Pozzolans & Additives

Pozzolanic Additives

The word pozzolanic is derived from the word Pozzuoli. This is the name of an area on the Italian coast near Vesuvius. This is where the Romans sourced pozzolanic additives, they mined the ashes deposited by the occasional eruptions of this volcano, and they then used these additives in mortars.

A "pozzolan" is defined as "a siliceous or siliceous and aluminous material, which in itself possesses little or no cementing property, but will in a finely divided form - and in the presence of moisture - chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties." Definition taken from "Pozzolanic and Cementitious Materials" by Malhotra and Mehta (Gordon and Breach Publishers, 1996)

Hydraulic reactions similar to those which occur in a hydraulic lime can be achieved in non- hydraulic lime by adding pozzolans. These finely powdered materials are added to lime mortar to increase the durability, performance, and to provide a chemical set, independent of carbonation, which also occurs in due course. For best results the pozzolans need to be 70 microns or less, pozzolans any larger only act as an aggregate.

The following list contains the most commonly used pozzolans:

Inorganic Additives

Metastar - A fine white ceramic powder from burnt English china clay (kaolin). This product is produced under a strict quality control environment. Add 10% Metastar by volume of mortar.

Brick dust - Crushed low-fired brick tile. It ranges in colour from red to buff. Add one part brick dust to four parts mortar. Brick dust also acts as a particulate and may colour the mortar.

PFA - Pulverised Fuel Ash. Dark grey coloured powder, suitable for grouting.

Trass - A khaki coloured volcanic powder sourced in Germany.

Volcanic dust - Original material used by the Romans as a pozzolan and still used in certain parts of Italy.

Organic Additives

Hair - Traditionally ox hair was the most commonly used although horse and goat was also used. The hair was mostly used for internal plaster to give tensile strength particularly when used for plastering onto laths. Today hair is sourced from goat, yak, horse, and less commonly cattle.

Tallow - This product is derived from the rendering of livestock. It is added for increased water resistance, particularly in limewashes and greater flexibility. It is added to slaking quicklime to achieve an even distribution throughout the limewash. The mix is allowed to cool and is then sieved. However, if used inside in humid conditions it may promote mould growth.

Raw linseed oil - This plant based additive is useful to inhibit the ingress of moisture through limewashes (weather resistance). It also reduces the dusting effect of limewash. It is usually added to the last two coats to help in the shedding of moisture.

Casein (or Skimmed Milk) – Casein is a protein in milk. This is added to limewash and the substances combine to form Calcium Caseinate, giving good resistance to dusting and improved weathering.

Other Additives

Plaster of Paris - This gypsum material is only to be gauged with mortar for internal use. This accelerates the initial set when mixed with fat lime mortars and is normally used for internal decorative plaster work.

Marble Dust – Used in the making of traditional plasters, and hand moulded work.

Pigments - Traditionally earth based pigments common to the particular locality were added to limewash for aesthetic reasons.

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