Civil Engineering Reference
In-Depth Information
limewashing becomes a surface decoration
rather than a protection.
like other limestones, but it is limestone
that contains a lot of clay and aluminium
silicates. These 'impurities' give the lime the
setting quality, depending on the amounts
present. It is possible to create artificially
a hydraulic lime by adding what are called
'pozzolans' to pure lime, so-called because
the Romans first discovered it by adding
ground-up volcanic dust from a place in
Italy called Pozzuoli. Many different cultures
around the world have used lime renders
and mortars, and have added their own
pozzolans: ground-up brick dust is a favour-
ite that works well. (Clay alone would not
work, because the clay minerals need to be
burnt at high temperatures first).
If you do need to patch in a section of
render for any reason, take out any loose
material, dampen down the surface of the
hole, and apply fresh render in thin layers
no more than 12mm (½”) thick, working
well into the sides. The patch will adhere
well to the rest of the render because it's
all the same stuff. Limewash over the top
when done. Old render can be pounded up
and used as well as or instead of sand for
aggregate in a new mix.
Hydraulic limes
Hydraulic limes on the whole are not suitable
to use on strawbale buildings because the
set makes them too rigid (like cements) and
their breathability is also reduced. Many
builders are tempted to use them because
they seem to be more like the cement that
they are used to - because they are powdered
- but in fact they have very different proper-
ties. They are sold as NHL (with a number),
which stands for natural hydraulic lime, and
they come in three strengths: NHL 2 (feebly
hydraulic), NHL 3.5 (moderately hydraulic)
and NHL 5 (eminently hydraulic). The higher
the number, the stronger the lime. The only
naturally occurring hydraulic lime available
in the UK is manufactured by Singleton
Birch in Lincolnshire, made from a Blue
Lias limestone, either NHL 2 or NHL 3.5.
St Astier, a French company, and Castle, a
German one, import their hydraulic limes
from France into the UK and Ireland and
offer all three strengths.
These should not be confused with hydrated
limes (see page 99), although they also
come in a bag as a white powder. Hydraulic
limes have a 'set' like cement does; that
is, they go hard to the touch before they
have carbonated. A feebly hydraulic lime
(the traditional terminology for a weak
hydraulic lime) may set in two days, but
an eminently hydraulic lime (a strong one)
may set in a few hours. The set is not the
same as carbonation, and these limes will
still carbonate over a long period. Eminently
hydraulic limes are used for such things as
the building of lighthouses and bridges,
because they will set under water and are
not affected by being in water, and also for
limecrete floors (see opposite), because
of their strength. Some of the strongest
of them are known as Roman cements,
but they are not the same as what we call
cement today.
Hydraulic limes would be used below damp-
proof course level and above the eaves, and
fat limes (made from putty) elsewhere. In
Hydraulic limes are made from naturally
occurring limestone, burnt in a kiln just
