Q.: We have been asked to comment on specifications that call for applying a fluosilicate hardener to concrete slabs at the time of construction. Some slabs are indoor and some outdoor, all part of an agricultural facility. Does such a hardener serve to both harden and cure? Does it act as a sealer? The specifications call for applying a membrane curing compound.
A.: ACI Committee 302 considers liquid floor hardeners (even including fluosilicate hardeners, which are probably the best) to be only remedial agents. Fluosilicate hardeners function by reacting with cement compounds to deposit crystalline fluorides and silica in the pores of the concrete. This makes the concrete harder, stronger and less porous than it had previously been. Fluosilicate hardeners are intended for use to improve the hardness of floors that are defective, such as floors that are too soft because the concrete mix was too low in strength design, floors that were not placed, finished or cured properly, or floors with surfaces that have become carbonated from use of unvented heaters. This type of hardener does not act as either a curing agent or sealer, and when it is applied it should not be used until after any liquid membrane curing agent has dissipated or been removed.
Fluosilicate hardener should really not be used as part of the planned construction of new floors. Instead, the concrete should be designed to produce enough strength to provide the hardness required. Good construction practices and good curing practices should be followed (see ACI 302, "Guide to Concrete Floor and Slab Construction"). A very high quality floor will have a low enough porosity so that it would not soak up much fluosilicate hardener anyway.