Under normal conditions, once cement and water are mixed the hydration process will begin and the concrete or mortar will gain strength and increase in durability with the passage of time. To some degree concrete is self-curing. However, the term "curing" has come to mean more than "self-curing." In fact, curing has come to mean managing the environment in which hydration takes place in order to enhance and optimize the properties of hardened concrete.
The hydration process is driven by three elements:
- the availability of unhydrated cement
After concrete has begun its hardening process, the relative distances between the hydrated and unhydrated cement particles are by-and-large fixed. The introduction of water into the system at this point is not likely to cause a change in distances between the hydrated and unhydrated cement particles.
It may be news to some people that not all the cement in concrete hydrates. In fact, it has been estimated that under normal conditions (self-curing) only 75% or so of the cement ever hydrates. Unhydrated cement, locked in the concrete, can hydrate only if it comes into contact with water.
How much water is needed is support hydration? The answer to this question lies in both the physical and chemical world. During the hydration process, water is chemically combined with cement in the ratio of approximately 25 pounds of water to 100 pounds of cement. The products of hydration, however, do not form a solid mass. Rather, their physical appearance is more like steel wool with a lot of empty space between the crystals. Within this mass lie both hydrated and unhydrated cement. Also within this mass is water that is physically bonded (gel water). The physically bonded water exists in the spaces between hydrated cement crystals.
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As stated above, in order for cement to hydrate, the cement grains must come in physical contact with water. Although the pore structure of cement paste is microscopic in size, its relative volume can be quite large. There is a lot of space in which water can be present without having every molecule of water in direct contact with cement. Therefore, to insure contact between unhydrated cement and water there must be more water present than the minimum required for complete chemical bonding with the cement.
The next question that must be answered is, "How much more water must be available for complete hydration?" The answer has been known for more that 50 years. It has been determined that the amount of water for complete hydration is equal to the chemically bonded water (25 pounds of water per 100 pounds of cement) plus extra water (physically bonded) that is required to maintain contact with all the cement. The physically bonded water required is an additional 20 pounds of water. Therefore, the total amount of water required for total hydration can be stated as 45 pounds of water per 100 pounds of cement, or a water/cement ratio of .45. In addition, there must be an accounting for the water lost from the system through evaporation. This would require the water:cement ratio to be something higher than .45. (See related article by Ken Hover, FACI, Spring 2002 issue of L&M's Concrete News.)
The water/cement ratio law states that as the water to cement ratio is reduced (to a minimum point) the strength is increased. As the water/cement ratio is increased the distance between hydrated cement crystals is increased, thereby reducing the bonding contact area between the hydrated cement crystals and the aggregate. This, in turn, reduces the strength of the concrete.
The primary factor determining concrete strength is the density of the hydrated cement paste. The ratio at which the unhydrated cement combines with water is predetermined by its unique stoichiometric ratio. The water content of the concrete does not affect the strength of the individual crystals formed during hydration. However, the water content of the concrete mix, at the time of hardening, will play a major role in determining the physical proximity of the individual crystals to each other and the overall density of the hydrated cement paste.
As the cement paste becomes more dense (before hardening) the higher will be the strength of the hardened concrete or mortar. It is for the same reason that the tighter the concrete is troweled, the harder the concrete at the surface will become. This is due to the unhydrated cement particles being physically pressed closer together and developing a stronger bond. It should be noted, however, a little knowledge can be dangerous. Over-troweling causes its own set of problems in the finished concrete surface.
The concept of the water/cement ratio and the need for more water than is required by the stoichiometric equation for hydration, teaches us two things about how to improve the strength of concrete:
- Keep the water to cement ratio as low as possible in order to produce the densest possible hydrated cement paste; and
- After the concrete has hardened, keep the internal moisture content of the concrete as high as possible, for as long as possible, by using proper curing procedures.
About 60 years ago the Portland Cement Association determined that when the internal relative humidity of concrete drops below 80 percent, virtually all hydration stops. In addition, as reported by the PCA, as the temperature of concrete falls below 50o F the rate of hydration begins to slow; when below 40o F it is greatly retarded; and at the freezing point, it virtually stops.
It should also be noted that for hydration to occur space is required for crystalline growth. Once this space within the cement system is filled, there is no longer any room for crystalline growth and all hydration stops.
American Concrete Institute's ACI 308R-01 "Guide to Curing Concrete" is an excellent source for information on this subject.
© 2003 L&M Construction Chemicals, Inc. | ConcreteNews Summer 2003.