Question

Discuss the cement hydration process and its importance in developing the strength of concrete. Explain how it works according to the stages (dormant,setting,hardening stages). Discuss the setting time, its importance in concreting, and a practical solution to delay the initial setting time. Explain how the cement fineness, water-to-cement ratio, and curing temperature can promote or hinder the hydration.

Answer 1

Generally the dry cement doesn't get bind together and aquire strength with fine or coarse aggregates.It aquires the adhesive property only when it is mixed with water.The chemical reactions taking place when water is added to cemnt is known as hydration of cement.The cement particle contains some compounds such as tricalcium silicate(C₃S).dicalcium silicate(C₂S),tricalcium aluminate(C₃A) and tetra calcium alumino ferrate(C₄AF) these compounds reacts with water the hydration of cement takes place.Hydration is one of the most important property of a cement that is usually considered while selecting cement for different construction works.Let us have a detail analysis of the hydration process.When water is added to a cement mixture the hydration products such as tobermorite gel(3CaO.SiO_2.4H₂O), calciumhydroxide and some minor compounds slowly starts depositing on the outer periphery of the nucleaus of the hydrated cement molecules..As the hydration proceeds further the hydration products slowly started to deposit on the orginal cement grains.This makes the diffusion of water to unhydrated nucleus more and more difficult.This reduces the rate of hydrationThen gradually the hydration products occupy the spaces that is occupied by the water.Once the entire water molecules are reacted or all the available cement molecules are get reacted with water  the hydration of cement stops.This is the detailed processes takes place in hydration of cement.

Quick hydration is useful for attaining the high early strength of concrete.But for better final strength of the concrete the hydration process have to be completed gradually.

Stages of hydration

When water is added to cement the tricalcium aluminate reacts with water to form a gel called tobermorite gel.This gel reacts with sulphate to form a rod like crystals called ettringite.The reaction of tricalcium aluminate isstrongly exothermic.But it lasts only for a few minutes.After this there will be a stage of comparitively low heat of evolution.This stage is called dormant period.it lasts for a few hours.It is better to transport the cement paste to the worksite at the dormant stage so that the paste remains workable during this stage.At this stage the cement paste is in a plastic state as the dormant period progresses the setting of the cement paste starts thus it becomes too stiff to workable.This stage at the end of dormant period is the setting stage.At the starting of this stage the paste begins to et stiffen but it is still soft and workable.This is the inial setting of cement and the time taken for the cement paste to reach this stage is called initial setting time.For a good quality cement it is about 30minutes.After this initial setting time the cement paste conitinues to be stiffen until a stage is reached that the cement paste completely losts its plasticity and becom a rigid solid this is the last stage of cement hydration which is called the hardening stage.

Setting time

The time required for a cement to react completely with the water and to get sets completely to form a hardened soild mass is called setting time.There are two setting times in the hydration of the cement they are initial setting time and final setting time.The time in which the cement paste can be moulded in any desired shape without losing its strength is called the initial setting time.The time upto which the cement paste remains fully in a plastic state after the addition of water into it is called initial setting time.Generally the initial setting time of good ordinary portland cement is maximum 30minutes.The time taken by the cement paste to loss its plasticity completely to form a hardened solid mass is called final setting time.Generally the final setting time of ordinary portland cement is minimum 10hrs.

Setting time is very important in concreting because the concrete is workable,mouldable and remains in the plastic state only before the initial setting time.So it is important for the contractors and workers to know the initial setting time to determine the time taken for mixing,compacting and transporting it without losing its homogenity.Final setting time is also important because it is the time at which the concrete loses its plasticity completely and form a strong hardened mass.So it is also important for the contractors and workers to know the final setting time to remove the scaffolding and formwork and to start the curing process.

The initial setting time can be dealyed by cooling the concrete by using ice or liquid nitrogen.By using this method the temperture is get reduced so that the hydration process become slower.By this method the initial setting time can be delayed only for a few hours. Addition of gypsum during the manufacture of cement can also delay the initial setting time.The use of proper chemical retarders at the timeof mixing itself this will delay the setting of the concrete upto 24hrs without affecting its final strength.But delaying the initial settimng time using these chemical retarders is not so esy.Some factors such as the temperature of the ingredients and the dosage system have to be kept in mind.

Factors affecting hydration of cement

Fineness of cement ,water cement ratio and curing temperature are thge major factors affecting hydration of cement.As the cement paticles are more finer so there will be more surface area for the particles to get reacted with water.More area of contact more will the reaction weith water.As aresult the hydration process takes place qiuckly resulting in high early strength of concrete.In hydration process the cement paste get hardened as a result of its reaction with water.so water cement ratio has a great influence on the hydration of cement.Usuallky water cement ratio is maintained with in the range 0.45 to 0.60.For high strength concrete lower water cement ratio is used.Along with it a plasticizer is used to increase the flowability of concrete.Increased curing temeperature will increase the early hydration rate but it adversely affect the final compressive strength.