Both harden the surface by reacting with free lime, but they do not behave the same on the floor. Lithium is denser, penetrates further and reacts faster; sodium is cheaper, reacts more at the surface and is more likely to leave a residue.

Side by side

Property	Lithium	Sodium
Typical dilution	1:3 to 1:4	1:4 to 1:8
Reaction speed	Fast, 2-6 h	Slower, 6-24 h
Residue risk	Low	Higher, especially in cool conditions
Efflorescence	Low	Moderate
Cost per m2	Higher	Lower
Best use	Dense, hard-trowelled slabs; gloss work	Porous, open slabs; budget work

Choosing by slab, not by price

On a hard-trowelled, low-porosity slab, sodium densifier has little free lime to react with near the surface and tends to sit, dry white and require re-wetting. Lithium penetrates that tighter surface better and finishes cleaner. On a porous, open slab, sodium works well because there is enough lime through the top layer and the cost difference is meaningful on large areas.

Timing before polishing

Do not polish into a densifier that is still reacting. Lithium needs 4-24 hours depending on slab temperature, sodium often 12-24. If the floor is tacky, still darkening or cool to the touch, wait. Polishing too early loads the resin pads with chemical and produces streaks that look like scratch marks.

Application rules that apply to both

• Two thin passes with a uniform wet film, never a flood.
• Keep the floor wet for the dwell window, then remove excess before it dries on the surface.
• Do not apply in direct sun or above 30 C without misting; flash drying leaves white residue.
• Test a 1 m2 patch and read it after 24 hours before committing the whole floor.

What about potassium and blends?

Potassium sits between the two on penetration and cost, and is common in blended products. Blends are fine when the manufacturer states the lithium content; be cautious with products that only say "densifier" without chemistry, because application rate and dilution are usually tuned for one chemistry.

What the reaction looks like on site

The difference between lithium and sodium is easiest to see in the ten minutes after application and the twenty-four hours after that. Reading those two windows tells you which product you actually have, and whether it reacted at all.

What you watch	Lithium	Sodium	What a bad sign looks like
Wetting out on the surface	Stays open longer, spreads evenly	Beads quickly on dense slabs	Dry patches within two minutes
Surface residue next day	Little or none	White powder common, needs removal	Chalky film that wipes off easily
Feel after cure	Slick, glassy	Slightly grippier, sometimes dusty	Soft or tacky hours later
Water absorption test	Drop stays on surface	Drop stays, edge may darken	Drop absorbs in under a minute
Response to polishing	Clarifies quickly at 400-800	Needs extra passes to clear residue	Haze that reappears after burnishing

None of those observations replace a hardness or absorption test, but they tell you within a day whether the application is on track. A floor that still absorbs water freely 24 hours after a densifier pass did not get enough product, went on too cold, or was applied over a surface that was too dry to pull it in.

Troubleshooting a densifier that did not harden

The most common failure is not the wrong product, it is the right product applied to a surface that could not accept it. Work through the list before ordering another drum:

• Surface was too dense or too polished. A densifier needs open pores. On a floor already polished to 800 or finer, most of the chemical stays on top and does nothing; apply before the last steps, not after.
• Application rate was too low. Check the record: most products specify 3-5 m² per litre. A rate of 8-10 m² per litre gives a third of the reaction.
• The slab was cold or the air was dry. Below about 10 C the reaction slows sharply, and in low humidity the product dries before it penetrates.
• Application was uneven. Two thin passes in opposite directions beat one flood pass every time, and a flood leaves puddle marks that no second application fixes.
• Residue was left on. Sodium products especially need the white powder removed by microfibre or a damp extraction pass, or it loads the first resin step and hazes the finish.

Test before re-applying: pour a cup of water on the floor and time how long it takes to disappear. Under a minute means the surface is still open and could take a second pass; over five minutes means it has hardened and the problem is elsewhere, usually in the polishing sequence rather than the chemistry.

For most polished concrete work where the finish is the product being sold, lithium is the lower-risk choice. Where the slab is open, the area is large and the finish is a honed industrial floor rather than a showroom polish, sodium is perfectly serviceable and cheaper.