Here is a compilation of essays on ‘Hydrated Cement’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Hydrated Cement’ especially written for school and college students.
Essay on Hydrated Cement
Essay Contents:
- Essay on the Process of Hydration of Cement
- Essay on the Water Requirement for Hydration of Cement
- Essay on the Structure of Hydrated Cement
- Essay on Heat Liberation during Hydration of Cement
- Essay on Strength Development of Hydrated Cement
- Essay on the Effect of Impurities in Calcium Silicate and Alkalies on Strength of Cement
- Essay on the Setting and Hardening of Cement
Essay # 1. Process of Hydration of Cement:
On adding water to the cement, the silicates and aluminates present in the cement start a chemical reaction and form a spongy mass known as gel. During this process a large quantity of heat is liberated. The quantity of heat liberated depends upon the amount of different constituents in the cement. In other words, hydration of cement can be defined as the reaction taking place in water and cement paste by virtue of which Portland cement becomes a bonding agent.
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There are two ways in which the compounds present in the cement may react with water. In the first case, on addition of water, the cement compounds dissolve to produce a super saturated solution from which different hydrated products are predicated, this is a true reaction of hydration. In the second type of reaction the water is hydrolysed. Actually the term hydration is applied to both true hydration and hydrolysis.
The products of hydration of cement chemically are the same as the products of hydration of individual compounds under similar conditions. This fact was stated for the first time by Le Chateliar. Main hydrates can be classified as calcium silicate hydrates and tri-calcium aluminate hydrate. The two calcium silicates are the main cementitious compounds in cement. The physical behaviour of cement during hydration is similar to that of these compounds alone. C4AF is supposed to hydrate into tri-calcium aluminate hydrate and CaO.Fe2O3 aqueous.
(a) Calcium Silicate Hydrates:
In modern cements the total amount of calcium silicate hydrate varies from 70-80%. The rates of hydration of C3S and C2S in pure state differ considerably.
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When water is added in a limited quantity as in the case of cement paste or cement concrete, C3S undergoes hydrolysis first, producing calcium silicate of lower basicity. Ultimately C3S2H3 is formed and lime as Ca(OH)2 is released. Then hydrates C2S making the approximate assumption that both C3S and C2S produce C3S2H3 as the final product of hydration, their equations of hydration can be written as follows-
From the above equations it is clear, that on weight basis both silicate require approximately the same amount of water for their hydration, but C3S produces more than two times the Ca(OH)2 as is formed by hydration of C2S. The amount of Ca(OH)2 is not a desirable product in the concrete mass as it is soluble in water and may get leached out making the concrete porous. Thus in hydraulic structures cement with higher percentage of C2S should be used. The physical properties of calcium silicate hydrates are useful in connection with setting and hardening properties of cement.
It has been observed that the hydration of C3S does not take place at a constant rate. The initial rapid release of calcium hydroxide into the solution leaves an outer layer about 10–6 cms thick of calcium silicate hydrate. After this, for some time, very little reaction takes place. This period of little reaction is called as dormant period. After some time, the coating on un-hydrated cement grain ruptures due to the pressure beneath it and hydration proceeds again.
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Further it has been observed that calcium silicate hydrates show strength development similar to that of Portland cement. A considerable strength is developed before the process of hydration is complete and a small amount of hydrate binds together the un-hydrated remaining particles.
(b) Tri-Calcium Aluminate Hydrate:
The amount of C3A in most cement is comparatively small, but its behaviour is very important. The reaction of pure C3A with water is very violent and leads immediate stiffening of paste. The immediate stiffening of paste is called flash-set. To prevent flash set 2-3% by weight of clinker, gypsum is added to the cement clinker. Gypsum (CaSO4.2H2O) reacts with C3A and forms insoluble calcium sulpho-aluminate (3CaO, Al2O3, 3CaSO4, 31H2O) and ultimately a tri-calcium aluminate hydrate is formed.
As more C3A comes into solution, the composition changes, the sulphate content decreases continuously. The rate of reaction of the aluminate is high, and if this readjustment in composition is not rapid enough direct hydration of C3A may take place. The peak rate of heat develops within 5 minutes of adding water to cement, shows that some calcium aluminate hydrate is formed directly during this period.
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The hydration of C3A is retared by Ca(OH)2, which is liberated by the hydrolysis of C3S. It happens due to the fact that Ca (OH)2 reacts with C3A and water to form C4AH12, which form a protective layer or coating on the surface of un-hydrated gram of C3A.
The equation can be written as:
C3A + 6H = C3AH6
The molecular weights show that 100 parts of C3A react with 40 parts of water by weight, which is a much higher proportion of water than that required by the silicates.
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The presence of C3A in cement is undesirable. It contributes to strength upto 3 days and more specially upto 24 hours strength of cement or concrete. When hardened, cement paste is attacked by sulphates, and disruption of cement paste takes place due to the formation of sulpho aluminate from C3A. The high rate of heat development also causes cracks in the concrete which is not desirable.
However C3A acts as flux, reducing the burning temperature of clinker which results in saving of fuel. It also facilitates the combinations of lime and silica. For these reasons C3A is useful in the manufacture of cement.
(c) C4AF:
It also acts as a flux. If C4AF is not present, the reaction in the kiln would progress much more slowly and would probably be incomplete. Gypsum with C4AF forms calcium sulpho ferrite as well as calcium sulpho aluminate and its presence accelerates the hydration of silicates. Gypsum reacts both with C3A and C4AF.
The amount of gypsum to be added to the cement clinker should be decided very carefully as excess amount of gypsum would lead to an expansion and consequent disruption of the set cement paste. The optimum content of gypsum is determined by observing the generation of heat of hydration. If the amount of gypsum is more, immediately after adding water a second peak in the rate of heat evolution would take place after 4 to 8 hours of the first peak of rate of heat evolution.
If the amount of gypsum is correct, there will be little C3A left to react after all the gypsum has combined and no second peak of rate of heat of hydration will occur. The amount of gypsum required increases with the C3A content and also alkali contents in the cement. Increasing the fineness of cement has the effect of increasing the quantity of C3A available at early ages and this increase the gypsum requirements. If C3A present is not more than 5%, then, 2.5% gypsum is sufficient, but if C3A is more than 5%, then 3% gypsum should be added.
(d) Calcium hydroxide Ca(OH)2:
Calcium hydroxide is produced during the hydration of C3S and C2S. It constitutes 20 to 25% of the volume of solids in the hydrated paste and makes the concrete porous, weak and undurable, Ca(OH)2 also reacts with sulphates present in water or soil to form calcium sulphates which further reacts with C3A and causes deterioration of concrete. The effect of calcium hydroxide can be reduced by using pozzolanic materials. The only advantage of Ca(OH)2 is that it being alkaline in nature, maintains the pH value of water around 13 in the concrete which reduces the corrosion of reinforcement.
Essay # 2. Water Requirement for Hydration of Cement:
For fully hydration of Portland cement on an average 23% of water by weight is required, whereas C2S requires 24% of water by weight of cement and C2S requires 21%, This 23% of water chemically combines with cement and is called bound water. A certain amount of water is filled with in the pores of the gel and is known as gel water. The bound water and gel water are complimentary to each other.
If the quantity of water is inadequate to fill the gel pores, the gel formation will stop. If the gel formation stops, there is no question of the gel pores presence. It has been estimated that about 15% water by weight of cement is required to fill up the gel pores. Thus for the complete chemical reaction and to fill up the gel pore space, a total 23% + 15% = 38% of water by weight of cement is required.
Thus it will be seen if only 38% by weight of cement water is used, the resultant paste will go full hydration and no extra water will be available for the formation of undesirable capillary cavities. On the other hand if more than 38% of water is used, then the excess water will form undesirable capillary cavities. Thus greater the amount of water above 38%, larger the undesirable capillary cavities.
On hydration the volume of cement particles increases to 2.06 to 2.1 times of its un-hydrated volume. Upto water cement ratio of about 0.6, the gel fully fills up the space occupied by water and the structure is homogeneous. If the cement ratio is more than 0.7, the increase in volume of hydrated product will not be sufficient to fill up the voids produced by water in the paste, resulting porous mass.
Essay # 3. Structure of Hydrated Cement:
Many of the mechanical properties of hardened cement and concrete depend not so much on the chemical composition of the hydrated cement but on the physical structure of the product of hydration known as ‘gel’. Fresh cement paste is a plastic network of particles of cement in water. After setting of the paste, its gross volume remains approximately constant.
At any stage of hydration, the hardened paste consists of hydrates of the various compounds known as gel, crystals of Ca(OH)2, un-hydrated cement and some other minor compounds and the residue of the water filled in the spaces of the fresh paste.
Most of the products of hydration are colloidal and the surface area of the solid phase increases more than twice its original volume and large amount of free water becomes on this surface. If no movement of water neither from outside to inside nor from inside to outside is permitted, the reaction of hydration use the water until very little water is left to saturate the solid surface and the relative humidity within the paste decreases.
This condition is known as self-desiccation, as gel can form only in water filled space, the self-desiccation leads to lower hydration compared with a moist cured paste. However in self-desiccated paste if the water cement ratio is in excess of 0.5, the amount of mixing water is sufficient for hydration at the same rate as for moist-cured paste.
For hydration of ail particles of cement, about 23% water of weight of dry cement is required to form gel. The hydrated product of cement or gel consists of solid products and water held physically or adsorbed on the large surface area of the hydrates. This water is called gel water. The amount of this water is about 15% of the weight of the dry cement.
The volume of this gel water is about 28% of the gel of cement. Thus if the water/cement ratio is less than 0.38 or 0.40, there is no extra water in the cement to provide workability to the concrete. If the water/cement ratio is more than 0.4, then this additional water forms capillary channels and can be evaporated.
In fully hydrated cement the water is held very firmly and cannot be evaporated. This water is known as combined water. The amount of this water is about 23% of the weight of the dry cement.
Capillary Water:
The solid products of hydration occupy a volume less than the sum of absolute volumes of the original dry cement and the combined water by about 18.5%. This residue space for fully hydrated cement with no excess water required for hydration is known as capillary pores. Water present in these pores is called capillary water.
Thus the total water added to cement can be classified into three groups:
1. Gel water,
2. Capillary water and
3. Combined water.
Essay # 3. Volume of Product of Hydration:
The gross space available for the products of hydration consists of the absolute volume of the dry cement together with the volume of water added to the mix. The small losses of water due to bleeding etc. have been neglected. It has been observed that the four compounds of cement namely C3S, C2S, C3A and C4AF need 24%, 21%, 40% and 37% water by weight for their chemical reaction respectively. The non-evaporable water in cement is found by experiment as 23% of the weight of unhydrous cement.
Further it has been observed that the specific gravity of the products of hydration of cement is such that they occupy a greater volume than the absolute volume of un-hydrated cement, but smaller than the sums of volumes of the dry cement and the non-evaporable water by approximately 0.254 of the volume of non-evaporable water. The average value of specific gravity of the products of hydration in a saturated state is assumed as 2.16.
The procedure of determining the volumes of different components is illustrated by the following example:
Let us consider the hydration of 100 gram of cement for simplicity in calculation.
Let the specific gravity of dry cement be 3.15
Then absolute volume of un-hydrated cement = 100/3.15 = 31.8 ml (c.c.)
Non evaporable water as assumed above = 23% = 23 ml (c.c.)
∴ Solid products of hydration will occupy a volume = 31.8 + 0.23 x 100 (1 – 0.254)
= 48.96 ml app.
It has been found that paste in this condition contains 28% porosity, and then volume of gel water Wg is found by the relation,
Wg/(48.96 + Wg) = 0.28
or Wg = 19.0 ml.
∴ Volume of hydrated cement = 48.96 + 19 = 67.96ml,
∴ Vol. of products of hydration of 1 ml of dry cement = 67.96/31.8 = 2.14
In the above case hydration is considered in a sealed tube. It has been observed that hydration in a sealed specimen can progress to a stage till the combined water becomes about half of the original water content. After this stage no hydration is possible. Thus full hydration in a sealed specimen is possible only when the mixing water is atleast twice the water required for chemical reaction, i.e. the mix has a water- cement-ratio of about 0.5 by weight. Actually full hydration is possible when for gel formation, sufficient water is available both for chemical reaction and for filling the pores of gel formed.
Hydration of Paste Cured under Water:
In this case water can be imbibed through capillaries. Let us consider hydration of 100 gram of cement. The specific gravity of cement being 3.15, the volume occupied by 100 gram of cement will be 31.8 ml. As shown in the above example this volume of dry cement will occupy a volume of 67.96 ml app. on full hydration. Thus original mixing water will be (67.96 – 31.8) = 36.15 ml. This corresponds to a water cement ratio of 0.36 by weight.
But by actual experiments it has been observed, (if water is also allowed for bleeding) that for water cement ratio less than 0.38 by weight, complete hydration is not possible as the volume of water and cement available is insufficient to accommodate all the products of hydration. Further if the water/cement ratio is reduced to 0.3 about 19% of the original weight of cement will remain un-hydrated and will never be able to hydrate as gel has occupied all available space.
Further it may be stated that this un-hydrated cement will not prove detrimental to the strength. Actually by water cement ratio law it has been found that lower the water/cement ratio, higher the strength, this is probably due to the fact that in such cases the layers of hydrated paste surrounding the un-hydrated grains are thinner.
On the other hand, if the water/cement ratio is higher than about 0.38 or 0.4, all the cement particles can hydrate but capillary pores will also be present. Some of the capillaries will contain excess water from the mix and the others will be filled by imbibing water from outside.
Capillary Pores during Hydration:
At any stage of hydration the capillary pores represent that part of the gross volume which has not been filled by the products of hydration. Hydrated products occupy more than two times the volume of dry cement alone, thus the volume of the capillary system is reduced with the progress of hydration. Capillary pores are much larger than gel pores.
The capillary porosity of paste depends upon the water/cement ratio of the mix and degree of hydration. Actually the type of cement has more influence on the degree of hydration than water/cement ratio. Water cement ratios higher than 0.38, the volume of gel is unable to fill all the space available in the mix and capillary pores are left even for complete hydration of cement.
In a good concrete, however continuous capillaries are not desirable. It has been seen that in mature and dense pastes, the capillaries are blocked by gel. Table 2.7 shows are required to produce maturity at which capillaries becomes blocked or segmented.
The product, of hydration of cement is called gel, which is porous in structure. The gel pores are inter-connected forming interstitial spaces between its particles. By experiments the space occupied by gel pores is found about 28%. In other words gel pores occupy space equal to about one third of the vol. of the gel solids.
Essay # 4. Heat Liberation during Hydration of Cement:
The hydration of cement is accompanied by liberation of heat. Such a chemical reaction is known as exothermic and there are many exothermic chemical reactions. Normal cement generally produces heat of hydration about 90 cal/gram in 7 days and 90 to 120 cal/gram in 28 days (360 to 500 joules/gram (as 1 cal/g = 4.2 joul/g). As the conductivity of concrete is very low it acts as an insulator and in the interior of a large mass of concrete such as a dam, hydration of cement can develop very high temperature.
At the same time heat dissipation takes place from the exterior surface of the concrete and a steep temperature gradient develops. During further cooling of interior, causes serious cracking of the concrete. In cold climate, heat produced by hydration of cement prevents freezing of water in the capillaries of the fresh concrete. In such situations high evolution of heat is advantageous.
The heat of hydration is the quantity of heat in calories/gram or joules/gram of un-hydrated cement developed on complete hydration of cement at a given temperature. The temperature at which the hydration takes place affects the rate of heat development as shown in the following Table 2.8.
Strictly speaking heat of hydration is a composite quantity, consisting of chemical heat of reaction of hydration and heat of adsorption of water on the surface of the gel formed by the process of hydration. The heat of adsorption being about 25% of the total heat of hydration. From practical considerations, total heat of hydration is not important, but the rate of heat evolution is important.
C3A hydrates most rapidly and liberates heat at a very high rate at the early age. 7% C3A liberates about 35 Cal/g heat at the age of 12 hours and 53 Cal/g at the age of 24 hours. The C3S contents are assumed as constant. The influence of C3S contents on the evolution of heat while C3A contents are considered constant.
As per Bogue, for usual range of Portland cements, about 50% of the total heat is liberated between 1 and 3 days, about 75% in 7 days and from 83 to 91% of the total heat in 180 days. In fact the heat of hydration depends on the chemical composition of cement and the heat of hydration of cement is very nearly equal to sum of the heats of hydration of individual compounds when hydrated separately.
The heat of hydration of main compounds of cement based on the study of VERBECK and C.W. FOSTER are given in Table 2.9 below:
The quantity of cement in a mix also will affect the total heat development. Thus by adjusting or reducing the quantity of C3A and C3S in cement, heat of hydration can be controlled.
Heat Liberation Pattern from Setting Cement:
Heat liberation from setting cement after adding water to the cement based on the work of W. Lerch. The reaction of water with cement is exothermic. The liberation of heat is known as heat of hydration. On mixing water with the cement, a rapid heat evolution occurs, which lasts for few minutes. This heat of evolution is due to aluminates and sulphates (C3A). This is shown by ascending peak A. On adding gypsum, the solubility of aluminate is depressed and the initial heat evolution eases quickly in about 2 hours’ time.
Next heat evolution is due to C3S and represented by B on the graph. This peak reaches in about 8 hours. The study and control of heat of hydration is very important in the construction of concrete dams and other massive structures.
Influence of Compound Composition on Properties of Cement:
Heat of hydration of cement is a simple additive function of the compound composition of cement. In general in modern cements the lime silicates compound C3S and C2S on an average vary from 70% to 80%. In a well burnt clinker the amount of C3S is about 45%, but a small increase in the total lime content in the raw materials increases the percentage of C3S appreciably and decreases C2S. C3A varies from 1 % to 12% on an average, which is responsible for the development of high rate of heat of hydration.
Essay # 5. Strength Development of Hydrated Cement:
On adding water to the cement, C3A is the earliest to hydrate and contributes to the strength of the cement paste at one to three days age but at the advanced age it causes retrogression in the development of strength, particularly in cements with high contents of C3A or (C3A + C4AF) both. Due to the development of large amount of heat of hydration, it causes cracks in the paste and disintegrates the hardened paste or concrete. Although C3A is least desirable in cement, but it acts as flux for the fusion of clinker and reduces the temperature of fusion, resulting in saving of fuel.
It also facilitates the combination of lime and silica. An increase in iron content decreases the percentage of C3A, but is causes higher formation of C4AF. Low C3A cement generates less heat and develops higher ultimate strength and exhibits greater resistance to the destructive elements than cements containing higher percentage of C3A. For these reasons C3A is useful in the manufacture of cement.
C4AF:
It is considered as an inert material, but it also acts as a flux for clinker fusing at lower temperature, resulting in the saving of fuel. Practically it does not contribute any strength.
C3S:
After adding water to the cement, C3S hydrates immediately next to C3A and contributes early strength to the cement paste or concrete. In general the early strength of Portland cement will be higher with higher percentage of C3S. Many researchers have found that C3S contributes most to the strength development during the first four weeks. The 7 days strength of neat C3S has been found about 400 kg/cm2 or 40 Newton per mm2 and after 18 months about 700 kg/cm2.
C2S:
The gain in strength of C2S has been found after 28 days on wards after adding water to the cement. At seven days no strength develops in C2S. At the age of 1 year, weight for weight C3S and C2S both compounds contribute equally to the ultimate strength. For moist curing after 180 days C2S has been found to develop greater strength, after about 18 months C2S has been found to develop about 700 kg/cm2.
Essay # 6. Effect of Impurities in Calcium Silicate and Alkalies on Strength of Cement:
The impurities in calcium silicate strongly affect the rate of reaction and strength development of the hydrates. Al2O3 has a marked effect on the strength development of C3S. It has been observed that 1% addition of Al2O3 to pure C3S increases the early strength of the paste very much upto about 35 days.
Test results have shown that the increase in strength beyond the age of 28 days is strongly affected by the alkali contents. The greater the amount of alkali present, lower the gain in strength. Thus the amount of alkalies should not be more than 0.4%. Table 2.10 shows the effect of principal compounds of cement on its properties.
Essay # 7. Setting and Hardening of Cement:
Term setting is used to describe the stiffening of the cement paste. Broadly speaking setting refers to a change from a fluid to a rigid state. On adding sufficient amount of water to cement, a paste is formed, which gradually becomes less plastic and finally becomes stiff and hard.
When the paste has lost its plasticity and becomes sufficient rigid to withstand an arbitrarily defined pressure, then the paste is said to have set. The setting period has been divided into two groups known as initial set and final set. After the final set, the cement paste further increases in strength and rigidity. This state of paste is called as hardening of cement paste.
The setting process is accompanied by temperature changes in the cement paste. The initial set corresponds to a rapid rise in temperature and the final set to the peak temperature. The setting time of cement is found to decrease with the rise in temperature, but at 30°C (85°F) the reverse effect is observed. At low temperatures setting is restarted.
Further cement set can be divided into the following two groups:
1. Flash set.
2. False set.
1. Flash Set:
If on adding water to the cement a very violent reaction takes place with the evolution of large amount of heat of hydration and cement sets within five minutes after adding water to it; then this set is known as flash set. After flash set the restoration of plasticity of paste is not possible.
2. False Set:
If the abnormal premature stiffening of cement paste takes place in few minutes after mixing water with the cement then it is called false set. In the case of false set no appreciable heat is evolved and remixing of the cement paste without adding any water and restoring plasticity of the paste is possible until it sets in the normal manner and without any loss of strength.
Causes of False Set:
1. If gypsum is added and inters ground in a very hot clinker, the dehydration of gypsum takes place and lime-hydrate (CaSO4, 0.5H2O) or anhydrites (CaSO4) are formed. When water is added to such cements, these hydrates form gypsum and the plaster set takes place resulting stiffening of paste.
2. If alkalies are present in the cement, during storage period they form alkali carbonates. These alkali carbonates react with calcium hydroxide Ca(OH)2 liberated by the hydrolysis of C3S to form CaCO3. This CaCO3 precipitates and produces rigidity in the paste.
3. At moderately high temperature, the aeration of C3S also can cause false set in the cement. In this condition water is adsorbed on the grains of the cement and these freshly activated surfaces can combine very rapidly with more water during mixing. This rapid hydration will cause false set.
If false set is found to occur in any cement, it can be dealt with by remixing the concrete without adding any water to it. By this process, workability will also increase and concrete can be laid in the normal way without any difficulty.