Cement chemist notation

Short description: Abbreviated notation for chemical formulas of common oxides

Cement chemist notation (CCN) was developed to simplify the formulas cement chemists use on a daily basis. It is a shorthand way of writing the chemical formula of oxides of calcium, silicon, and various metals.

Abbreviations of oxides

The main oxides present in cement (or in glass and ceramics) are abbreviated in the following way:

CCN	Actual formula	Name
C	CaO	Calcium oxide, or lime
S	SiO2	Silicon dioxide, or silica
A	Al2O3	Aluminium oxide, or alumina
F	Fe2O3	Iron oxide, or rust
T	TiO2	Titanium dioxide, or titania
M	MgO	Magnesium oxide, or periclase
K	K2O	Potassium oxide
N	Na2O	Sodium oxide
H	H2O	Water
C	CO2	Carbon dioxide
S	SO3	Sulfur trioxide
P	P4O10	Phosphorus pentoxide

Conversion of hydroxides in oxide and free water

For the sake of mass balance calculations, hydroxides present in hydrated phases found in hardened cement paste, such as in portlandite, Ca(OH)₂, must first be converted into oxide and water.

To better understand the conversion process of hydroxide anions in oxide and water, it is necessary to consider the autoprotolysis of the hydroxyl anions; it implies a proton exchange between two OH⁻, like in a classical acid–base reaction:

OH⁻acid 1 + OH⁻base 2 → O²⁻base 1 + H₂Oacid 2

or also,

2 OH⁻ → O²⁻ + H₂O

For portlandite this gives thus the following mass balance:

Ca(OH)₂ → CaO + H₂O

Thus portlandite can be written as CaO · H₂O or CH.

Main phases in Portland cement before and after hydration

These oxides are used to build more complex compounds. The main crystalline phases described hereafter are related respectively to the composition of:

• Clinker and non-hydrated Portland cement, and;
• Hardened cement pastes obtained after hydration and cement setting.

Clinker and non-hydrated Portland cement

Four main phases are present in the clinker and in the non-hydrated Portland cement.
They are formed at high temperature (1,450 °C) in the cement kiln and are the following:

CCN	Actual formula	Name	Mineral phase
C3S	3 CaO · SiO2	Tricalcium silicate	Alite
C2S	2 CaO · SiO2	Dicalcium silicate	Belite
C3A	3 CaO · Al2O3	Tricalcium aluminate	Aluminate or Celite
C4AF	4 CaO · Al2O3 · Fe2O3	Tetracalcium alumino ferrite	Ferrite

The four compounds referred as C₃S, C₂S, C₃A and C₄AF are known as the main crystalline phases of Portland cement. The phase composition of a particular cement can be quantified through a complex set of calculation known as the Bogue formula.

Hydrated cement paste

Hydration products formed in hardened cement pastes (also known as HCPs) are more complicated, because many of these products have nearly the same formula and some are solid solutions with overlapping formulas. Some examples are given below:

CCN	Actual formula	Name or mineral phase
CH	Ca(OH)2 or CaO · H2O	Calcium hydroxide
C-S-H	0.6–2.0 CaO · SiO2 · 0.9–2.5 H2O, with variable composition within this range, and often also incorporating partial substitution of Al for Si	Calcium silicate hydrate
C-A-H	This is even more complex than C-S-H	Calcium aluminate hydrate
AFt	C6AS3H32, sometimes with substitution of Fe for Al, and/or CO2−3 for SO2−4	calcium trisulfoaluminate hydrate, or ettringite
AFm	C4ASH12, often with substitution of Fe for Al, and/or various other anions such as OH− or CO2−3 for SO2−4	Calcium monosulfoaluminate
C3AH6	3CaO · Al2O3 · 6 H2O	Hydrogarnet

The hyphens in C-S-H indicate a calcium silicate hydrate phase of variable composition, while 'CSH' would indicate a calcium silicate phase, CaH₂SiO₄.

Use in ceramics, glass, and oxide chemistry

The cement chemist notation is not restricted to cement applications but is in fact a more general notation of oxide chemistry applicable to other domains than cement chemistry sensu stricto.

For instance, in ceramics applications, the kaolinite formula can also be written in terms of oxides, thus the corresponding formula for kaolinite,

Al₂Si₂O₅(OH)₄,

is

Al₂O₃ · 2 SiO₂ · 2 H₂O

or in CCN

AS₂H₂.

Possible use of CCN in mineralogy

Although not a very developed practice in mineralogy, some chemical reactions involving silicate and oxide in the melt or in hydrothermal systems, and silicate weathering processes could also be successfully described by applying the cement chemist notation to silicate mineralogy.

An example could be the formal comparison of belite hydration and forsterite serpentinisation dealing both with the hydration of two structurally similar earth -alkaline silicates, Ca₂SiO₄ and Mg₂SiO₄, respectively.

Calcium system

belite hydration:

2 C2S + 4 H → C3S2H3 + CH

(Reaction 4b)

Magnesium system

forsterite serpentinisation:

Forsterite2 Mg2SiO4 + water3 H2O → serpentine Mg3Si2O5(OH)4 + bruciteMg(OH)2

(Reaction 4c)

2 M2S + 3 H → M3S2H2 + MH

(Reaction 4d)

The ratio Ca/Si (C/S) and Mg/Si (M/S) decrease from 2 for the dicalcium and dimagnesium silicate reagents to 1.5 for the hydrated silicate products of the hydration reaction. In other term, the C-S-H or the serpentine are less rich in Ca and Mg respectively. This is why the reaction leads to the elimination of the excess of portlandite (Ca(OH)₂) and brucite (Mg(OH)₂), respectively, out of the silicate system, giving rise to the crystallization of both hydroxides as separate phases.

The rapid reaction of belite hydration in the setting of cement is formally "chemically analogue" to the slow natural hydration of forsterite (the magnesium end-member of olivine) leading to the formation of serpentine and brucite in nature. However, the kinetic of hydration of poorly crystallized artificial belite is much swifter than the slow conversion/weathering of well crystallized Mg-olivine under natural conditions.

This comparison suggests that mineralogists could probably also benefit from the concise formalism of the cement chemist notation in their works.

See also

• Hydration of belite in cement (analogous to forsterite hydration)
• Hydration reaction of forsterite (olivine) in serpentinisation

References

• Locher, Friedrich W. (2006). Cement: Principles of production and use. Düsseldorf, Germany: Verlag Bau + Technik GmbH. ISBN 3-7640-0420-7. 

• Mindess, S.; Young, J.F. (1981). Concrete. Englewood, NJ, USA: Prentice-Hall. ISBN 0-13-167106-5. 

External links

• Cement and Concrete Glossary