Hydrated Lime in Mortar: What It Is, Why It's Used, and When to Choose It

Your bricklayer uses a bag of white powder to make the mortar more workable. You watch them add it alongside the cement, and you have no idea whether it's the right product, the right amount, or even the right type of lime. Get it wrong and the mortar either won't set (if they've used hydrated lime without cement), or it will be too strong and too brittle (if they've skipped it entirely on cavity wall blockwork). These are avoidable problems.

What it is and what it's for

Hydrated lime is calcium hydroxide (Ca(OH)2), produced by burning limestone to make quicklime (calcium oxide), then slaking it with water. The result is a fine white powder sold in 25 kg paper bags. In the UK it's classified as CL90-S under BS EN 459-1:2015, the British Standard governing building limes. The CL90 designation means the calcium oxide plus magnesium oxide content exceeds 90%.

The key fact about hydrated lime: it is non-hydraulic. Left on its own mixed with sand, it will not set. Not slowly, not partially. It won't set at all in any useful timescale. Hydrated lime in modern construction is an additive to cement mortar, not a standalone binder. The cement does the setting. The lime changes how the mortar behaves before and after it sets.

Why add it? Seven genuine reasons:

Workability. Lime makes the mix more plastic and easier to spread. Bricklayers describe it as making the mortar "creamy." It behaves on the trowel more like old-fashioned hand-mixed mortar than the harsher, stickier consistency of straight cement-and-sand.

Water retention. Lime holds water in the mix longer, which prevents premature drying caused by absorbent bricks or blocks pulling moisture out of the mortar too quickly. When mortar dries before the cement has hydrated properly, bond strength drops significantly.

Flexibility. Hardened cement:lime:sand mortar is slightly more flexible than cement-only mortar. It can accommodate small amounts of building movement without cracking through the joint. At standard mix ratios the effect is modest, but it accumulates over the life of the structure.

Self-healing. Free lime in the hardened mortar can dissolve in rainwater, migrate into micro-cracks (in the range of 10-200 micrometres), and redeposit as calcium carbonate, sealing the crack. This happens automatically over time through a process called autogenous healing or carbonation. At standard 1:1:6 ratios, the effect is real but modest. Don't overstate it. It's not a structural repair mechanism, just a benefit that reduces long-term moisture ingress through hairline cracks.

Rain resistance. Research from Wienerberger/Lucideon and BRE Digest 362 shows that cement:lime mortars outperform cement-only mortars for resistance to rain penetration. The lime fills fine voids in the mortar matrix that would otherwise allow capillary water movement.

Reduced efflorescence. Efflorescence (the white salt deposits that sometimes appear on new brickwork) is reduced when mortar has fewer hairline cracks for water to travel through. The lime contribution to crack resistance helps here.

Improved bond. Lime improves adhesion between mortar and masonry units, particularly with dense materials that don't absorb much water.

All seven benefits are real and documented by the Brick Development Association and Wienerberger's technical teams. They are also why the Designation (iii) cement:lime:sand mortar is the standard UK general-purpose mix for brickwork and blockwork above DPC, not a historical leftover.

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The confusion that causes real problems

The biggest mistake homeowners and DIYers make with lime is using the wrong type. This comes up in forum discussions constantly, and the consequences range from mortar that turns to sand overnight to wasted materials and re-pointing jobs that need redoing.

Hydrated lime (CL90-S) - white powder in paper bags, sold at builders merchants and B&Q. Used as a plasticiser in cement:lime:sand mortar. Will NOT set without Portland cement.

Natural Hydraulic Lime (NHL) - a different product entirely. NHL2, NHL3.5, and NHL5 are classified by compressive strength. NHL is a standalone binder. Mix it with sand and no cement and it will set, including underwater. Used for conservation work on pre-1920s solid-wall buildings, lime render, heritage repointing.

Quicklime (CaO) - raw burned limestone before it's been slaked with water. Industrial product. Highly reactive, extremely hazardous. Not sold for construction use directly. Not what you want.

Lime putty - hydrated lime that's been soaked in excess water for 24 hours or longer. Smoother and more workable than powder, used in specialist lime mortar work. Not the same as bag hydrated lime.

A cement:lime:sand mortar is a cement mortar with lime as a plasticiser. It is not a lime mortar. These are different products with different properties, different applications, and different performance characteristics. For a modern masonry extension with cavity walls above the DPC, cement:lime:sand is correct. For repointing an 1880s solid-wall house with soft handmade bricks, you need NHL-based mortar. If you confuse the two, you'll either produce mortar that won't set, or you'll damage historic masonry with a mix that's too strong.

Standard UK mortar mix ratios

For most domestic extension work, Designation (iii), the 1:1:6 mix, is what your bricklayer will be using for above-DPC blockwork. This is the most common UK general-purpose mortar. For below-DPC courses and exposed locations, Designation (ii) at 1:0.5:4.5 is more appropriate.

In a 1:1:6 mix, the volumes are equal parts cement and lime, with six parts sand. So one bag of cement gets one bag of lime. Per cubic metre of mixed mortar you'll need approximately 6-7 bags of cement, 6-7 bags of hydrated lime, and 0.8 m3 of sand (roughly 1.3 tonnes). For a typical cavity wall extension, lime consumption is directly proportional to cement: if your build needs 30 bags of cement for the blockwork, it needs around 30 bags of lime.

For Designation (iii) - the most common mix for domestic blockwork - the ratio is 1:1, one bag of lime per bag of cement, not the 1:0.5 ratio used in stronger Designation (ii) mixes.

How much do you need

Calculate lime requirements from your mortar volume, which follows from your blockwork area.

For a standard cavity wall (two leaves, each 100mm block):

1. Calculate face area of each wall leaf in m2 (length x height)
2. Multiply by two for both leaves
3. Each m2 of blockwork requires approximately 0.025 m3 of mortar (10mm joints, standard 440x215mm blocks)
4. For every m3 of Designation (iii) mortar: 6-7 bags of lime
5. Add 10% for wastage and mixing losses

Worked example for a simple 4m x 2.4m cavity wall section:

• Face area per leaf: 4 x 2.4 = 9.6 m2
• Both leaves: 19.2 m2
• Mortar volume: 19.2 x 0.025 = 0.48 m3
• Lime required: 0.48 x 6.5 = 3.1 bags, rounded up to 4 bags (including wastage)

For a whole extension with 60 m2 of wall face area across both leaves, expect 15-20 bags of hydrated lime in total. It's not expensive enough to warrant excessive precision in the estimate, but getting caught short mid-build with a bricklayer on site is more expensive than carrying an extra bag or two.

Cost and where to buy

£8 – £28 per 25 kg bag, but the spread matters.

Independent builders merchants sell hydrated lime at £8-12 per bag ex VAT. That's the price your bricklayer's merchant will charge if they have a trade account. National chains are a different story: Travis Perkins list Blue Circle Hydralime at £22-27 inc VAT, B&Q at around £20, Jewson at the top of the range. The product is identical. The price gap between a local independent and B&Q for the same 25 kg bag of hydrated lime can be £10-15 per bag.

For a build needing 15 bags, that's £150-225 extra if you're shopping at a DIY chain rather than a builders merchant. Call your local independent merchant first.

Main UK brands are Blue Circle Hydralime (Tarmac), Heidelberg Materials (formerly Hanson/Castle), and White Rhino. All conform to BS EN 459-1 CL90-S. There is no practical performance difference between brands for cement:lime:sand mortar work. Buy whatever your merchant stocks.

Lime vs plasticiser

The alternative to hydrated lime for improving mortar workability is liquid plasticiser (Febmix Admix or similar), added to the mix water. The lime vs plasticiser question comes up on every forum thread about bricklaying and the honest answer is: it depends on the application.

The below-DPC point is not a minor caveat. Liquid plasticiser introduces microscopic air bubbles into the mortar to improve workability. Below ground where water pressure is present, those air voids can allow water absorption into the mortar matrix. For below-DPC courses, lime is the correct plasticiser. Most bricklayers know this; confirm with yours if you're not sure.

Above DPC on a new-build extension, both work. Many bricklayers prefer plasticiser for speed and simplicity. Lime gives a marginally better result on quality and durability metrics, and is the choice for any visible face brickwork where long-term appearance matters.

How to handle it safely

Hydrated lime is caustic. Its pH is 12.4, which is in the range of concentrated bleach. Direct contact with wet lime or lime-rich mortar can cause chemical burns. The insidious thing about alkali burns is they don't always hurt immediately. The burn develops over hours as the alkali saponifies (dissolves) tissue. By the time it becomes painful, the damage is done.

Required PPE per the COSHH assessment for hydrated lime:

• Eyes: Full-seal wide-vision goggles with anti-mist coating. Not safety glasses, not a visor alone.
• Hands: Chemical-resistant rubber gloves or fabric composite gloves. Nitrile or latex exam gloves are not adequate for sustained contact.
• Body: Long-sleeved overalls, boots. If handling in an enclosed space, an impervious apron.
• Respiratory: An FFP2 dust respirator when opening bags or in dusty conditions. Fine lime powder is a respiratory irritant.

Sweat inside gloves activates lime on skin. Change gloves regularly on hot days and wash hands before removing gloves. Anyone with cuts or broken skin on their hands should be particularly careful, as lime penetrates more aggressively through damaged skin.

A COSHH assessment is legally required before using hydrated lime on any UK construction site, including self-managed domestic projects where workers are present. The site supervisor (that's you, if you're project managing) is responsible for this.

Mixing and application

Hydrated lime is added dry to the mix, not dissolved in water first. Standard machine mixer sequence:

1. Add half the sand to the dry mixer
2. Add the cement
3. Add the lime
4. Add remaining sand
5. Mix dry briefly to combine
6. Add water gradually until the mix is workable

The correct consistency is a mix that holds its shape when cut with a trowel but flows slightly when the trowel is shaken. Not stiff and crumbly, not sloppy and running. Lime makes the mix sticky in a way cement-only mixes don't. First-time bricklayers sometimes add too much water to compensate for the stickiness, which weakens the mix. Don't.

Mix only what can be used within 1-2 hours. Lime mortar that sits too long stiffens and cannot be re-tempered (adding more water to a stiffened lime mix compromises its strength).

Storage

Store lime bags in a dry, ventilated area, off the ground on pallets. Keep bags sealed until use.

Shelf life is approximately 12 months for sealed bags. Once a bag is opened, hydrated lime begins absorbing carbon dioxide from the air and slowly converts back to calcium carbonate (chalk), the reverse of the manufacturing process. This is called carbonation. As it happens, the lime loses its plasticising properties. A partly used bag left open on site for a few weeks will have degraded noticeably.

If you have bags that have been open or improperly stored: if the contents are still free-flowing white powder, it's likely still usable. If the bag contains hard lumps or the powder has a gritty, chalky texture, discard it. Don't use degraded lime in structural mortar. It's cheap enough that the cost of replacement is trivial compared to the risk of poor-quality mortar in your walls.

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Where you'll need this

Hydrated lime is used wherever cement:lime:sand mortar is specified in masonry work:

• Walls and blockwork - added to mortar for above-DPC cavity wall construction, approximately one bag per bag of cement in a 1:1:6 Designation (iii) mix

Lime appears across the structure phase of any project involving brickwork or blockwork. It is also used in render mixes (6:1:1 sand:lime:cement for scratch coat) and in some pointing applications.

Common mistakes

Confusing hydrated lime with hydraulic lime. This is by far the most common error, documented in dozens of UK DIY forums. Hydrated lime (CL90-S, white powder in paper bags) will not set without Portland cement. Natural hydraulic lime (NHL2, NHL3.5, NHL5) sets without cement. If a homeowner buys hydrated lime thinking it's a substitute for hydraulic lime and mixes it with sand alone, they will end up with mortar that crumbles to sand. The products look similar, both are "lime," both come in bags, and their names are confusingly similar.

Using it below DPC as a plasticiser alternative. Liquid plasticiser below DPC encourages water absorption. Lime is the correct additive for below-ground mortar mixes. Many bricklayers default to plasticiser for everything because it's easier. Push back on this for below-DPC courses.

Not wearing eye protection when opening bags. Fine lime powder in the air during bag opening is a genuine hazard, not a theoretical one. It takes one wrong movement to get dust in an unprotected eye. Goggles go on before the first bag is opened.

Using an opened bag that's been sitting on site for weeks. Degraded lime produces weaker, less workable mortar. The £10 saving from using an old bag is not worth the quality risk. Check bags before use. If in doubt, open a fresh one.

Getting the ratio wrong for the designation required. A 1:1:6 Designation (iii) mix and a 1:0.5:4.5 Designation (ii) mix are different products for different applications. Don't use Designation (iii) below DPC or in exposed locations where (ii) is specified. The difference in lime content (one bag vs half a bag per bag of cement) has real consequences for mortar durability.