Heat of Hydration, Test and Chemical composition test
Heat of Hydration ?
Heat of hydration in common with many chemical reactions, the hydration of cement with water compounds is exothermic, and the quantity of heat (in joules) per gram of unhydrated cement, evolved upon complete hydration at a given temperature, is defined as the heat of hydration. For the usual range of Portland cements, about one - half of the total heat is liberated between 1 and 3 days, about three - quarters in 7 days, and nearly 90 percent in 6 months. In fact, the heat of hydration depends on the chemical composition of the cement, and is approximately equal to the sum of the heats of hydration of the individual pure compounds when their respective proportions by mass are hydrated separately;
Methods of determining its value are described in BS 4550: Part 3: Section 3.8: 1978, and in ASTM C 186-05.
In this Articles :-
- What is heat of hydration ?
- Heat of hydration test.
- Chemical composition test.
The reaction liberates a considerable quantity of heat. This liberation of heat is called heat of hydration. This is clearly seen if freshly mixed cement is put in a vaccum flask and the temperature of the mass is read at intervals. The study and control of the heat of hydration becomes important in the construction of concrete dams and other mass concrete constructions.
Four major component of cement heat of hydration C2S, C3S, C3A and C4AF since the heat of hydration of cement is an additive property, it can be predicted from an expression of the type
H = aA + bB + cC + dD
Where H represents the heat of hydration, A, B, C, and D are the percentage contents of C3S, C2S, C3A and C4AF. and a, b, c and d are coefficients representing the contribution of 1 percent of the corresponding compound to the heat of hydration.
| Compound | Heat of Hydration to the age | ||
| 3 Day’s | 90 Day’s | 15 Year’s | |
| C3S | 58 | 104 | 122 |
| C2S | 12 | 42 | 59 |
| C3A | 212 | 311 | 324 |
| C4AF | 69 | 98 | 102 |
The hydration process is not an instantaneous one. The reaction is faster in the early period and continues idenfinitely at a decreasing rate. Complete hydration cannot be obtained under a period of one year or more unless the cement is very finely ground and reground with excess of water to expose fresh surfaces at intervals. Otherwise, the product obtained shows unattacked cores of tricalcium silicate surrounded by a layer of hydrated silicate, which being relatively impervious to water, renders further attack slow. It has been observed that after 28 days of curing, cement grains have been found to have hydrated to a depth of only 4ÎĽ. It has also been observed that complete hydration under normal condition is possible only for cement particles smaller than 50ÎĽ.
Normal cement generally produces 89-90 cal/g in 7 days and 90 to 100 cal/g in 28 days.
- Heat of hydration - C3A > C3S > C4AF > C2S
- Rate of hydration - C4AF > C3A > C3S > C2S
NOTE - If in any engineering construction low heat of hydration case required then proportion of C3A and C3S is reduction, ex - Mass Concreating, Bridge, Foundation etc.
Heat of Hydration Test :-
The temperature at which hydration occurs greatly affects the rate of heat development, which for practical purposes is more important than the total heat of hydration; the same total heat produced over a longer period can be dissipated to a greater degree with a consequent smaller rise in temperature. Fineness of cement affects the rate of heat development but not the total amount of heat liberated, which can be controlled in concrete by the quantity of cement in the mix (richness). It may be noted that there is no relation between the heat of hydration and the cementing properties of the individual compounds.
We have said, the two compounds primarily responsible for the strength of hydrated cement are C, S and CS, and a convenient rule assumes that CS con tributes most to the strength development during the first four weeks and C, S influences the later gain in strength. At the age of about one year, the two compounds, mass for mass, contribute almost equally to the strength of hydrated cement. However, in contrast to the pre-diction of heat of hydration of cement from its constituent compounds, it has not been found possible to predict the strength of hydrated cement on the basis of compound composition.
The reaction of cement with water compounds is exothermic. This can be easily observed if a cement is gauged with water and placed in a thermos flask. Much attention has been paid to the heat evolved during the hydration of cement in the interior of mass concrete dams. It is estimated that about 120 calories of heat is generated in the hydration of 1 gm. of cement. From this it can be assessed the total quantum of heat produced in a conservative system such as the interior of a mass concrete dam. A temperature rise of about 50°C has been observed. This unduly high temperature developed at the interior of a concrete dam causes serious expansion of the body of the dam and with the subsequent cooling considerable shrinkage takes place resulting in serious cracking of concrete.
The use of lean mix, pozzolanic cement, artificial cooling of constituent materials and incorporation of pipe system in the body of the dam as the concrete work progresses for circulating cold brine solution through the pipe system to absorb the heat, are some of the methods adopted to offset the heat generation in the body of dams due to heat of hydration of cement.
Test for heat of hydration is essentially required to be carried out for low heat cement only. This test is carried out over a few days by vaccum flask methods, or over a longer period in an adiabatic calorimeter. When tested in a standard manner the heat of hydration of low heat Portland cement shall not be more than 65 cal/gm. at 7 days and 75 cal/g, at 28 days.
Chemical Composition Test :-
In the Chemical composition test table are given below, This table show the Chemical characteristics of various types of cement or Chemical requirement of various types of cement.
| Sr No | Type of Cement | Lime Saturation Factor (%) | Alumina Iron Ratio (%) Min | Insoluble Residue (%) Max | Magnesia (%) Max | Sulphuric Anhydride | Loss on Ignition (%) Max.
|
| 1. | 33 Grade OPC (IS 269-1989) | 0.66 Min 1.02Max | 0.66 | 4 | 6 | 2.5% Max. When C3A is 5 or less3% Max. When C3 A is greater than 5
| 5 |
| 2. | 43 Grade OPC (IS 8112-1989) | 0.66 Min 1.02Max | 0.66 | 2 | 6 | 2.5% Max. When. C3A is 5 or less3% Max. When C3A is greater than 5
| 5 |
| 3. | 53 Grade OPC (IS 12269-1987) | 0.8 Min 1.02Max | 0.66 | 2 | 6 | 2.5% Max. When C3A is 5 or less3% Max. When C3A is greater than 5
| 4 |
| 4. | Sulphate Resisting Cement (IS 12330-1988)
| 0.66 Min 1.02Max | . N S | 4 | 6 | 2.5% Max | 5 |
| 5. | Portland Pozzolana Cement (IS 1489-1991) Part I
| N S | NS | X+4(100-x)/100 | 6 | 3% Max | 5 |
| 6. | Rapid Hardening Cement (IS 8041-1990) | 0.66 Min 1.02Max | 0.66 | 4 | 6 | 2.5% Max When C3A is 5 or less3% Max. When C3A is greater than 5
| 5 |
| 7. | Slag Cement(IS 455-1989)
| N S | N S | 4 | 8 | 3% Max | 5 |
| 8. | High Alumina Cement (IS 6452-1989)
| NS | NS | NS | NS | NS | NS |
| 9. | Super Sulphated-Cement (IS 6909-1990)
| NS | NS | 4 | 10 | 6% Min | NS |
| 10. | Low Heat Cement (IS 12600-1989) | NS | 0.66 | 4 | 6 | 2.5% Max. When C3A is 5 or less3% Max. When C3A is greater than 5
| 5 |
| 11. | IRS-T40 | 0.8 Min 1.02Max.
| 0.66 | 2 | 5 | 3.5% Max | 4 |
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