21-12-2012, 06:23 PM
The Use of Silicate Solutions to Cure Concrete
The Use of Silicate.pdf (Size: 40.92 KB / Downloads: 364)
Introduction
Sometimes the action of concrete hardening is referred to as the curing process. But for the
purpose of this white paper, concrete curing will be defined as in the American Concrete Institute
(ACI) Report 116: “The action taken to maintain moisture and temperature conditions in a
freshly placed cementitious mixture to allow hydraulic cement hydration and (if applicable)
pozzolanic reactions to occur so that the potential properties of the mixture may develop.”
When concrete is not properly cured, its strength, durability, appearance, and resistance to
freeze-thaw damage will suffer. Here’s why: Cement in concrete needs water in order to
chemically react and form a hard, durable matrix, or “glue” that holds the concrete together.
This reaction between cement and water is called hydration, and it can continue for a long period
of time after concrete placement. It is beneficial to keep the concrete curing method in place for
as long as possible, since a long, slow hydration process will result in concrete with the best
strength and durability. If water evaporates too quickly from concrete (as it will if no legitimate
curing method is used) there will be cement in the concrete that did not properly hydrate. This
can lead to the durability and strength problems mentioned earlier.
More specifically, improper curing can cause plastic shrinkage cracking, which occurs when
moisture rapidly evaporates from a fresh concrete surface faster than the bleed water can appear
to replace it. Stress in the new concrete surface resulting from this rapid drying causes the short,
irregular cracks known as plastic shrinkage cracking.
Concrete Industry Statements on Using Silicate to Cure Concrete
The application of a curing compound to a freshly finished concrete surface will help ensure that
the water required for proper hydration of the cement stays in the concrete, instead of
evaporating out. A quality curing compound does this by forming a thin, continuous film on the
concrete surface that greatly reduces the amount of internal water that can pass through as vapor.
Think of it as stretching a large, thin piece of plastic wrap over the concrete to hold the moisture
in.
The American Concrete Institute (ACI) in its Guide for Concrete Floor and Slab Construction
(ACI 302.1R), as well as the American Society for Testing and Materials (ASTM) specification
C 309 clearly define concrete curing compounds as “membrane-forming”. Solutions of
sodium, potassium, or lithium silicate are not membrane forming, and therefore do not comply
with the above industry standards for concrete curing compounds.
The Intended Purpose of Silicate on Concrete
Silicate solutions are very useful materials that can improve the durability of concrete when used
properly. First and foremost, silicate solutions are penetrating materials on concrete. In order to
work properly, silicates need to be able to soak into the concrete surface to form a hard, glassy
substance (calcium silicate) within the pores of the concrete. When applied to fresh concrete,
when the surface pore structure is filled with water, silicates will not penetrate properly and will
not chemically react as intended.
Appropriate Curing Options
There are three general methods for curing new concrete. The first method is called water
curing and involves keeping a continuous flow, ponding, or fog of water on the concrete surface
for at least 7 days. Often, this is not practical for concrete work as it makes it difficult to
impossible for other trades to continue work on the project while the curing is taking place. The
second method is the placement of moisture-retaining coverings such as plastic sheeting, wet
burlap, or curing blankets over the freshly finished concrete (also recommended to stay in place
for a minimum of 7 days). These coverings can be challenging to work and walk on, and can
leave stains or marks on the concrete surface if place improperly. The third method of curing is
the application of liquid membrane-forming curing compounds or curing and sealing
compounds. Curing and sealing compounds, besides retaining moisture in fresh concrete, have
the added benefit of providing a longer-lasting protective and decorative seal on the surface.
By their nature, curing compounds and cure and seals leave a film on the concrete surface that
can interfere with the adhesion of other materials to the treated surface such as resilient floor
coverings, protective coatings, sealers, or liquid densifying treatments. Because of this, it is
tempting to use a silicate solution as the curing compound on a new project, as silicates do not
form a film that can interfere with adhesion. However, the use of a dissipating curing
compound is a much more appropriate option to cure new concrete that will later be coated or
covered. Dissipating curing compounds meet all the industry standards and guides for concrete
curing including ACI 302.1R and ASTM C 309, and over time they lose adhesion from the
concrete and wear away under traffic and UV exposure. Concrete cured with a dissipating
curing compound need only a simple wash to ready them for covering or sealing.
Summary
Sodium silicate solutions do not provide the two primary functions of a concrete curing
compound: they do not form a continuous film on the concrete surface and they do not retain
moisture or promote proper cement hydration in freshly placed concrete. Concrete industry
standards clearly state that the use of a silicate solution as a curing compound puts the concrete
at risk for low strength development and surface durability issues. Used for their intended
purpose, silicates can densify a well-cured concrete surface but should not be mistaken as a
suitable option for curing new concrete.