When the water is added to cement, a chemical reaction starts which is exothermic in nature and produces a significant amount of heat. This is known as hydration of cement and the liberated heat is called the heat of hydration.
The process of hydration is not an instantaneous. Hydration of cement is faster in the early periods and continues indefinitely at slower rate. In about a month’s time, 85 to 90% of the cement hydrates and the cement attains almost its full strength. The hydration still continues and cement grows stronger with time.
The process of hydration of cement may be explained as follows:
In a cement paste i.e. a thorough mixture of cement and water, chemical reaction soon starts and after a lapse of some time, the paste consists of hydrates of various compounds, collectively called gel, unhydrate cement, water and residue of the water-filled spaces in the fresh paste. These voids are called capillary pores. Hydration of cement is very important.
The cementing gel consists of thin fibrous crystals which is porous in nature. These pores are called gel-pores. About 23% of water by mass of cement is required for chemical reaction of cement with water and is known as bound water.
About 15% of water by mass of cement is required to fill the gel-pores and is known as gel-water. Thus, a total of 38% of water by mass of cement is required for complete hydration of cement. In this discussion, it is assumed that the reaction takes place in a sealed container.
The remaining water mixed with cement causes undesirable capillary cavities. If only 38% of water is added, the capillary cavities can be eliminated.
The products of hydration of cement are colloidal and because of this, during hydration the surface area of solid phase increases enormously. This absorbs a large amount of free water. If water added is 38%, all the colloids are not sufficiently saturated which decreases the relative humidity of the paste.
This leads to a lower hydration of cement as the gel can be formed only in water filled space. This requires a minimum of 50% of water by mass of cement or in other words, a water-cement ratio in excess of 0.5 is sufficient for hydration.
In actual conditions, the reaction does not take place in a sealed container and with lower percentage of water, the concrete mix would not be workable. A mix is workable if it can be easily mixed, placed and compacted at the required place.
Usually about 50 to 60% of water by mass of cement is added to manufacture the concrete.
The rate of hydration of cement is mainly influenced by following points
The temperature at which hydration takes place: At high temperatures the reaction is rapid. Only 10 to 15 minutes of hydration at higher temperature is equivalent to 10 to 12 hours of hydration at lower temperatures. It is for this reason that in cold weather, sometimes the aggregates are heated before they are used for making concrete.
The fineness of cement: The finer the cement, the more rapid is the reaction. As the hydration starts at the surface of the cement particles, the larger the available surface area, the more rapid is the hydration. Finer cements have larger surface areas and therefore the hydration is rapid. However, a very fine ground cement is susceptible to air-set and deteriorates earlier.
The ingredients of cement: The reaction can be made rapid or slow by changing the properties of the ingredients of the cement.