H YDRATION PROCESS

Portland cement is what is called a hydraulic cement, and these types of cement set and develop strength as a result of hydration. The hydration process consists of chemical reactions between the cement mixture and water. This happens because the compounds present in the cement mixture are anhydrous, meaning that they are decomposed when in contact with water and will as a result form hydrated compounds. The solubility of the hydrated compound is lower than that of the original anhydrous compound, and thus, complete hydration saturation will eventually occur. For Portland cement, it is shown that a volumetric shrinkage will occur during the hydration process. [2]

There are four main components of Portland cement; C3S, C2S, C3A and C4AF. These structure different hydration products and demonstrate different hydration kinetics. Due to this fact, the research regarding cement hydration has mainly focused on the hydration of each individual clinker phase in water. A table displaying the approximate weight percent of each of the components are seen in table 2.1.

The most abundant phase, constituting 70-90 wt% of the Portland cement, is the silica phase.

The main component consisting of 55-65wt% is C3S whereas 15-25wt% is comprised of C2S.

Hydration of these silica phases therefore plays an important role in the properties of the cement. The resultant hydrated compound from the hydration process of both these phases are calcium silicate hydrate and calcium hydroxide. Shown below are the idealized chemical equations; [2]

2C3S C3S2H3 + 3CH 2C2S + 4H C3S2H3 + CH

strength development is the hydration of the C3S present, whilst the hydration of the C2S mainly contributes to the final strength of the hardened cement. As the mechanism of the hydration of these phases are very similar, the Portland cement hydration behavior is often analog to C3S hydration. [2]

The hydration rate of C3S can be measured by heat release as it is an exothermic process and it is defined in five different stages:

• Preinduction period

• Induction period

• Acceleration period

• Deceleration period

• Diffusion period

From figure 2.1 one can observe the evolution of heat in the different stages of the cement hydration.[2]

Figure 2.1 Evolution of heat vs time of hydration[2]

Stage I: The Preinduction period:

Immediately after the water is added to the cement mix the pre-induction period starts. The period is active during mixing and only a few minutes after mixing is complete. Addition of water to the cement powder starts a fast hydration reaction between the C3S and the water.

During this phase, the first layer of C-S-H will precipitate on the surface. It can be observed as a large exothermic reaction on figure 2.1.[2]

Stage II: The induction period:

During the induction period, relatively little hydration can be observed. The release of heat is nearly non-existent which translates to very little hydration and slow precipitation of additional C-S-H. When the mixture is critically supersaturated, precipitation of calcium hydroxide takes place, thereby resuming the hydration at a higher rate and marking the end of the induction period. The length of the period varies with temperature, but at atmospheric conditions it lasts a few hours.[2]

Stage III: The acceleration period:

During the acceleration stage the initial setting occurs. This period contains the interval with the most rapid hydration as can be seen in the heat signature in figure 2.1. During stage III calcium hydroxide crystalizes from the solution whilst the C-S-H phase deposits into the remaining water filled phase leading to a network to be formed which causes the mix to start developing strength.[2]

Stage IV: The deceleration period:

As this network continues to grow, the porosity of the system decreases. The end result of this is a network where water transportation to the C-S-H phase is no longer possible, and the hydration rate decelerates which can be observed in the heat signature in figure 2.1. The combined duration of stage III and IV at atmospheric conditions can be several days. [2]

Stage V: The diffusion period:

During the diffusion period, hydration continues but at a very relaxed pace. This can be observed on figure 2.1. The hydrated products become denser, and as a result the strength increases. The duration of stage V at atmospheric conditions are indefinite, but no major structural changes takes place during this period. [2]

2.2.1 Effect of temperature on hydration process

While there are various aspects affecting the hydration of Portland cement, one of the major

shortens the duration of the induction and setting period and increases the rate of hydration of the setting period substantially, but often reduces ultimate strength and degree of hydration upon extended curing as can be observed in figure 2.2.

Figure 2.2 Temperature effect on hydration [2]

For degrees below 40°C the resultant products created by hydration will be the same as those created at atmospheric conditions, whereas at higher temperatures (T>40°C) the morphology and microstructure of the CSH phase will change. As a result of these changes, the material becomes more fibrous, and polymerization of silicate is observed at a higher degree. For temperatures exceeding 110°C which cannot be observed on figure 2.2, the formation of crystalline calcium silicate hydrates will occur due to the fact that the CSH phase is no longer stable. As a result of this, the matrix may shrink which results in decreased compressive strength and increased permeability of the set cement.[2]