Lithium silicate chemical hardeners for concrete have been growing in market share in comparison to sodium or potassium silicates, increasing in direct proportion to diamond-polished floor work. So why are lithium silicate densifiers sales increasing?
How silicates work
Sodium, potassium, magnesium, and lithium silicates all react with calcium hydroxide (also referred to as “portlandite”)—a byproduct of cement hydration—to produce calcium silicate hydrate (C-S-H), the same binder that results from adding water to cement and gives concrete much of its strength and hardness. In the hydration process, calcium hydroxide dissolved in water moves to the surface region of a slab where the silicates can react with it. This newly created C-S-H is deposited primarily in the pores and canals on the surface of a slab.
Normally when calcium hydroxide comes to the surface of fresh concrete, it reacts with atmospheric carbon dioxide, producing carbonation (calcium carbonate). The reaction is greater when the concrete humidity is high and when bleeding is prolonged. It's also accelerated by construction heaters that produce carbon dioxide.
This reaction of soluble silicate with calcium hydroxide in concrete also produces alkali metal hydroxide, lithium hydroxide, potassium hydroxide, or sodium hydroxide, all of which could be detrimental to concrete if reactive aggregates and moisture are present. There is also the potential of the silicate to form efflorescence, which is highest with sodium, lower with potassium, and lowest with lithium. The function of the sodium, potassium, or lithium part of the silicate's function only is to stabilize and solubilize the silicate so it can remain in solution until it penetrates the concrete and then can react with the abundant calcium hydroxide found in the concrete. Sodium, potassium, or the lithium ions typically do not react in concrete to any degree, so they are incidental to the primary benefits.
But the hydroxides from sodium, potassium, and magnesium in combination with laitance from the scrubbing process must be removed before they crystallize on the surface. The advantage of lithium—when applied in the correct amount—is it dries to a dust. It also is considerably more alkaline, raising the pH of the surface concrete and reducing the possibility of alkali silica reaction (ASR).
A compelling reason to consider using lithium densifiers instead of sodium is that the application is much easier and proceeds quicker. It's typically sprayed lightly on a slab—compared to saturating a floor surface with other silicates—leaving no residue to clean up. If applied excessively it must be removed before it crystallizes, just like other silicates. The higher reactivity of lithium compared to sodium also means you don't have to scrub it into concrete to encourage the reaction. The concentration of sodium densifiers must be higher than lithium to achieve the same effect. The Environmental Protection Agency (EPA) rates silicate residues as hazardous materials because pH levels are high (10+) so other silicate residues must be disposed of as hazardous waste, adding to the contractor's expense. When these residues dry on the surface of slabs, they deposit whitish crystal silicates that are difficult to remove.