Mortar Plasticiser: How It Works, Correct Dosage, and When Not to Use It

Your bricklayer pours a glug of dark liquid into the mixer's water bucket and the next batch of mortar comes out smoother, easier to spread, and faster to lay. They've added plasticiser, and there's a fair chance they've added too much. Overdosing this stuff is the single most common mortar mistake on UK building sites. Mortar that feels like chocolate mousse on the trowel is exactly the mortar that crumbles after three winters. Worse, plasticised 1:6 mortar in the wrong location, at copings, chimneys, or below the DPC on an exposed site, is explicitly flagged by the Brick Development Association as a frost-attack risk regardless of how carefully it was mixed.

What it is and what it's for

Mortar plasticiser is a liquid admixture you add to gauging water before mixing cement and sand. The active ingredient is an anionic surfactant, usually a modified lignosulfonate (a wood-derived compound) or a synthetic detergent-like molecule, and it does one job exceptionally well: it traps millions of microscopic air bubbles in the wet mortar.

Those bubbles, between 0.02 and 1.0 mm across, do two things. While the mortar is still wet they act as ball-bearings, letting cement and sand particles slide past each other in shear. The mix becomes "buttery" on the trowel without you adding more water. Once the mortar has set, the same bubbles work as expansion chambers. When water inside the cured mortar freezes, it expands into the air voids instead of generating destructive pressure inside the cement matrix. This is why plasticised mortar resists freeze-thaw damage better than plain cement-and-sand of the same proportion.

Two British Standards govern the product. BS EN 934-3:2009+A1:2012 is the conformity standard for masonry mortar admixtures. BS EN 998-2:2016 is the masonry mortar specification standard, and Table NA.1 of its UK National Annex sets out the M1 to M12 mortar designations. NHBC Standards Chapter 6.1.14 explicitly permits plasticiser in M4 and M6 cement:lime:sand mortars (the 1:1:5½ and 1:½:4½ designations), but with two non-negotiable conditions: the admixture must be chloride-free, and it must be dosed strictly per manufacturer instructions. Calcium chloride based "frost-proofers" are a different product category and are banned for use with mortar that contains wall ties or reinforcement, because the chloride ions corrode embedded steel.

The product is functionally an alternative to hydrated lime in the workability role. Lime makes mortar plastic by adding fines that hold water; plasticiser makes mortar plastic by adding air. They are not chemically equivalent and they are not interchangeable in every situation, but for general above-DPC blockwork on a modern UK extension, plasticiser is what most bricklayers reach for.

Standard liquid, concentrate, or powder

Plasticiser ships in three forms and confusing them produces serious mistakes.

The most common product on a UK site is the standard 5-litre liquid bottle, and that's what the rest of this guide assumes unless stated otherwise. The screw cap on most 5L bottles holds roughly 60 to 80 ml, which is conveniently inside the recommended dosage band for one 25 kg bag of cement. One cap full per bag of cement is the practical heuristic.

Concentrates such as Everbuild Opti-Mix are a different proposition. A 1-litre concentrate bottle is roughly equivalent to 25 litres of standard liquid, and you need around 5–12 ml per 25 kg cement bag (a teaspoon, not a cap full). Bricklayers swap between standards and concentrates without always reading the label, and a cap full of concentrate poured into a mixer turns the mortar into the kind of frothy, hollow disaster that crumbles within a year. Read the bottle every time.

Powder sachets pre-portioned for one 25 kg cement bag are foolproof and cost a few pence more per bag. For a homeowner running a single weekend wall-repair job, a box of sachets removes the dosage risk entirely.

Pre-plasticised cements (Mastercrete, Multicem) contain the plasticising compound as a powder ingredient already mixed into the cement. If you're using one of these you do not add separate liquid plasticiser. Doing so doubles the air-entrainment and produces the overdosed, structurally weak mortar this guide keeps warning about.

The right dosage and how to actually measure it

The single most important number on this page: standard liquid plasticiser is dosed at 60 to 125 ml per 25 kg bag of cement. SikaMix's technical datasheet specifies 125–250 ml per 50 kg bag, which works out to 62–125 ml per 25 kg. Bostik Cemplas and Everbuild Febmix sit in the same band. The exact figure varies slightly between brands but the range is consistent.

How to measure in practice:

1. Add water to the gauging bucket first (a 14-litre black builders bucket is standard).
2. Pour the plasticiser into the water using the bottle cap as your measure. One cap per 25 kg bag of cement is correct for standard liquid.
3. Pour the dosed water into the running mixer.
4. Add cement, then sand, allowing the dry materials to be wetted gradually.

The plasticiser must dissolve into the water before the dry mix is added. Pouring plasticiser straight onto dry cement and sand causes localised concentration, uneven distribution, and a mix that's over-plasticised in some pockets and under-plasticised in others.

The real failure mode is not under-dosing, it's over-dosing. Mortar that's been over-plasticised feels brilliant on the trowel: light, fluffy, easy to spread, no effort. That feel is the bricklayer's enemy, because it's the feel of air voids. The cured mortar will be measurably weaker (compressive strength drops with air content) and the voids let water and frost penetrate. One thread on BuildHub describes a professional bricklayer who'd been routinely overdosing by 80% for years without realising. That's the warning: trade familiarity with plasticiser doesn't guarantee correct usage.

Where plasticiser is and is not appropriate

This is the section that separates good guidance from bad. Plasticised mortar is fine in some locations and explicitly wrong in others.

The Brick Development Association's Severely Exposed Brickwork Technical Guide 2023 is unambiguous on this: "Mortars generally used in the walls of buildings, such as 1:1:6 cement/lime/sand or 1:6 plus plasticiser, when used in exposed detailing such as chimneys, cills, plinths, boundary walling and below DPC on exposed sites are at risk of suffering frost attack." The recommendation for those locations is M12 designation (1:0 to ¼:3 sulfate-resisting cement to sharp sand) with F2/S2-class clay bricks or engineering bricks. M12 is too strong to need a workability aid; it's mixed dense and worked harder.

The reason matters. Air entrainment improves freeze-thaw resistance compared to a plain 1:6 cement:sand mix at the same exposure, but the improvement is from "very poor" to "merely poor" at exposed locations. It does not bring 1:6 plasticised mortar up to the durability of M12. The physical chemistry says the same thing: at copings and parapets, water sits on the masonry, freezes, thaws, and cycles repeatedly. A mortar with 15-20% deliberately introduced air voids hands the freeze-thaw cycle a head start.

Plasticised mortar below DPC is a more nuanced question than the forum chorus of "never use it below ground" suggests. NHBC permits plasticiser in the M6 designation, which is the below-DPC standard. The real issue is that homeowners and inexperienced builders sometimes use M4 plasticised (the above-DPC mix) below DPC, where M6 with sulfate-resisting cement is required. The mortar designation must match the location; using plasticiser doesn't reduce the strength requirement.

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How much do you need

Calculate plasticiser quantity from the cement quantity, which follows from the mortar volume, which follows from the wall area.

For a standard cavity wall (two leaves of 100mm block) with 10 mm joints, each square metre of single-leaf blockwork takes around 0.012 m³ of mortar. A whole cavity wall (both leaves) is about 0.025 m³ per m² of wall face area. One cubic metre of mixed 1:6 mortar uses approximately 6 bags of 25 kg cement.

Worked example for a typical 30 m² rear extension:

• Total wall face area, both leaves: roughly 100 m²
• Mortar volume at 0.025 m³ per m²: 2.5 m³
• Cement at 6 bags per m³: 15 bags of 25 kg cement
• Plasticiser at 125 ml per bag: 1,875 ml, plus 10% wastage = roughly 2 litres
• Buy a single 5-litre bottle. You'll have plenty left over for repointing or future jobs.

Most domestic extension builds need one 5-litre bottle and finish with two-thirds of it unused. Plasticiser keeps for 12–24 months sealed and frost-protected, so the leftover is usable on the next job as long as it hasn't frozen. For very small jobs (a garden wall, a repair) a 1-litre concentrate bottle or a box of powder sachets works out cheaper and removes the dosage gamble.

Cost and where to buy

£6 – £9 for a 5-litre bottle of standard liquid plasticiser at major UK retailers in April 2026.

Per bag of cement the cost is £0 – £0, which is roughly a quarter of what hydrated lime costs as a workability aid. The brand differences are real but small. SikaMix has the strongest brand reputation among professionals; Bostik Cemplas and Bond It deliver effectively the same chemistry at a third less.

B&Q does not stock liquid mortar plasticiser as of April 2026 (only hydrated lime). For DIY-counter purchases, Wickes, Screwfix, and Toolstation are your options. Travis Perkins and Jewson stock 25-litre trade containers but pricing requires a logged-in trade account.

For a 25-litre trade container you'll pay around 25 to 30 pounds from a builders merchant, which is genuinely cheaper per litre than the 5L retail bottles. Worth doing only if you're building enough to use 25 litres before the shelf life expires (one large extension is well under 25 litres of plasticiser).

Plasticiser vs hydrated lime: which to use

This is the recurring question on every UK bricklaying forum and the honest answer is that for most modern domestic extension work, both work, and bricklayers prefer plasticiser for speed.

The decision is straightforward in most cases. For hidden cavity-leaf blockwork on a modern extension, plasticiser is the right choice: faster, cheaper, simpler, no caustic dust to deal with. For visible facing brickwork where long-term appearance matters, lime gives a marginally better result. For heritage repointing on pre-1920 solid-wall buildings, neither hydrated lime nor liquid plasticiser is appropriate; you need natural hydraulic lime (NHL) or lime putty mortar. For copings, chimneys, and parapets in any building, neither is appropriate; you need M12 designation mortar without air entrainment.

Many bricklayers default to plasticiser-only and avoid lime entirely, which is fine for inner-leaf blockwork but suboptimal for visible facework. If your extension has facing-brick external walls and you care about how the joints look in five years, ask the bricklayer to use a 1:1:6 cement:lime:sand mix on the outer leaf. Lime gives a more uniform colour and a finer joint texture than plasticised cement:sand.

Storage

Plasticiser is water-based and freezes at roughly 0°C. A frozen bottle may or may not reconstitute; some products work fine after thawing, others lose their air-entraining properties permanently. Keep plasticiser indoors over winter, in an unheated but frost-free space (a garage that doesn't drop below 5°C, an outbuilding with mass).

Shelf life on the bottle is typically 12 months for Bostik Cemplas and Bond It, 24 months for Everbuild Febmix. Once the bottle is opened the chemistry doesn't degrade rapidly the way opened lime does, but evaporation and contamination accumulate. Always shake the bottle well before pouring; the surfactant can settle slightly during long storage.

If you have a part-used bottle that's older than the printed shelf life, mix a trial batch and check the consistency before using it on structural work. Mortar that doesn't develop the buttery feel after a few minutes of mixing has lost its air entrainment and the plasticiser should be replaced.

Where you'll need this

• Walls and blockwork - added to the gauging water for blocklaying mortar on cavity-wall construction. Standard practice for inner-leaf blockwork above DPC.

Plasticiser appears on every wet-trade phase of any extension or renovation that involves new masonry, including blockwork, brickwork, and repointing of existing walls. The dosage rules and exposure restrictions in this guide apply equally whether you're laying a single garden wall or the structural envelope of a two-storey rear extension.

Common mistakes

Over-dosing because the mortar feels better. The "chocolate mousse" texture is a warning sign, not a goal. One cap of standard liquid per 25 kg cement bag is the right dose. Don't add a second cap because the first batch felt slightly stiff. If the mortar needs more workability, add water within the cement:water ratio limits, not more plasticiser.

Using washing-up liquid as a cheaper substitute. Detergents introduce uncontrolled bubble sizes, contain salts that cause efflorescence, and produce mortar that fails freeze-thaw cycling. Plasticiser is roughly 17 pence per bag of cement; this is not a saving worth chasing.

Mixing standard liquid with concentrate at the same dosage. A cap of concentrate is roughly 25 caps of standard liquid in plasticising power. Read the label every time.

Using plasticised 1:6 mortar at copings, chimneys, parapets, or below DPC on exposed sites. These locations need M12 (1:0–¼:3 SRPC) or M6 SRPC, not 1:6 plus plasticiser. The BDA explicitly names this as a frost-attack failure mode. If the exposure rating of the location is severe, the mortar designation must be raised regardless of any workability aid.

Adding plasticiser to pre-plasticised cement. Mastercrete and Multicem already contain the plasticising compound. Adding liquid plasticiser on top doubles the air entrainment and produces the same fluffy, structurally weak mortar as overdosing.

Adding plasticiser direct to the dry mix. Plasticiser must dissolve into the gauging water first. Pouring it onto dry cement causes uneven distribution and pockets of over-plasticised mortar.

Laying plasticised mortar in frost. The set mortar resists freeze-thaw better than plain cement:sand, but this benefit only kicks in once the mortar has cured. Fresh plasticised mortar laid below 2°C, or laid when the temperature will drop below 0°C within 24 hours, suffers the same frost damage as any other mortar. Don't lay below 2°C and check the forecast for the curing window.

Treating the bottle cap as a precise measure on an unfamiliar product. Most 5-litre standard liquid bottles have caps in the 60–80 ml range, which is conveniently close to the recommended dose. But cap sizes vary between brands and between standard and concentrate products. Check the label, and if you're switching brands, measure one cap into a kitchen measuring jug to confirm what you're working with.