Pozzolans are materials that are not themselves cementitious, but contain constituents that will combine with lime at ordinary temperatures in the presence of water to form stable insoluble hydraulic compounds. Pozzolans have been subjected to heat either naturally (volcanic earth) or artificially (crushed fired clay brick and tile). Heating has resulted in certain constituents, principally silicates and aluminates, becoming reactive towards alkalis. When mixed with lime, hydraulic compounds are produced by the reaction between these constituents and the lime.
The pozzolanic reaction is not entirely understood, and indeed each pozzolan reacts differently. Fast-reacting pozzolans normally result from a reaction between lime and aluminates, and slower progressive reactions normally result from the reaction between lime and silicates. The resulting calcium aluminate hydrates and calcium silicate hydrates are similar, but not necessarily the same as the hydrates formed during the hydration of hydraulic limes.
The practice appears to start in the Minoan civilisation of 1500BC Crete when crushed fired clay pot and tile were deliberately incorporated in mortars, presumably as a beneficial effect was recognised. The practice was continued by the ancient Greeks and then adopted by the Romans, and the presence of pozzolans is a principal explanation for the remarkable durability of Roman mortars. The Romans found that the naturally occurring volcanic earth and ash from the areas around Vesuvius, Naples and Rome had a similar effect as fired clay. As the Roman Empire expanded, deposits of similar materials were located – German trass from Coblenz and Bavaria and earth from Santorini being perhaps the best-known examples.
Finely ground/milled pozzolans are more reactive due to the larger surface area.
Pozzolanic additives available ex stock are:
Crushed brick and tile
Argical M1000 Metakaolin (calcined china clay)
PFA Pulverised fuel ash
Pozzolans must be carefully gauged and mixed either as powder or as a slurry normally at 5%, 7.5%, 10% or 15% by volume to the mixed mortar. Alternatively, they can be gauged by volume to the lime content – the English Heritage Smeaton trials on Hadrians Wall found that 1: 3: 1 (lime putty: sand: pozzolanic brick dust) was the best performing mortar in samples based on non-hydraulic lime, and a marked improvement noted when brick dust had been added to hydraulic lime samples.