If you want to be picky, then the word pozzolan only applies to
volcanicash from Pozzolano in Italy. I shall use the word pozzolan in
a much more general way and the pozzolan which is normally used today
in UK is brick dust.

Pozzolans are impurities which are added to a pure lime mortar to give
it some cementitious properties; to make it harder and more
impermeable to water. When lime putty mortar is used outdoors, then
pozzolans are always added to give it some extra weather resistance.

Hydraulic lime mortar contains natural pozzolans, so is fine for using
outdoors without any more additives.

Hydrated lime should be put in a tub of water for a couple of days
when lo and behold it becomes lime putty. Pozzolans can then be added
for external work.

Once pozzolans have been added to lime putty mortar, it will start to
set and so normally is used the same day.

To say that hydraulic lime (or lime putty mortar + pozzolan) have
cementitious properties is misleading because todays cement is MUCH
stronger than either. I don't have the figures to hand but my dodgy
memory thinks that todays cement is about 10 times as strong as
cements of 150 years ago and 20 times as strong as hydraulic lime
mortar. Castle Cement have done lots of research on this.

Until a few years ago, a little Portland cement was often added to
lime mortar to act as a pozzolan but there were too many mortar
failures so English Heritage commissioned some research.

The results of the research were published as "The Smeaton Report" and
in summary the report says that that cement+lime mixes will succeed if
there is enough cement in the mix to get a _cement_ set. The lime is
there just to provide plasticity.

What does NOT work is a mix where 'just a little' cement is used as a
pozzolan because the cement clogs up the pores of the lime so the air
and water which are required for a _lime_ set can't get into the
matrix.
Mortar failure is likely because there is a poor lime set and
not enough cement for a cement set.


--
~~ Anna Kettle, Suffolk, England
|""""| ~ Lime plasterwork, plaster conservation
/ ^^ \ // Freehand modelling and pargeting
|____| www.kettlenet.co.uk 07976 649862