Basic Cement Chemistry

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Learning Objectives

• Know the meaning and the way of presentation of chemical and mineralogical composition of cement specific materials
• Know the chemical moduli applied for cement specific materials
• Know the Bogue-calculation for clinker mineral phases and the relationship with chemical moduli
• Chemical Composition
• Chemical Formulae
• Mineralogical Composition
• Chemical Parameters for Cement-Specific Materials
• Contents of Clinker Minerals According to Bogue
• Relationship between Chemical Moduli and Clinker Mineral Contents
• Exercise for Calculation

Elemental Composition (weight – %)

Chemical Composition

• Elemental composition in weight percentage. By convention, the elements are expressed in form of their oxides (exception; Cl, F).
• Usual sequence of elements in cement chemistry
• L.o.I.: e.g. at 1050 °C, mainly due to CO2 and H2O

Chemical Composition – Examples

Chemical Formula

• The chemical formula indicates the elements occurring in a chemical compound:

For a molecular compound, type and absolute number of elements are given

For a mineralogical compound, type and relative number of elements are given

Note: In mineralogical compounds, the elements need not  necessarily occur in simple numerical ratios (impurities,        solid solution)

Chemical Formulae

In the cement chemistry, shorthand’s are often used:

Mineralogical Composition

Composition of material, expressed in weight-percentage of the occurring minerals

Note: Difference between chemical and mineralogical  composition

Chemical Parameters for Cement-Specific Materials

Titration

Content of carbonates as determined by acid-base titration, expressed as CaCO3

% Titration = 1.786 CaO + 2.48 MgO

Applied for:   Limestone Marl Raw Meal

Lime Saturation

The LS is a measure to which extent the CaO-richest compounds C3S, C3A and C4AF can be formed without the necessary presence of free lime. At LS >  100, free lime will unavoidably be present after burning.

Applied for:

Raw meal

Clinker

Cement:  neat OPC only

CaO = CaO(total) – 0.7 SO3

Usual range in clinker: 85 – 100

Note:  The influence of MgO can be accounted for

Max. 2% MgO may be introduced in formula (not applied in cement specifications)

Silica Ratio

Applied for:

Siliceous-argillaceous raw   components

Raw Meal

Clinker

Cement

Usual range in clinker: 1.8 – 3.6

Alumina Ratio

Applied for:

Siliceous-argillaceous raw   components

Raw Meal

Clinker

Cement

Usual range in clinker: 1 – 3

Na2O-equivalent

Total alkali content, expressed as Na2O

Na2O-equivalent = Na2O + 0.658 K2O

Note:

Limit for low alkali cement

Na2O-equivalent  <  0.6 %

Applied for:

Clinker

Cement

Moduli of Main Clinkers in Holcim group

Moduli of Main Clinkers

Content of clinker Minerals according to Bogue

• Percentage content of clinker minerals, assuming that chemical equilibrium is attained, and that no impurities are present.

C3S     =    4.07 CaO – 7.6 SiO2 – 6.72 Al2O3 – 1.43 Fe2O3

C2S     =   8.6 SiO2 + 5.07 Al2O3 + 2.08 Fe2O3 – 3.07 CaO

or 2.87 SiO2 – 0.754 C3S

C3A     =   2.65 Al2O3 – 1.69 Fe2O3

C4AF   =   3.04 Fe2O3

In reality, the mineralogical composition of industrial clinkers differs to some extent from that calculated according to Bogue.

Applied for   Cement

– OPC only (excl. blended cements)

– Correction for CaO in CaSO4:    CaO = CaOtot – 0.70 SO3

– For AASHTO: TiO2 and P2O5 to be           added to Al2O3 (no longer for ASTM)

Clinker

– CaO can be corrected for free lime; depending on objective of calculation CaO = CaOtot – CaOfree

Significance of Clinker Minerals for Cement Properties

Significance of Clinker Minerals for ASTM Cement Types

• Type I Portland no restrictions regarding clinker minerals
• Type II Portland with moderate sulphate resistance (and moderate heat of hydration)

C3A max. 8 %

(C3S + C3A max. 58% for moderate   heat of hydration)

• Type III Portland with high early strength

C3A max. 15%  (C3A max. 8% for moderate sulphate resist.  C3A max. 5% for high sulphate resist.)

• Type IV Portland with low heat of hydration

C3S max. 35%

C2S min.  40%

C3A max.   7%

• Type V Portland with high sulphate resistance

C3A max. 5.0 %

C4AF + 2 C3A max. 25%

or  C4AF + 2 C2F max. 25%

Clinker Minerals as Function of LS

Clinker Minerals as Function of SR

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