What densifiers don’t do—curing
Reactive chemical surface treatments are not effective curing compounds. They do not form a membrane that retains moisture to promote the hydration of portland cement. At best, they densify the surface and, along with troweling, may slow the rate of drying of the slab interior. But at the surface, they are no more effective than water. Like water, once the liquid is removed or dries, curing of the surface ceases.
Curing issues rarely, if ever, arise at mid-depth in a concrete slab because it dries so slowly there. But curing is essential for surface integrity where the concrete can dry rapidly. The problems caused by not curing may not be readily apparent when ambient conditions are favorable, but in warm dry conditions, the resulting surface deficiencies, such as dusting, crazing, and mortar flaking, can be obvious. After all, it’s the curing at the surface that really matters.
The claim that a reactive chemical penetrant “complies with ASTM C309 on hard troweled surfaces” is misleading. ASTM C309, Specification for Liquid Membrane-Forming Compounds for Curing Concrete, requires a water retention of 0.55 kg/m2 in 72 hours when tested in accordance with ASTM C156. This test evaluates the effectiveness of the curing product, not the slab finish or a combination of the two. In ASTM C156, the potential curing compound is applied to a standard wood-float finish and the water-retention capability is measured. If the test is altered to a densified hard-troweled finish, no product at all is often needed to retain 0.55 kg/m2 in 72 hours. This implies that simply hard-troweling a floor meets the ASTM C309 requirement for water retention. But, since the top surface of the concrete does not get cured, it will not achieve its potential strength and durability properties posing, a risk of surface problems. Unless you plan to grind or polish the weak surface off, curing of the slab is critical.
Separate, effective curing is still required on hard-troweled floors that are to receive a silicate treatment. The treatment can be applied before or after curing, but if a curing compound is used first, it must be removed before applying the treatment or the silicate compound cannot penetrate. Unfortunately, complete removal of a curing compound from the surface pores is a challenge and the removal process removes some of the calcium hydroxide so the effectiveness of the surface treatment is decreased.
But if the surface treatment is applied first—before the curing compound—the fresh slab is saturated so initial penetration is minimal. Also, any byproduct must be flushed away, which also removes some silicates that are no longer available to react. Some densifiers, however, such as those that are lithium-based, do not form a byproduct that requires removal so they can be applied and covered with a curing compound without flushing. During the curing period, the silicates are drawn into the saturated pores and react.
In any case, reactive silicate treatments should not be used to cure slabs, since they do not have moisture retention capabilities. If they are applied too heavily, they will result in an unreacted white residue on the surface that is objectionable to most owners and difficult to remove.
What densifiers don’t do—moisture vapor
Just as reactive penetrants are not effective curing compounds, they also do not make reliable topical moisture vapor mitigation products. Water vapor transmission is measured in permeance by ASTM E96, Test Method for Water Vapor Transmission of Materials. To be effective, a moisture mitigation product must reduce vapor transmission below about 0.1 perms to allow the installation of most floor coverings (that is, if one is unable or unwilling to wait long enough to let the slab dry naturally).
Surface densifiers/hardeners do not form a membrane or confluent layer of any kind. At best, they densify the surface by creating additional CSH within the pore system, but there is no basis to claim they fully fill the pores with reaction product. In the worst case, there is no calcium hydroxide remaining to react with and they do nothing at all.
When tested using ASTM E96, concrete slabs that contain integral silicate admixtures or topical silicate treatments result in no measurable decrease in water vapor transmission when compared to equivalent slabs without the silicate. In addition, the reaction product is not hydrophobic, so it is not an effective water repellent. Without supporting test data, the use of reactive penetrants as an integral or topical moisture vapor mitigation should only be attempted when the risk of a floor covering failure is acceptable.
Conclusion
To use or not to use? That is the question regarding liquid chemical treatment products. In my opinion, the potential advantages outweigh the potential disadvantages with the understanding that these products are intended to harden and densify the immediate surface of the slab only. They are not effective curing compounds or reliable moisture vapor mitigation options.
The degree to which hardener/densifiers perform depends on the amount of available calcium hydroxide which is also consumed by pozzolans and carbonation. But while their effectiveness cannot be quantifiably measured, surface hardener/densifiers are generally requested by owners and their tenants to create attractive floors that are easy to keep clean. These surface treatments don’t weaken the surface and, if applied properly, do not result in discoloration. Given sufficient calcium hydroxide, they can improve the abrasion resistance of weak surfaces prone to wear. However, they cannot restore a poor surface to the quality of a properly finished and well-cured slab. In most repair cases, they slow surface wear but need to be re-applied occasionally as additional calcium hydroxides are exposed. On properly-finished well-cured slab surfaces, their benefit may be minimal but their harm can be negligible.
Scott Tarr, P.E., FACI, is president of North S.Tarr Concrete Consulting, Dover, N.H. He is the author of “Concrete Floors on Ground” published by the Portland Cement Association.
