Almost all the concrete leaving a batching plant today contains at least one admixture. Concrete admixtures are added in very small amounts during mixing and change how the concrete behaves: they make it flow without extra water, bring setting forward or delay it, protect it against frost or make it less permeable.
This guide covers the types of concrete admixtures defined in the standard, what each one is for, how to choose one for the job and what graphene-based admixtures such as our SmartADDITIVES range add.
What an admixture is (and is not)
An admixture is a product added during mixing, in a small amount relative to the mass of cement, to modify the properties of fresh or hardened concrete. EN 206 (applied in the UK together with BS 8500) limits the total admixture content to 50 g per kg of cement (5 %), unless the effect of a higher dosage on performance and durability has been established.
It should not be confused with other materials that also go into concrete:
- Additions such as fly ash, silica fume or ground granulated blast-furnace slag (GGBS). They are used in much larger quantities and can count as part of the binder.
- Fibres of steel, polypropylene or PVA. They have their own standards and a mechanical role: controlling cracking or replacing part of the reinforcement.
- Release agents and curing compounds. They are applied to the formwork or the concrete surface, not mixed into the concrete.
Types of admixture under EN 934-2
EN 934-2 is the European standard for concrete admixtures. It defines each family by what it must achieve in a test concrete, and requires this performance to be declared for CE marking.
| Admixture | What it does | When it is used |
|---|---|---|
| Water-reducing admixture (plasticiser) | Reduces the mixing water for the same consistence, or increases workability without adding water | Standard ready-mixed concrete, better workability |
| High-range water-reducing admixture (superplasticiser) | The same, with a much stronger effect | Pumped, flowing or self-compacting concrete, precast, high-strength concrete |
| Water-retaining admixture | Reduces water loss through bleeding | Mixes prone to bleeding |
| Air-entraining admixture | Creates stable, evenly distributed microscopic air bubbles in the paste | Concrete exposed to freeze–thaw and de-icing salts |
| Set accelerator | Brings forward the start of setting | Concreting in cold weather |
| Hardening accelerator | Increases early-age strength | Precast, early striking of formwork |
| Set retarder | Delays the start of setting | Hot weather, long haul times, large pours, avoiding cold joints |
| Water-resisting (waterproofing) admixture | Reduces capillary water absorption | Tanks, foundations, basements, precast cladding units |
| Viscosity-modifying admixture | Stabilises the mix and prevents segregation | Self-compacting concrete and difficult pumping |
The standard also covers combined admixtures, such as set-retarding water reducers. Accelerators for sprayed concrete have their own standard, EN 934-5.
Outside these families there are other products in common use: corrosion inhibitors for the steel, shrinkage-reducing admixtures, so-called antifreeze admixtures and clay inhibitors for aggregates of variable quality.
Retarding admixtures
A retarder keeps the concrete workable for longer. It is used in hot weather, on long hauls from the plant and in large pours where each new layer has to be compacted into the previous one before that layer sets, so that no cold joint forms. Concrete is normally placed within about 90 minutes of batching; a retarder helps the mix stay workable within that window when conditions work against it. The trade-off is slower early strength and later striking of formwork. Many retarders are supplied combined with a water reducer.
Waterproofing admixtures
Waterproofing admixtures (water-resisting admixtures in EN 934-2) reduce capillary absorption, and some also block pores or form crystals inside them. They help, but they do not make a structure watertight on their own. In basements and other below-ground structures, watertight concrete (Type B, structurally integral protection, under BS 8102) depends above all on a low water/cement ratio, adequate section thickness, crack-width control and properly detailed joints with waterstops.
Plasticisers and superplasticisers
These are by far the most widely used. Both do the same job, dispersing the cement particles so that the paste flows with less water, and they differ in strength. Under EN 934-2, a plasticiser must allow at least 5 % less water for the same consistence, and a superplasticiser at least 12 %.
By chemistry, they are usually grouped into generations:
- Lignosulfonates. Wood-derived, inexpensive and moderately effective. The classic plasticisers.
- Sulfonated naphthalene and melamine. The first superplasticisers, which made flowing concrete possible.
- Polycarboxylate ethers (PCE). They disperse by steric repulsion (their side chains physically hold the cement grains apart) and reduce water far more. They are the basis of today’s self-compacting and precast concrete.
- Multifunctional admixtures with nanomaterials. As well as reducing water, they act on cement hydration. This is where graphene-based admixtures fit.
Why does reducing water matter so much? Because the water/cement ratio has the greatest influence on strength and durability. Water that the cement does not need for hydration leaves capillary pores behind, and those pores let in the water, chlorides and CO₂ that attack the concrete and the reinforcement.
How to choose an admixture for the job
No single admixture does everything. This table sums up the most common situations:
| Situation | Usual admixture | Why |
|---|---|---|
| Pumped concrete in buildings | Superplasticiser | Flowing consistence without raising the water/cement ratio |
| Precast with rapid demoulding | Superplasticiser + hardening accelerator | High strength within hours so that moulds can be reused |
| Summer or long haul | Retarder or retarding superplasticiser | Keeps the mix workable until discharge |
| Cold-weather concreting | Set accelerator | Shortens the time during which the concrete is vulnerable to frost |
| Freeze–thaw exposure | Air-entraining admixture | The microscopic bubbles relieve the pressure of water as it freezes |
| Tanks and foundations in contact with water | Waterproofing admixture + low water/cement ratio | Less absorption and lower permeability |
| Marine or chloride environment | Low water/cement ratio + corrosion inhibitor | Delays chlorides reaching the reinforcement |
| Sprayed concrete | Alkali-free accelerator + water reducer | Rapid setting at the surface and thicker layers |
| Self-compacting concrete | Superplasticiser + viscosity-modifying admixture | Flows and compacts under its own weight without segregating |
In cold weather the aim is to get through the vulnerable period quickly: concrete must not freeze before it has reached a compressive strength of about 5 N/mm². Products sold as antifreeze for concrete mostly work as accelerators. They shorten that period rather than lowering the freezing point to any useful extent, and they do not replace covering and insulating the fresh concrete.
In marine environments there is another option: removing the problem at source with reinforcement that does not corrode. We explain it in the guide to GFRP rebar vs steel.
Costly mistakes
- Adding water on site. The most common and most damaging mistake: it raises the water/cement ratio, lowers strength and cancels out what the admixture did. If more workability is needed, superplasticiser is used, never water. Some superplasticisers can be added to the truck mixer before discharge, when the supplier allows for it and the concrete is remixed for long enough.
- Overdosing. More admixture is not better: it can delay setting severely, entrain too much air or cause segregation and bleeding.
- Combining admixtures without checking compatibility. Some combinations cancel each other out or change the setting chemistry.
- Expecting the admixture to fix what it cannot. It will not make up for poor aggregate grading, inadequate compaction or a lack of curing.
Graphene-based admixtures: what they change
Graphene-based admixtures do more than disperse the cement. In SmartADDITIVES, graphene reacts with the free lime during cement hydration, bonds chemically to its structure and densifies the cementitious matrix. The result is more strength with less water, which makes it possible to reduce cement while maintaining performance.
Cement is the component of concrete with the largest carbon footprint, so lowering its content is the most direct way to cut emissions without losing strength. The admixtures also have an Environmental Product Declaration verified by The International EPD System, which can be used in tenders with environmental criteria.
The range is organised by application:
- Solid MECHANIC: ready-mixed and plant concrete.
- Solid PRECAST: precast, with high early strength.
- Solid HARD: extreme exposure, impermeability and protection against chlorides and carbonation.
- Solid DRY: semi-dry concrete.
- Solid GUNITE: sprayed concrete.
- Solid FIBER: fibres to reduce or replace reinforcement.
- Solid SUPPORT: air entrainer, antifreeze, retarder, viscosity modifier and clay inhibitor.
We do not publish generic dosages: they depend on the cement and on each plant’s mix design. They are set through trial mixes with our technical department, which also checks compatibility with the admixtures the plant already uses and that the mix still meets the limiting values for its exposure class.
