Water Required for Hydration of Cement

Water Required for Hydration of Cement

Water Required for Hydration of Cement

Water Required for Hydration of Cement: C3S needs 24% water by weight of cement, while C2S requires 21%. Additionally, it has been estimated that approximately 23% of the water in cement by weight is needed for chemical reaction with Portland cement compounds. Since this 23% of water chemically reacts with cement, it is referred to as bound water.

Within the gel-pores, a certain amount of water is absorbed. This water is referred to as gel-water. One might argue that bound water and gel-water are complementary. If there is insufficient water to fill the gel-pores, the forming of gel will stop, and if the formation of gel ceases, there is no possibility of gel-pores being present. Additionally, it has been estimated that approximately 15% by weight of cement is needed to fill the gel-pores. Thus, a total of 38% water by weight of cement is needed to complete the chemical reactions and fill the space inside the gel-pores.

If only water equal to 38% of the cement’s weight is used, the resulting paste would experience complete hydration and no further water will be required for the forming of undesirable capillary cavities. When, on the other hand, more than 38% of water is used, the extra water results in undesirable capillary cavities. Thus, the more water used beyond the minimum needed (38%), the more unwanted capillary cavities may form. All of this is based on the assumption that hydration occurs in an enclosed jar that prevents moisture from entering or leaving the paste.

If the water/cement ratio increases, the capillary cavities gets larger. When the w/c ratio is reduced, the cement particles get closer together. As the amount of anhydrous cement increases during hydration, the products of hydration also increases. The volume rise in the gel caused by full hydration could fill the area previously filled by water up to a w/c ratio of around 0.6.

If the w/c ratio is greater than 0.7, the hydrated product’s volume will never be adequate to fill the voids formed by water. This kind of concrete will never retain its porous nature. This is to suggest the gel takes up an increasing amount of space formerly used by mixing water. The amount of gel is measured to be about twice that of unhydrated cement.

Figure 1 illustrates the progression of hydration diagrammatically.

Water Required for Hydration of Cement
Figure 1: The hydration mechanism and the forming of cement gel are depicted diagrammatically (source).

The condition of cement particles immediately after dispersion in an aqueous solution is depicted in Figure 1 (a). The reaction is fast within the first few minutes, and the calcium silicate hydrate forms a covering around the cement grains. Refer to Figure 1 (b). If the hydration process progresses, hydration compounds such as calcium hydroxide precipitate from the saturated solution and bridge the distance between the cement grains, stiffening the paste to its final form, as shown in Figure 1 (c). Additional hydration results in increased cement gel deposition at the expense of unhydrated cement and capillary porewater [Figure 1 (d)].

The estimated structure of hardened cement paste has been identified briefly due to the hydration of some of the main compounds. The result of the hydration of the other main and minor compounds in cement receives little attention. Continued analysis is being conducted on the morphology of the hydration products and the nature of hardened cement paste in its entirety.

The development of high voltage electron microscopy, along with advances in the ability to make very thin sections, enables high quality imaging and diffractometry while minimizing damage to the specimen during observation. The scanning electron microscope produces stereographic photographs and a clear image of the cement paste’s composition. These provide further insight into the aggregate cement bond, microfracture, and porosity of cement gel.

Read Also: ASTM C403: Setting Time of Concrete

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