Cob: cob:water resistant plasters Natural Cements

[Charmaine R Taylor]

Here is a note from Mike Wye in the UK:

 Parker's (roman cement) was patented in 1796 by Parker, and was based
on burning
clay bearing limestones - it was a form of eminently hydraulic lime.

 The Romans used burnt forms of clay such as volcanic ash, crushed brick

tile not unburnt clay as burning makes highly reactive kaolins which
react
with the lime to make cemetitious compounds.  The general term for such
a
 reactive burnt clay is a pozzolan.

If you make a 3/1 mix of coarse sand to lime putty you can change it
from a relatively porous, low compression strength material to a virtual

concrete  depending on how much pozzolan you choose to add.

> Nowadays many commercial pozzolans are very reactive and react with
their
> own weight of lime, so for many lime renders and mortars you don't
need
to
> add a lot, 10-15% by volume is typical in English conditions.
> We supply a pozzolan called Metastar which is available in the US and
> Europe.
> Mike Wye
> www.mikewye.com




To which Charmaine adds:

When the Romans couldn't get good ash they did dig up clay in the fields
and burn it for making their lime based cements. Roman cement is a
generic term mostly used to mean clay bearing additives with lime.