Cob: a question about renders...

[root]

lime mortars and plasters harden by carbonation (exposure to air).
Pozzolanic materials react with lime putty as a result of the silica
(SiO2) and alumnia (Al2O3) of the pozzolan reacting with lime (Ca(OH)2).
This reaction is quicker than carbonation so producing a quicker setting
mortar. Also, Calcium silicate hydrate (the product of the pozzolan/lime
reaction) forms a net of fibrous crystals, giving a 'harder' mortar set.
The Romans used volcanic ash from the region of Pozzuoli in Italy as a a
pozzalan for their concretes. Basically any material containing reactive
silicates can be used. Modern pozzalans are made from pulverised fuel
ash, calcined clay, and brick dust and powder. More traditionaly,
crushed brick, crushed burnt coal, ash, crushed shells were used. If you
look at old mortars a high percentage of the aggregate was made up of
such materials. The use of powdered brick or ash will also affect the
colour of the mortar depending on the colour of the pozzalan used. Coal
and wood ash produces a slightly blue/grey mortar for example. Larger
chunks of pozzolanic material also add to the visual appearance of the
mortar or render. Hope this is more food for thought.

- Paul