this, the coats would have to be applied thinly,
requiring more than one layer for the base coat hence: extra rendering
work.

We use Brickies (crushed) sand which contains a large variety of
aggregate sizes anywhere between 0.2 - 3mm, great to built up the base
coat and great for interlocking with clay or lime respectively. Brickies
sand here in Australia is available in two qualities depending on the
proportion of clay it contains. It is either 70:30 sand:clay or 80:20
sand:clay, we prefer the former.

For the much thinner final or finish coat (this is only 3-5mm thick) of
either lime or clay render, we use washed Beach sand, works beautifully
for a smooth finish and great surface effects can be achieved if
worked/polished with sponges, rags or sheep skins. Because this coat is
only thin we find the fine sand works as the mix is not heavy and spread
on only thinly.

We usually spray the first and second coat, the final coat is applied
with trowel etc. for a smooth finish.

By the way, we only use slaked quick lime, stored in 44 gallon drums,
the putty covered with water. We found this to be the best quality lime
for exterior render/finish coats, it is flexible and holds up great to
most weather conditions.

It has to be noted that we mostly use processed engineered clay which
has the same properties, quality and appearance each time. We also use
different types of this engineered clay, one for the first, one for the
second coat and a superior quality clay with a beautiful natural colour
for the finish coat. Using engineered or bagged clay would probably make
rendering life much easier for everyone as you would know what you get,
how the material behaves and how it will look. In Europe it is actually
possible to buy bagged clay (for example from Topleem), it is expensive
but easy and ready to use and there is no need to experiment over and
over again, saving time & money (and relationships) in the long run.
Using engineered clay may also be the answer for building officials as
we could show many examples of how the material behaves and it would be
easier to conduct research on strengths, durability etc. if the same
material was used each time. Will leave it here as the bagged clay issue
is one very close to our heart, meaning this mail could get very long
and drift from the sand issue.

Best wishes, Frank & Ingrid


Yesterday-Today-Tomorrow, Strawbale Construction
Ph. & Fax: + 61 2 4869 3302 Mobile: 0408415806
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