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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 ( SiO 2 ), alumina ( Al 2 O 3 ), and iron ( Fe 2 O 3 ).
- Also Portland cement contains small amount of magnesia ( MgO ), alkalies (Na 2 O and K 2 O ), and sulfuric anhydrite ( SO 3 ).
Approximate Composition Limits of Oxides in Portland Cement
Oxide Common Name Content, % CaO SiO 2 Al 2 O 3 Fe 2 O 3 MgO Na 2 O and K 2 O SO 3
Lime Silica Alumina Iron Magnesia Alkalies Sulfuric anhydride
Oxide Composition
Mass Percentage Oxide Cement 1 Cement 2 Cement 3 CaO 66 63 66 SiO 2 20 22 20 Al 2 O 3 7 7.7 5. Fe 2 O 3 3 3.3 4. Others 4 4 4
Major Compounds of Portland Cement (Bogue’s Compound Composition)
Name Chemical formula Abbreviation
- Tricalcium silicate
- Dicalcium silicate
- Tricalcium aluminate
- Tetracalcium alumino ferrite
3CaO.SiO 2 2CaO.SiO 2 3CaO.Al 2 O 3 4CaO.Al 2 O 3 .Fe 2 O 3
C 3 S
C 2 S
C 3 A
C 4 AF
Bogue’s Compound Composition
- C 3 S=4.07(CaO)-7.6(SiO 2 )- 6.72(Al 2 O 3 )-1.43(Fe 2 O 3 ) – 2.85( SO 3 )
- C 2 S= 2.87 (SiO 2 ) - 0.75( 3Cao. SiO 2 )
- C 3 A= 2.65(Al 2 O 3 ) – 1.69 (Fe 2 O 3 )
- C 4 AF = 3.04 (Fe 2 O 3 )
Significance of Compound Composition
Mass Percentage Compound Cement 1 Cement 2 Cement 3
C 3 S 65 33 73
C 2 S 8 38 2 C 3 A 14 15 7 C 4 AF 4 10 14
Characteristics of Hydration of the Cement Compounds
Compounds Reaction Rate Amount of Liberated Strength Heat Liberation
C 3 S Moderate Moderate High High
C 2 S Slow Low
Low initially, high later
Low
C 3 A Fast Very high Low Very high
C 4 AF Moderate Moderate Low Moderate
- Reactions of Hydration
- 2C 3 S + 6H = C 3 S 2 H 3 + 3Ca(OH) 2 (100 + 24 = 75 + 49 )
- 2 C 2 S + 4H = C 3 S 2 H 3 + Ca(OH) 2 (100 + 21 = 99 + 22 )
- C 3 A + 6H = C 3 AH 6 [C 3 A + CaSO 4. 2H 2 O = 3Cao. Al 2 O 3. 3CaSO 4. 31H 2 O] Calcium Sulfoaluminate
- Strength gain of cement phases
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 C 3 S and C 3 A 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
Hydration curve from conduction calorimetry
0
20
40
60
80
100
% of phase hydrated
1 3 7 28 90 180 Age in Days
C S 3 β C S 2 C A 3 C AF 4
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
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:
- “Properties of Concrete”, A.M. Neville, Fourth Edition, Pearson Education Asia pvt., Ltd., 2000.
- “Concrete- Microstructure, Properties and Materials”, P.K. Mehta and Palulo J.M. Monteiro, Tata Mcgraw Hill
