5.1      Overview of the Hydration Process

     The hydration of cement can be thought of as a two-step process. In the first step, called dissolution, the cement dissolves, releasing ions into the mix water. The mix water is thus no longer pure H2O, but an aqueous solution containing a variety of ionic species, called the pore solution. The gypsum and the cement minerals C3S and C3A are all highly soluble, meaning that they dissolve quickly. Therefore the concentrations of ionic species in the pore solution increase rapidly as soon as the cement and water are combined. Eventually the concentrations increase to the point that the pore solution is supersaturated, meaning that it is energetically favorable for some of the ions to combine into new solid phases rather than remain dissolved. This second step of the hydration process is called precipitation. A key point, of course, is that these new precipitated solid phases, called hydration products, are different from the starting cement minerals. Precipitation relieves the supersaturation of the pore solution and allows dissolution of the cement minerals to continue. Thus cement hydration is a continuous process by which the cement minerals are replaced by new hydration products, with the pore solution acting as a necessary transition zone between the two solid states. The reactions between portland cement and water have been studied for more than a hundred years, and the fact that hydration proceeds by a dissolution-precipitation process was first elaborated by the famous chemist Le Chatelier [1].
     There are two reasons that the hydration products are different from the cement minerals. One reason is that there is a new reactant in the system: water. Not only does the water facilitate the hydration process by dissolving the cement minerals, but it also contributes ions, in the form of hydroxyl groups (OH-), to the hydration products. The second reason is the tendency for all processes to approach thermodynamic equilibrium. This dictates that the solid phases that precipitate out of the pore solution are the ones that are the most stable under the current conditions. The stability of a phase is defined by a parameter called the free energy, which can be roughly defined as the amount of chemical and thermal energy contained in the phase. The lower the free energy, the more stable the phase. As discussed in Chapter 3, the cement minerals are formed at temperatures exceeding 1400?C, because they have the lowest free energy under those extreme conditions. At the much lower temperatures present during cement hydration, the cement minerals are actually quite unstable, meaning that there are many other solid phases that will form preferentially in their place once they dissolve. In fact, the whole point behind the high-temperature cement manufacturing process is to create solid phases that will readily dissolve in water, allowing new phases to form. When one phase is converted into another phase with a lower free energy, there is usually a release of excess energy in the form of heat. Such a reaction is termed exothermic, and the exothermic heat associated with cement hydration has already been defined as the heat of hydration.
    Figure 5-1 shows a graph of the rate of cement hydration over time, with the hydration process divided into four somewhat arbitrary stages. Information about the rate of a reaction is called kinetics, and the kinetics of cement hydration are covered in detail in Chapter 9. Here we will use the general behavior shown in Figure 5-1 to discuss the various processes that occur during hydration.


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Figure 5-1:  Schematic of the rate of hydration or heat evolution as a function of time.

    As noted above, some of the cement minerals and constituents are very soluble, and thus when cement and water are first combined there is a short period of fast reaction and heat output as the cement dissolves, lasting for less than one minute (Stage 1). Stage 1 is brief because of the rapid formation of an amorphous layer of hydration product around the cement particles, which separates them from the pore solution and prevents further rapid dissolution. This is followed by the induction period, during which almost no reaction occurs (Stage 2). The precise nature of the induction period, and in particular the reason for its end, is not fully known, or perhaps it should be stated that it is not fully agreed upon, as there are strongly held but differing opinions among cement chemists. The induction period is discussed in Section 5.2.
    During Stage 3, the rapid reaction period, the rate of reaction increases rapidly, reaching a maximum at a time that is usually less than 24 hours after initial mixing, and then decreases rapidly again to less than half of its maximum value. This behavior is due almost entirely to the hydration of the C3S, and the rate of hydration is controlled by the rate at which the hydration products nucleate and grow. Both the maximum reaction rate and the time at which it occurs depend strongly on the temperature and on the average particle size of the cement. This reaction period is sometimes divided into two stages (before and after the maximum rate) but as the rate-controlling mechanism is the same throughout (nucleation and growth) it is preferable to treat this as single stage.
    At the end of Stage 3 about 30% of the initial cement has hydrated, and the paste has undergone both initial and final set. Stage 3 is characterized by a continuous and relatively rapid deposition of hydration products (primarily C-S-H gel and CH) into the capillary porosity, which is the space originally occupied by the mix water. This causes a large decrease in the total pore volume and a concurrent increase in strength. The microstructure of the paste at this point consists of unreacted cores of the cement particles surrounded by a continuous layer of hydration product, which has a very fine internal porosity filled with pore solution, and larger pores called capillary pores.
    In order for further hydration to take place, the dissolved ions from the cement must diffuse outward and precipitate into the capillary pores, or water must diffuse inward to reach the unreacted cement cores. These diffusion processes become slower and slower as the layer of hydration product around the cement particles becomes thicker and thicker. This final period (Stage 4) is called the diffusion-limited reaction period.
    Figure 5-2 shows the microstructure of a cement paste at it hydrates, as simulated by a realistic digital image based model [ ]. The yellow phase is the main hydration product, C-S-H gel. At the end of Stage 3, the yellow rims if hydration product have become interconnected, causing final set and giving paste some minimal strength. By 28 days the image is dominated by C-S-H gel and the porosity has noticeably decreased. The final amount of porosity will depends strongly on the initial w/c of the paste.

Figure 5-2: Results of a realistic digital model of cement hydration. Phases are color coded: Black=water (pores), Red = C3S, Blue = C2S, Yellow = C-S-H gel. a) Cement particles dispersed in water just after mixing. (Stage 1). b) 30% hydration, ~ 1 day (end of Stage 3). c) 70% hydration, ~ 28 days (Stage 4). (Images courtesy of NIST).

    The overall progress of the hydration reactions is described by the degree of hydration, , which is simply the fraction of the cement that has reacted. Complete hydration of all the cement gives = 1. The degree of hydration can be measured in a few different ways, including x-ray measurements to determine how much of the minerals remain and loss on ignition measurements to determine how much bound water the paste contains. Another common method is to sum the amount of heat given off by the paste (as measured by thermal calorimetry) and divide this value by the total amount of heat given off for complete hydration. The latter value will depend on the mineral composition of the cement. Another parameter that can be used to monitor the progress of hydration is the compressive strength. This is not a precise measure, since the strength depends on many factors other than the progress of the chemical reactions, but it is very practical since the development of strength is the primary reason for using cement and concrete in the first place. Figure 5-3 shows the degree of hydration ( ), and the strength of a Type I OPC paste plotted as a function of time on the same graph. Note that the time is plotted on a log scale.
    From Figure 5-3 it can be seen that the degree of hydration and the strength track together, particularly at later times. This is because the strength of cement paste depends primarily on the amount of capillary porosity, and the amount of capillary porosity decreases in proportion to the amount of hydration that has taken place. This decrease occurs because the C-S-H gel phase (including its internal gel pores) occupies significantly more volume than the cement minerals it forms from.

Figure 5-3: Typical development of the degree of hydration and compressive strength of a Type I portland cement over time.

    In Figure 5-3 alpha has reached a value of 1 (complete hydration) after one year, but this will not always be the case, as many cement pastes will never reach full hydration. Depending on why hydration terminates, incomplete hydration may or may not be a bad thing. The final degree of hydration will depend on the w/c of the paste, the cement particle size, and the curing conditions. Hydration will continue at a slow rate during Stage 4 until one of the three following criteria is met:
1) All of the cement reacts. This is the situation shown in Figure 5-3. This indicates that the paste has a moderate or high w/c and was cured correctly. While it is the best possible outcome for the given mix design, it does not guarantee high quality concrete as the w/c may have been too high. If the cement contains some large particles, full hydration of these particles may not occur for years. However this is generally not the case with modern cements.
2) There is no more liquid water available for hydration. If the cement has a w/c less than about 0.4, there will not be enough original mix water to fully hydrate the cement. If additional water is supplied by moist curing or from rainfall, hydration may be able to continue. However, it is difficult to supply additional water to the interior of large concrete sections. If the cement is improperly cured so that it dries out, hydration will terminate prematurely regardless of the w/c. This is the worst-case scenario, as the strength will be lower (perhaps significantly) than the value anticipated from the mix design.
3) There is no more space available for new reaction product to form. When the capillary porosity is reduced to a certain minimal level, hydration will stop even if there is unreacted cement and a source of water. This is the best possible outcome, and it is only possible if the w/c is less than about 0.4. Not only will the cement paste or concrete have a high strength, but it will also have a low permeability and thus be durable.

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