Chemical Composition of Cement

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Cement

Ancient Romans were probably the first to use concrete – a word of Latin origin – based on hydraulic cement that is a material which hardens under water. This property and the related property of not undergoing chemical change by water in later life are most important and have contributed to the widespread use of concrete as a building material.

Roman cement fell into disuse, and it was only in 1824 that the modern cement, known as Portland cement, was patented by Joseph Aspdin, a Leeds builder.

Portland cement is the name given to a cement obtained by intimately mixing together calcareous and argillaceous, or other silica-, alumina-, and iron oxide-bearing materials, burning them at a clinkering temperature, and grinding the resulting clinker.

The definitions of the original British and new European Standards and of the American Standards are on those lines; no material, other than gypsum, water, and grinding aids may be added after burning.

Basic chemistry of cement

We have seen that the raw materials used in the manufacture of Portland cement consist mainly of

  • lime CaO
  • silica SiO2
  • alumina Al2O3
  • iron oxide Fe2O3

These compounds interact with one another in the kiln to form a series of more complex products, and, apart from a small residue of uncombined lime which has not had sufficient time to react, a state of chemical equilibrium is reached.

However, equilibrium is not maintained during cooling, and the rate of cooling will affect the degree of crystallization and the amount of amorphous material present in the cooled clinker.

The properties of this amorphous material, known as glass, differ considerably from those of crystalline compounds of a nominally similar chemical composition. Another complication arises from the interaction of the liquid part of the clinker with the crystalline compounds already present.

Nevertheless, cement can be considered as being in frozen equilibrium,

i.e. the cooled products are assumed to reproduce  the equilibrium existing at  the  clinkering  temperature.  This assumption   is, in fact, made in the

Calculation of the compound composition of commercial cements: the ‘potential’ composition is calculated from the measured quantities of oxides present in the clinker as if full crystallization of equilibrium products had taken place.

Main compounds in Portland cement

Four compounds are regarded as the major constituents of cement: they are listed in following table together with their abbreviated symbols.

  • CaO  =  C
  • SiO2   =  S
  • Alz03 =  A
  • Fe2O3 =  F
  • H2O     =   H
Name of compound Oxide composition Abbreviation
Tricalcium silicate 3CaO.SiO2 C3S
Dicalcium silicate 2CaO.SiO2 C2S
Tricalcium aluminate 3CaO.Al2O3 C3A
Tetracalcium aluminoferrite 4CaO.Al2O3.Fe 2O3 C4AF

Bogue’s Equations

The calculation of the potential composition of Portland cement is based on the work of R. H. Bogue and others, and is often referred to as ‘Bogue composition’. Bogue’s equations for the percentages of main compounds in cement are given below. The terms in brackets represent the percentage of the given oxide in the total mass of cement.

C1S  = 4.07(CaO)  – 7.60(SiO2)    —   6.72(Al2O3)    —       l.43(Fez03)    —  2.85(SO3) C2S = 2.87(SiO2)    0.754(3CaO.SiO2)

C1A = 2.65(Al2O3)        —    l.69(Fez0 3)

C4AF = 3.04(Fe2O3)

Silicates

  • The silicates, C3S and C2S, are the most important compounds,
  • They are responsible for the strength of hydrated cement paste.
  • In reality, the silicates in cement are not pure compounds
  • But contain minor oxides in solid solution.
  • These oxides have significant effects on the atomic arrangements, crystal form, and hydraulic properties of the silicates.

Aluminates

  • The presence of C3A in cement is undesirable:
  • It contributes little or nothing to the strength of cement except at early ages
  • When hardened cement paste is attacked by sulfates.
  • The formation of calcium sulfo­ aluminate (ettringite) may cause disruption.
  • C3A is beneficial in the manufacture of cement in that it facilitates the combination of lime and silica.
  • C4AF is also present in cement in small quantities, and, compared with the other three compounds, it does not affect the behavior significantly
  • It reacts with gypsum to form calcium sulfoferrite and its presence may accelerate the hydration of the silicates.

Importance of Gypsum (CaSO₄·2H₂O)

The amount of gypsum added to the clinker is crucial, and depends upon the C3A content and the alkali content of cement.

Increasing the fineness of cement has the effect of increasing the quantity of C3A available at early ages, and this raises the gypsum requirement.

An excess of gypsum leads to expansion and consequent disruption of the set cement paste. The optimum gypsum content is determined on the basis of the generation of the heat of hydration.

So that a desirable rate of early reaction occurs, which ensures that there is little C3A available for reaction after all the gypsum has combined.

ASTM C 150-05 and BS EN 197-1 specify the amount of gypsum as the mass of sulfur trioxide (SO3) present.

Minor Compounds

In addition to the main compounds listed in there exist minor compounds, such as MgO, TiO 2, Mn2O3, KP, and Na2O; they usually amount to not more than a few per cent of  the mass of cement.

Two of the minor compounds  are of interest:  the oxides of sodium  and potassium, and  K20   ,  known  as  the  alkalis  (although  other  alkalis  also  exist in cement).

They have been found to react with some aggregates, the pro­ ducts of the alkali-aggregate reaction causing disintegration of the concrete (see page 267), and have also been observed to affect the rate of the gain of strength of cement.

It should, therefore, be pointed out that the term ‘minor compounds’ refers primarily to their quantity and not necessarily to their importance.

A general idea of the composition of cement can be  obtained  from which gives the oxide composition limits of Portland cements.

Oxide Content, per cent
CaO 60-67
SiO2 17-25
Al2O3 3-8
Fe2O3 0.5-6.0
MgO 0.1-4.0
Alkalis 0.2-1.3
so1 1-3

Two terms used in require explanation. The insoluble residue, determined by treating with hydrochloric acid, is a measure of adulteration of cement, largely arising from impurities in gypsum. BS EN 197-1 limits the insoluble residue to 5 per cent of the mass of cement and filler; for cement, the ASTM C 150 limit is 0.75 per cent. The loss on ignition shows

the extent of carbonation and hydration of free lime and free magnesia due to the exposure of cement to the atmosphere. The specified limit both of ASTM C 150-05 and of BS EN 197-1 is 3 per  cent, except  for  ASTM Type IV cement (2.5 per cent) and cements with fillers of BS EN (5 per cent). Since hydrated free lime is innocuous, for a given  free lime content of cement, a greater loss on ignition is really advantageous.

Oxide and compound compositions of a typical Portland cement

Typical oxide composition per cent   Hence, calculated compound (using formulae of page 10), composition per cent
CaO                                63   C 3A 10.8
Si0 2                                20   C 3S 54.1
Al20 3                                6   C2S 16.6
Fe 2O3                               3   C 4 AF 9.1
MgO                                  1.2l   Minor compounds  
S0 3                                                     2      
K2O N a20      
Others                               1      
Loss on ignition               2      

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