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Cement (Chemical Composition and Hydration) Flipbook PDF
1 Cement (Chemical Composition and Hydration) Oxide Composition of Portlant Cement • Portland cement is composed of four
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Cement (Chemical Composition and Hydration) Oxide Composition of Portlant Cement •
Portland cement is composed of four major oxides: lime ( CaO ), silica ( SiO2 ), alumina ( Al2O3 ), and iron ( Fe2O3 ).
•
Also Portland cement contains small amount of magnesia ( MgO ), alkalies (Na2O and K2O ), and sulfuric anhydrite ( SO3 ).
Approximate Composition Limits of Oxides in Portland Cement
Oxide
Common Name
Content, %
CaO
Lime
60-67
SiO2
Silica
17-25
Al2O3
Alumina
3-8
Fe2O3
Iron
0,5-6
MgO
Magnesia
0,1-4
Na2O and K2O
Alkalies
0,2-1,3
SO3
Sulfuric anhydride
1-3
Oxide Composition
Mass Percentage Oxide
Cement 1
Cement 2
Cement 3
CaO
66
63
66
SiO2
20
22
20
Al2O3
7
7.7
5.5
Fe2O3
3
3.3
4.5
Others
4
4
4
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Major Compounds of Portland Cement (Bogue’s Compound Composition)
Name
Chemical formula
Abbreviation
1. Tricalcium silicate
3CaO.SiO2
C3 S
2. Dicalcium silicate
2CaO.SiO2
C2 S
3. Tricalcium aluminate
3CaO.Al2O3
C3 A
4. Tetracalcium alumino ferrite
4CaO.Al2O3.Fe2O3
C4AF
Bogue’s Compound Composition •
C3S=4.07(CaO)-7.6(SiO2)- 6.72(Al2O3)-1.43(Fe2O3 ) – 2.85( SO3 )
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C2S= 2.87 (SiO2) - 0.75( 3Cao. SiO2)
•
C3A= 2.65(Al2O3) – 1.69 (Fe2O3 )
•
C4AF = 3.04 (Fe2O3 )
Significance of Compound Composition
Mass Percentage Compound
Cement 1
Cement 2
Cement 3
C3 S
65
33
73
C2 S
8
38
2
C3 A
14
15
7
C4AF
4
10
14
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Hydration of cement •
When Portland cement is mixed with water its chemical compound constituents undergo a series of chemical reactions that cause it to harden. This chemical reaction with water is called "hydration". Each one of these reactions occurs at a different time and rate. Together, the results of these reactions determine how Portland cement hardens and gains strength.
OPC hydration
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Hydration starts as soon as the cement and water are mixed.
•
The rate of hydration and the heat liberated by the reaction of each compound is different.
•
Each compound produces different products when it hydrates.
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Tricalcium silicate (C3S). Hydrates and hardens rapidly and is largely responsible for initial set and early strength. Portland cements with higher percentages of C3S will exhibit higher early strength.
•
Tricalcium aluminate (C3A). Hydrates and hardens the quickest. Liberates a large amount of heat almost immediately and contributes somewhat to early strength. Gypsum is added to Portland cement to retard C3A hydration. Without gypsum, C3A hydration would cause Portland cement to set almost immediately after adding water.
•
Dicalcium silicate (C2S). Hydrates and hardens slowly and is largely responsible for strength increases beyond one week.
•
Tetracalcium aluminoferrite (C4AF). Hydrates rapidly but contributes very little to strength. Its use allows lower kiln temperatures in Portland cement manufacturing. Most Portland cement color effects are due to C4AF.
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Characteristics of Hydration of the Cement Compounds
Compounds
Reaction Rate
Amount of
C3 S
Moderate
Moderate
Liberated
Strength
High
Heat Liberation High
Low C2 S
Slow
Low
initially,
Low
high later
•
C3 A
Fast
Very high
Low
Very high
C4AF
Moderate
Moderate
Low
Moderate
Reactions of Hydration •
2C3S + 6H = C3S2H3 + 3Ca(OH)2
(100 + 24 = •
49 )
2 C2S + 4H = C3S2H3 + Ca(OH)2
(100 + 21 = •
75 + 99
+
22 )
C3A + 6H = C3AH6
[C3A + CaSO4 . 2H2O = 3Cao. Al2O3. 3CaSO4. 31H2O] Calcium Sulfoaluminate •
Strength gain of cement phases
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Heat of Hydration •
The heat of hydration is the heat generated when water and Portland cement react. Heat of hydration is most influenced by the proportion of C3S and C3A in the cement, but is also influenced by water-cement ratio, fineness and curing temperature. As each one of these factors is increased, heat of hydration increases.
•
For 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.
•
The heat of hydration depends on the chemical composition of cement.
Hydration rate of cement phases % of phase hydrated
100 80
C3 S
60
βC2 S
40
C3 A C4 AF
20 0 1
3
7
28
90
180
Age in Days
Hydration curve from conduction calorimetry
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Temperature rise curve in practice
Microstructure of cement paste Phases in Microstructure
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Socket where a sand grain has been pulled away from cement paste in 1-day old mortar. The sand grain was originally at the top of the picture. Note the open structure and the presence of crystals of calcium hydroxide in this region. “In portland cement mortars, the microstructure of the interfacial zone, extending to about 20 to 50 µm from the sand grain surface, is significantly different from that of the bulk paste matrix away from the sand grain. It is characterized by a massive CH layer engulfing the sand grain and by some channel type gaps.” “The formation of this zone may be the result of the presence of some water-filled gaps around the sand grains in the fresh mortar. These gaps may be the result of bleeding and inefficient filling with cement particles of the 20-µm space around the grain surface.” Solids in the Cement Paste
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Microstructure -role of Water
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Summary •
Bogue’s compound composition can be used to identify the variations in cement as a second level of quality control.
•
Understanding of the role of different compounds of cement during hydration is important for engineers to identify the behaviour of cement concrete.
•
Heat produced during hydration can cause damage to the concrete if not attended properly.
•
Knowledge of the microstructure and properties of the individual components of concrete and their relationship to each other is useful for exercising control on the properties of Concrete.
Reference: 1. “Properties of Concrete”, A.M. Neville, Fourth Edition, Pearson Education Asia pvt., Ltd., 2000. 2. “Concrete- Microstructure, Properties and Materials”, P.K. Mehta and Palulo J.M. Monteiro, Tata Mcgraw Hill
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