Hydration of Portland Cement

Hydration of Portland Cement

Portland cement is a hydraulic cement, hence it derives its strength from chemical reactions between the cement and water. The process is known as hydration.

Properties of cement compounds

These compounds contribute to the properties of cement in different ways.

Tricalcium aluminate, C3A

• It liberates a lot of heat during the early stages of hydration but has little strength contribution.

• Gypsum slows down the hydration rate of C3A. Cement low in C3A is sulfate resistant.

Tricalcium silicate, C3S

This compound hydrates and hardens rapidly. It is largely responsible for Portland cement’s initial set and early strength gain.

Dicalcium silicate, C2S

C2S hydrates and hardens slowly. It is largely responsible for strength gain after one week. 

Ferrite, C4AF

This fluxing agent reduces the melting temperature of the raw materials in the kiln. It hydrates rapidly but does not contribute much to the strength of the cement paste. By mixing these compounds appropriately, manufacturers can produce different types of cement to suit several construction environments.

Chemical reactions during hydration

When water is added to cement, the following series of reactions occur

The tricalcium silicate (alite) is hydrated to produce calcium silicate hydrates, lime and heat

Tricalcium silicate + water calcium silicate hydrate + lime + heat

2 (3Cao.SiO2) + 6H2O ------>3Cao.2SiO2.3H2O + 3Ca(OH)2 

Delta H = 120 cal/g

The belite (dicalcium silicate) also hydrates to form calcium silicate hydrates and heat: 

Dicalcium silicates + water calcium silicate hydrate + lime

2 (2Cao.SiO2) + 4H2O ------>3Cao.2SiO2.3H2O + Ca(OH)2     

Delta H = 62 cal/g

The calcium silicate hydrates contribute to the strength of the cement paste.

This reaction generates less heat and proceeds at a slower rate, meaning that the contribution of C2S to the strength of the cement paste will be slow initially.

This compound is however responsible for the long-term strength of Portland cement concrete.

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 mixed 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

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.

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.

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.

• 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).