The Power of Pozzolans

 

The use of lime dates back to pre-historic times. Lime is derived from limestone, a sedimentary rock formed mainly of skeletal fragments of marine organisms such as coral and molluscs. Its major materials are the minerals calcite and aragonite, which are different forms of calcium carbonate.

It is generally held that the Greeks began the large scale production of lime-based mortars in Europe and it was from there that the technology spread to Rome.

The Romans early recognized the need for a mortar that could be used under ground and under water – the development of hydraulic mortars is credited to them and the use of Pozzolans was crucial for this.

 

Pozzolana

 

Pozzolans would include volcanic ash or clay brick/tile dust – these would be added to the lime mortar mix to create a faster set and reduce the mortar’s vulnerability to frost and rain.

Vitrivius describe Pozzolans as producing “astonishing results” and he explains the process behind them:

 

The Ten Books on Architecture, Vitrivius

 

Lime Pozzolan binders are obtained by the addition of a Pozzolan (natural or artificial) to the lime while mixing mortar. A natural Pozzolan is a volcanic material, which originally derives from Pozzuoli, an Italian region around Vesuvius. Pozzuoli earth was used in the Roman mortars but other natural Pozzolan are Santorini earth (Greece) and trass (Germany).

Artificial Pozzolans include metakaolin, silica fume, brick dust (preferably low fired brick) and others such as fly ash.

 

Caesarea is the earliest known example to have used underwater Roman concrete technology on such a large scale. Photo by James Cocks www.jamescocks.com.

 

Pozzolans became the backbone of Roman construction and were incorporated in the ‘Roman Concrete’.

Pozzolans of Pozzuoli were used to build ‘La Via Appia’, the Colosseum and the Pantheon of Rome. The fact that the mix could harden under water allowed the Romans to extend their empire along their coastines which gave them a strategic advantage.

Interestingly, lime Pozzolan concrete still has a place in today’s construction technology, not only because of its original characteristics but particularly because it can also offer significant carbon savings and potentially present huge environmental benefits. After all, lime is a remarkably efficient natural absorber of carbon dioxide and it could sequester carbon emissions in a very effective way.

Interesting readings:

 

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