Which mortar and concrete additives actually do anything?

Additives are sold as if each one solves a problem, and mostly they adjust a property at the cost of another. Knowing what each one actually does stops you buying five of them for a shed base, and stops you believing a bottle can substitute for correct mixing, correct timing and correct protection.

Safety

Step by step

1. Start by asking whether the mix design is the real answer.Most problems people reach for an additive to solve — a harsh mortar, a slab that cracked, a render that would not stick — are ratio, sand, water or preparation problems. An additive corrects a property; it does not rescue a wrong mix.
2. Add plasticiser as a matter of course in mortar.Dose per the bottle, into the mixing water, not sprinkled onto the dry pile. It goes further than people expect, and over-dosing produces a soft, over-aerated mortar with poor bond.
3. Do not use masonry cement and plasticiser together.Masonry cement already contains an air-entraining agent. Adding plasticiser on top entrains far too much air and gives a weak joint.
4. Treat accelerators and frostproofers as timing tools only.They shorten the window before the work can survive a frost. They do nothing whatever for a mix that freezes inside that window, so the covering-up and the weather forecast still govern whether you work at all.
5. Keep chlorides away from steel.Any accelerator containing calcium chloride will corrode reinforcement mesh, wall ties, lintels and fixings embedded in the mix. Check the bottle says chloride-free before using it anywhere near metal.
6. Use SBR as a primer and in the mix for repairs.Brush a slurry coat of SBR and cement onto the prepared surface, then apply the repair mortar gauged with diluted SBR while the slurry is still tacky. This is what makes a patch stay on rather than ring hollow in two years.
7. Never use washing-up liquid.It is the most persistent myth in DIY building. It entrains air in an uncontrolled and unstable way, it contains surfactants that interfere with the cement's hydration, and it produces a weak joint. Proper plasticiser costs a few pounds a bottle and does the job correctly.
8. Measure the dose.Almost all admixtures are dosed by volume per bag of cement or per mix, and the figure is small. Use the cap, a syringe or a measuring jug, and dose into the water so it disperses evenly.

Alternatives

• Mortar plasticiser (air-entrainer): Entrains millions of tiny air bubbles. Makes mortar creamy and easy to work, reduces water demand, and substantially improves frost resistance in the hardened joint because the bubbles give freezing water somewhere to expand into. The one additive worth having as standard.
• Integral waterproofer: A water-repelling agent that reduces capillary uptake through the hardened mix. Useful in a render below a damp proof course or in a mortar for a retaining structure. It does not make anything waterproof, it does not stop water through a crack, and used in a render on an old solid wall it traps moisture in the wall.
• Accelerator: Speeds the chemical set so the mix gains early strength faster. Genuinely useful in cold weather to get a job frost-resistant before nightfall. Most are chloride-free; a chloride-based accelerator must never go anywhere near reinforcement or a wall tie, because it corrodes steel.
• Retarder: Slows the set. Used in hot weather and on large pours where the mix would otherwise go off before it can be placed and finished. Also the basis of ready-mixed mortar sold in tubs that stays workable for days.
• Frostproofer: In practice an accelerator sold under a reassuring name. It shortens the time to a frost-resistant set. It does not lower the freezing point of the mix and it does not protect fresh work from a frost that arrives before the set is achieved. Cover up regardless.
• Polypropylene fibres: Fine plastic fibres dispersed through concrete to control plastic shrinkage cracking in the first hours. Genuinely effective at that and cheap. They are not a substitute for reinforcement and carry no structural load.
• SBR bonding agent: A styrene-butadiene latex used as a primer coat and as a gauging liquid in the mix. It improves adhesion of a repair mortar or a render to an existing surface and improves the repair's flexibility and water resistance. The standard choice for patching concrete and rendering onto a difficult background.
• Waterproof render additive systems: Proprietary combinations of waterproofer and bonding agent supplied as a system with a specified mix. Used in tanking and in damp-proofing renders. Appropriate on a modern wall, and a decision that needs care on an old solid one.

Questions people ask