Hydration process

Hydration process

Concrete is a mixture prepared by mixing cement, aggregates, and water that forms a workable paste. The paste is molded and consolidated as desired before being left to harden. The concrete acquires its strength through a series of chemical reactions between water and the cement compounds. The process by which the cement compounds react to form chemical bonds with the molecules of water is referred to as hydration. The hydration process is critically important in concrete manufacture.

Ordinary Portland Cement constitutes five major components; tricalcium silicate, dicalcium silicate, tricalcium aluminate, tetracalcium aluminoferrite, and gypsum. An addition of water to the cement initiates a hydration reaction with each of this compounds. The hydration of calcium silicates contributes more to the overall concrete strength in comparison to the other compounds.

Upon addition of water, there occurs a rapid reaction with the tricalcium silicate, resulting in the formation of calcium silicate hydrate crystals and calcium hydroxide. As the process continues, these crystals grow thicker and prevent water from reaching the tricalcium silicate. Due to this, the process is slowed down considerably. The hydration of dicalcium silicate is much similar to that of tricalcium silicate but is at a slower pace since it is much less reactive in comparison. Dicalcium silicate contributes significantly to the long term strength of the concrete. The other compounds undergo hydration but do not bring about a considerable amount of strength to the concrete.

The overall concrete strength will be dependent on the amount of water used. Therefore, to ensure strength, a sufficient amount of water must be used to allow complete hydration and for the workability of the concrete paste in the initial stages.

During hydration and drying, there is a constant loss of water in the concrete mass. The volumetric changes in the structure of concrete due to internal loss of moisture is referred to as concrete shrinkage. Shrinkage of concrete is a time-dependent deformation and is independent of any applied loads. This may result in the formation of cracks within the concrete mass. The factors that affect shrinkage include; time, water to cement ratio, humidity, type of aggregates used, and the drying conditions..

Plastic shrinkage is a decrement of the volume of the concrete before it sets and manifests momentarily after the placing of concrete. Methods that reduce rapid water loss in the concrete are usually applied to counter the effects, such as covering the concrete with polythene to reduce evaporation. Drying shrinkage is the volume decrement of hardened concrete due to water loss in the gel pores existing within the concrete. Autogenous shrinkage is volume loss due to the hydration reactions and is most applicable in the interior of concrete dams. Carbonation shrinkage is caused by the disintegration of calcium hydroxide and a subsequent calcium carbonate deposition in its place.  This occurs due to a reaction with carbon (IV) oxide in the atmosphere. The smaller volume of calcium carbonate causes shrinkage.

The deformation of concrete due to applied loads over a sustained time is referred to as concrete creep. This type strain occurs in the direction of the load applied, and may not necessarily lead to concrete failure. The factors that contribute to concrete creep include; the size of the applied stress, the age and strength of the concrete, the type and properties aggregates, environmental conditions, amount of steel reinforcement, and conditions during curing. The concrete mix proportions also lead to creep since a poorer paste will tend to lead to higher concrete creep. The same applies to higher water to cement ratio in the initial paste.

References

Bazant, Zdenek P., and Folker H. Wittmann. “Creep and shrinkage in concrete structures.” (1982): 12-16.

Gartner, E. M., et al. “Hydration of Portland cement.” Structure and performance of cements 2 (2002): 57-113.

Principles, Scientific. “Concrete: Scientific Principles.” University of Illinois.