HYDRATION IN CEMENT PRODUCTION

We were discussing “Cements and its types” and " Raw materials for manufacturing of cements” and “pyroprocessing in cement industry” in our previous posts.

Today we are going to start here one very important topic i.e. hydration of cement in cement manufacturing. We will understand the various terminologies and engineering concepts used in cement technology with the help of this category.

Further we will go for discussion of properties of cement compounds, selection of the process, process technology, burning technology, firing technology, clinker cooling and grinding, cement packing and dispatch, pollution control and much more facts about cement technology in our next post in this category of cement technology.

Hydration of cement

Setting and Hardening

Hydration is the process in which water is combined with the reacting substance. The hydration of cement is accompanied by solidification.i.e.an initially liquid or plastic system progressively turns into a stone-like solid (hardened cement paste). The process of solidification comprises two stages - setting and hardening.

On setting, the cement paste stiffens into a solid, but as yet of negligible strength. In the following stage of hardening, the paste gradually develops considerable strength. There is no sharp difference between setting and hardening. The transition is gradual.

The hydration reactions can be represented as follows

Clinker phases + water = hydrate phase + energy

(High in energy) (Low in energy) (Heat of hydration)

The hydration reactions of aluminate phase are:

C₃A + 6 H₂O = C₃A.6 H₂O

C₃A + Ca (OH)₂ + 12 H₂O = 4CaO.Al₂O₃.13 H₂O

Both these reactions cause rapid setting.

In the presence of gypsum,

C₃A + 3(CaSO₄) + 26 H₂O  C₃A.3CaSO₄.32 H₂O

Alite reacts with water to form cal. silicate hydrates containing less lime and one molecule of Ca (OH)₂ split off.

6(3CaO.SiO₂) + 18H₂O  5CaO.6SiO₂.5H₂O + 13Ca (OH)₂

Bleeding

After concrete (an artificial conglomerate of cement and gravel / stone chips) has been placed in position and while still plastic, water appears at the surface. This is essentially a sedimentation phenomenon, the solids setting. Laws governing it are those of liquid flow in capillary system.

Bleeding is increased by vibration and is reduced by addition of plasticizing agents to the concrete mix and air entrainment.

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We will see other topic i.e. Properties of cement compounds in our next post in the category of Steel and cement technology.

I am very thankful to Mr. Subrata Bhaumik, Independent cement consultant, for providing such beautiful information and contents about cement technology.

Mr. Subrata Bhaumik has more than 50 (Fifty) Years (1965 - 2016) of Experience in Cement and other related Industry covering more than 100 assignments in cement plants with capacities ranging from 100 tpd to 10,000 tpd in India and abroad involving visit to 25 countries overseas in connection with work.

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