What Are The 3 Main Types of Lime Used in Construction?

limestone quarry
A limestone quarry where the raw material, calcium carbonate is extracted to make lime with.

Lime is a very versatile building material. There are a few different types of lime that are used in construction. Each lime has different benefits and drawbacks for certain applications. So what are the 3 main types of lime?

The 3 main types of lime are quicklime, hydraulic lime and hydrated lime. Quicklime is slaked with water to create lime putty and then mixed with aggregates to make mortar or plaster. Hydraulic lime comes as a powder like Portland cement. The powder is mixed with aggregates and water to create mortar or plaster. Hydrated lime is often seen as an inferior lime and shouldn’t be used for historical renovations. It is usually added to cement mortar and plaster to help plasticise the mix, giving it more workability. It should never be used as a substitute for Hydraulic or Putty lime.

So now we have a brief overview of the different types of limes, let’s take a closer look at each type individually to see what it is, how it sets and where it should be used. But before that, let’s understand what lime is, and how its created.

How Lime is Manufactured And The Lime Cycle

All lime building materials come from the raw material, calcium carbonate. This is usually quarried as limestone, but can also be found in chalk, coral rocks and shell rocks. The rocks are heated in a kiln which causes a chemical reaction, giving off carbon dioxide gas and forming calcium oxide. Calcium oxide is commonly known as quicklime or lump lime. When quicklime is mixed with water it forms calcium hydroxide. This is also known as slaked lime, lime putty or hydrated lime.

Singleton Birch quicklime, made in the UK.

When limes are exposed to the air they slowly absorb carbon dioxide to form back into the raw material, calcium carbonate. Although this is chemically the same material, it is often very different from the stone or original material. This is because additives such as sand have been combined with lime to make mortars and plasters. Most of the time, calcium hydroxide-based lime is used, especially for historical restoration. The absorption of carbon dioxide by lime is known as carbonation.

Hydraulic limes are different to quicklime and hydrated lime. Pure limes will not set underwater or in extremely wet conditions as they will not be able to absorb carbon dioxide properly. This is where Hydraulic limes come in. The limestone and chalk rocks that hydraulic limes are made from contain varying amounts of fine clay materials. When the stone is appropriately fired in a kiln these clays combine with the lime to create active compounds.

Related article: What are the Differences Between Lime and Cement?

These active compounds combine in water to give a chemical set in addition to regular carbonation. Where proportions of clay are minimal, less hydraulic lime is produced and carbonation still occurs to a large degree. If the proportions of clay are large, a more hydraulic lime is produced and significantly less carbonation occurs.

There are 3 types of hydraulic lime with varying strengths. Feebly hydraulic lime (NHL 2, around 8% clay), Moderately hydraulic lime (NHL 3.5, around 15% clay) and Eminently hydraulic lime (NHL 5, around 25% clay). NHL means Natural Hydraulic Lime, and the number stands for the compressive strength in N/mm2 after 28 days, which is the British standard for measuring compressive strength. For comparison, Portland cement can have up to 45% clay and is fired at a much higher temperature.

Quicklime/Lime Putty’s Uses And Benefits

Quicklime as discussed is manufactured from a calcium carbonate-containing rock such as limestone or chalk. The rock is heated in a kiln which produces quicklime. Quicklime is corrosive to certain materials and can burn skin and cause blindness. When handling quicklime, always wear protective goggles, a respirator and skin protection.

Slaking Quicklime

When quicklime is combined with water, it can react violently. This process is called slaking and the reaction causes a great amount of heat to be produced and will likely spit hot lime, always take care when slaking lime. Once the reaction has finished, the resulting lime putty (which looks like very thick cream) is usually matured for at least 1 month for mortars, and around 3 for plasters.

Mature lime putty.

Lime putty is usually matured to ensure there are no ‘drowned particles’ of quicklime within the putty. A drowned particle is a particle of quicklime that didn’t properly react with water in the initial slaking process. Drowned particles can cause mortars and plaster to ‘blow out’. If you see a small crater or pit in recent plastering work, you may notice a tiny white spot at its centre. This is the drowned particle of lime. Maturing lime putty ensures all of the quicklime has fully reacted and makes the mortar or plaster more workable.

Quicklime should always be kept stored in some kind of airtight container and away from flammable materials. Although lime itself is not flammable, when it comes into contact with water it will generate lots of heat and swell up. The heat generated by the lime has been known to ignite flammable materials if they are stored closely enough.

Quicklime can also react with the moisture in the air in a process called air slaking. Air slaking is a fairly slow process but it will render the quicklime less reactive over time, eventually creating hydrated lime which can no longer be used to create mortar, plaster or render.

Using Putty to Make Mortar, Plaster or Render

Once the putty is sufficiently aged, it can be mixed with various aggregates and additives to create a mortar, plaster or render. Plasters and renders will tend to be more lime rich than most building mortars. Plasters and renders can be a ratio of 1:1 lime: fine sand for skim coats, and down to a ratio of 1:2.5 lime: aggregate for base coat plasters. Lime mortars are often around a 1:3 or 1:2.5 lime: aggregate for most building and renovation projects.

Lime putty is very versatile and there are thousands of different mortars and plasters that can be produced. Additives such as horse hair, goat hair and other fibres can be added to plasters to help give extra tensile strength where it is required. Pozzolanic additives such as ash can be added to help improve set times and make the material set harder. The Romans used volcanic ash in their lime-building materials to create a hard, fast-setting mortar!

What Are The Main Uses For Lime Putty Mortars And Plasters?

Lime putty creates the softest lime mortar, plaster and render. Whilst many see this as a negative it is actually a very important characteristic, especially for renovation projects. Mortars and plasters should always be softer than the stone, brick or masonry of the building. If they aren’t, instead of the mortar taking the brunt of any flexing or movement within the building, the stonework will. A hard mortar may not break, but instead, cause the stonework around it to break. This is why it’s important that the mortar is the weakest element, it can be easily replaced if needed, but the masonry cannot.

Older and historical buildings were often constructed with soft masonry or bricks. This is why a soft lime mortar needs to be used for these old renovation projects. Using a very hard portland cement mortar would cause extensive damage to the building. In today’s construction work, the masonry and bricks used are much harder, and this is why it has become acceptable to use extremely hard cement-based mortars.

In addition, lime putty-based mortar, plaster and render are very breathable. Again, this is very important for old buildings susceptible to water ingress and damp issues. It can even be helpful in modern buildings which have damp issues. Lime building materials help to absorb and expel trapped water from the building. Often getting rid of mould and unsightly damp areas.

Related article: How To Stop Lime Mortar From Cracking

Hydraulic Lime’s Uses And Benefits

Hydraulic limes are made out of calcium carbonate-containing rocks in a similar manner to quicklime. The difference is, these rocks contain more clay impurities. When the clays are fired in a kiln alongside the calcium carbonate they combine to create active compounds. As discussed, these compounds react with water to create a chemical set which is much faster than lime putty.

hydraulic lime in paper bags
A pallet of hydraulic lime powder.

The Main Uses For Hydraulic Limes

Hydraulic limes are used in a similar way to lime putty, the difference is, hydraulic limes come in powder form, and putty is a ready-mixed wet material. Hydraulic limes have the ability to set underwater, and should only be used where conditions would not allow for lime putty mortar or plaster to set. For example, an area that is constantly damp or very exposed to the elements.

There are 3 strengths of hydraulic limes. The weakest, NHL 2 is excellent for internal works where lime putty would not set, or would take too long to set, such as a constantly wet wall. NHL 3.5 is a general building lime, it can be used in moderately exposed areas where putty or NHL 2 would not work well. Finally, NHL 5 is the hardest hydraulic lime. NHL 5 should be used in areas with severe exposure where NHL 3.5 would not work well.

It’s important to note, that the higher the strength of the hydraulic lime, the less flexible and breathable the mortar or plaster will be. Breathability and flexibility requirements should always be considered before choosing which lime you should use. And it’s still important to remember, a mortar which is stronger than the stonework of the building should never be used, especially on older historical buildings.

When making a lime mortar or plaster with hydraulic limes, use similar proportions of lime: aggregate as with lime putty. It should be stated, however, that lime putty is almost always better for making plaster. Weak hydraulic limes (NHL 2) will work well in coarser base coats, but fine skim coats should be made with putty.

As a general guide, hydraulic limes will keep in their bags for 6-12 months when stored correctly.

Hydrated Lime’s Uses And Benefits

Hydrated lime is made using the same process as quicklime. A relatively pure calcium carbonate-containing rock is fired in a kiln to produce quicklime. The quicklime produced is then combined with a controlled amount of water so that it remains a powder. Hydrated lime can be found at most builders’ merchants and is usually far more readily available than quicklime or hydraulic lime.

The Main Uses For Hydrated Lime

Hydrated lime is a far inferior lime for building and renovation works. You should never make a lime mortar purely with hydrated lime as it probably won’t carbonate strongly enough to set. Hydrated lime is most often used as an additive to portland cement mortars to help improve workability. Many people often confuse hydrated lime with hydraulic lime, whilst this is easy to do, hydrated lime should never be used as a substitute for quicklime, lime putty or hydraulic lime.

Hydrated lime is also sometimes used in road construction and can be used as garden lime. If you do use hydrated lime for your garden please be aware it is not the same as garden lime, which is normally some kind of crushed limestone. Hydrated lime will be much stronger than garden lime, so you will likely need to use less to get the same results.