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The Pozzolan Guide

What is a Pozzolan?

Pozzolans are certain types of finely ground materials which can be added to an "air lime" (non-hydraulic) mortar to induce hydraulic setting characteristics. This happens through a chemical reaction in the presence of water.

Why should I use a Pozzolan in lime mortar?

A downside of lime mortar is that it has a slow setting time. And the longer that the mortar takes to set or strengthen, can leave it vulnerable to damage from frost. Any damage can result in repairs, which may then impact project budgets and schedules.

However, by adding a pozzolan it lessens the time that lime mortar takes to gain an initial hydraulic set, and achieve its eventual compressive strength and flexibility. Notably, there is a trade-off when using any pozzolan, as it will decrease lime mortar or lime render's vapour permeability and breathability.

What Types of Pozzolans are there?

Pozzolans come in three common types, these are:

Pulverised Fly Ash (PFA)

"Fly Ash" as it is more commonly known, is a by-product of the combustion of coal (and permitted co-combustion materials) for electricity generation. It is removed from the flue gasses and collected for use as an additive in building products, such as in lime mortar.

Pulverised Fly Ash (PFA), when added to lime mortar in small amounts, increases strength and improves curing time.

Ground Granulated Blast Furnace Slag (GGBS)

Ground granulated blast furnace slag (GGBS) is produced as a by-product in the iron and steel making process. This iron-rich slag, once quenched and ground, can be added to lime mortar to improve its strength and setting characteristics.

Notably, GGBS is not technically a pozzolan but a direct cement replacement. This is because of how it chemically reacts differently with water than pozzolans do, forming calcium silicate hydrates (CSH). This is unlike pozzolans which react with calcium hydroxide to impart strength (and cannot be used as a direct cement replacement). †

Nevertheless, GGBS can be used as an alternative to pozzolanic additive because ultimately it will strengthen and accelerate the curing time of non-hydraulic lime mortars.

GGBS is similar to Pulverised Fly Ash (PFA), but is lighter in colour. This may make GBBS a better choice than PFA where a finished lighter colour is required.

Please remember to get expert advice if you are thinking of using GBBS as cement substitute.


Unlike other pozzolanic materials - such as GGBS or PFA - metakaolin is manufactured directly for use. This means that it is made to exact specifications to maintain high whiteness, high reactivity, ultrafine particle size distribution and consistency.

Metakaolin is preferable over Fly Ash, if a lighter colour is important or where no change in colour is required.

Hydraulic lime mortar does already contain natural Pozzolans.

There is a compelling argument for primarily choosing a hydraulic lime mortar that contains a high quality natural hydraulic lime (NHL). This is because hydraulic lime mortar with NHL already contains naturally occurring pozzolans at source. However, the use of hydraulic lime mortar may not always be possible.

So why choose non-hydraulic lime mortar?

Non-hydraulic lime mortars are softer and more flexible than hydraulic lime mortars. This means that they are best suited to any masonry that is soft, delicate or decayed, and where conservation is a principal requirement. This last point is significant: if your area's conservation rules or project adviser stipulates the use of non-hydraulic lime mortar, then you may need to use a pozzolan due to certain conditions - for example, in wet or damp environments, or where schedule is a significant factor.

Which Pozzolan should I choose, and how much do I need?

Individual types of pozzolans can be effectively used to replicate different grades of hydraulic lime mortar. So by adding varying amounts of a specific pozzolan, you can induce the desired grade or characteristic of hydraulic lime mortar that is best suited to the individual project requirements.

Hydraulic lime mortar has a standard requirement for set times and compressive strength characteristics that categorises it into three separate grades: NHL 2, 3.5 and 5. The MPa number refers to the minimum compressive strength recorded in Megapascals (a basic unit of measurement of pressure or tension) at 28 days as specified in British Standard EN 459.

Table 1: Compressive Strength of Natural Hydraulic Lime

Type of
Hydraulic Lime
Minimum Compressive Strength
MPa (91 days)
NHL 2 (feebly hydraulic) 2*
NHL 3.5 (moderately hydraulic) 3.5*
NHL 5 (eminently hydraulic) 5*

* Approximate typical strength in MPa

Each of the tables below provides a guide for how much pozzolan in weight can be mixed per 1kg or a single 25kg bag of lime mortar, to achieve each NHL grade. Adding the appropriate amount of pozzolan will result in the desired grade of mortar or render that is sympathetic to the masonry type, but with a maximum durability when needed.

Table 2: Pulverised Fly Ash (PFA)

NHL Grade % of Pozzolan to Non-hydraulic Mortar Mix Mix Ratio by weight (per 1kg / per 25kg bag) 56 Day Min. Compessive Strength(MPa) 91 Day Min. Compessive Strength(MPa) 180 Day Min. Compessive Strength(MPa)
2 2.5% 25 / 625g - 2.5 3.6
3.5 5% 50g / 1.25kg - - 3.9
5 NA NA - - -

PFA is a suitable choice of pozzolan if NHL 2 is required, and slower setting times are beneficial. The typical use for NHL 2 is either internal or soft, permeable masonry and where conservation is the primary concern.

Table 3: Ground Granulated Blast furnace Slag (GGBS)

NHL Grade % of Pozzolan to Non-hydraulic Mortar Mix Mix Ratio by weight (per 1kg / per 25kg bag) 28 Day Min. Compessive Strength (MPa) 56 Day Min. Compessive Strength (MPa) 91 Day Min. Compessive Strength (MPa)
2 4% 40g / 1kg 2.1 2.6 3.1
3.5 6% 60g / 1.5kg 4.6 5.1 5.8
5 8% 80g / 2kg 5.1 6.0 6.5

GGBS is a good general purpose pozzolan for all grades of NHL requirements, particularly NHL 3.5. The typical use for NHL 3.5 is for moderately permeable masonry materials, such as bricks, blockwork, sandstone, limestone, terracotta, general building work, cavity & solid wall construction, bedding, pointing and re-pointing. Lime mortar with GGBS initialises a set quickly.

Table 4: Metakaolin

NHL Grade % of Pozzolan to Non-hydraulic Mortar Mix Mix Ratio by weight (per 1kg / per 25kg bag) 56 Day Min. Compessive Strength(MPa) 91 Day Min. Compessive Strength(MPa) 180 Day Min. Compessive Strength(MPa)
2 2% 20g / 500g 2 2 2.3
3.5 3.5% 35g / 875g 3.2 4.2 4.9
5 4% 40g / 1kg 5.6 6.1 7.2

Metakaolin is similar to GGBS, but its high reactivity is best for use where NHL 5 properties are desired. The most suitable use for NHL 5 is for dense, impermeable masonry that is exposed to severe weather or wet conditions. Typical uses include paving, roofing, chimneys, parapets, cornice, balustrades, marinas, canals, dams, and sea-fronts.

About this Guide

Please note that all information and data is provided in good faith to serve as a general guide only. Data is drawn from a range of sources and includes practical experience and laboratory testing which can not be guaranteed as various factors, such as weather conditions, temperature and specific materials will vary for each individual project or site.

Appropriate testing and trials are required on site to verify materials and works are suitable on a case by case basis. Conserv accept no liability for any consequential delay, loss, cost or damage arising from the use of this guide.

Further Information

If you would like more detailed information on pozzolans, below are a selection of the sources used to produce this guide: