PORTLAND CEMENT CHEMISTRY

... and now for a little bonding. (If you have the valence for it.)

MAKING CEMENT

... or 'mortar he wrote.' Holnam
First of all, because we deal with so few chemical compounds, we abbreviate our chemical symbols, so throw away the periodic table! But, they make sense. Check out the abbreviated symbols so you can understand the rest of this.

CompoundAbbrevActual
Tricalcium SilicateC3S3 CaO, SiO2
Dicalcium SilicateC2S2 CaO, SiO2
Tricalcium AluminateC3A3 CaO, Al2O3
Tetacalcium AluminoferriteC4AF4 CaO, Al2O3, Fe2O3

What does each compound contribute to cement?
C3S - ("alite") Gives cement/concrete most of its early strength developing qualities.
C2S - ("belite") Hydrates more slowly than C3S & provides concrete's late strength because of this long tern hydration.
C3A - Responsible for the workability of the mortar. Also aids kiln burning as a flux that melts assisting in reactions of other compounds.
C4AF - No significant hydraulic properties, but melts at low temperatures (2,440 'f) promoting other reactions. Considered to be responsible for the gray color cement.

These are the primary compounds required for cement & their "cement abbreviations."

CompoundAbbrevActual
Calcium Oxide
C
CaO
Silicon Dioxide
S
SiO2
Aluminum Oxide
A
Al2O 3
Ferric Oxide
F
Fe2O 3

The CaO comes from the limestone, while the other 3 compounds required are obtained by mixing proportions of limestone, clay, sometimes iron ore &/or sand, & coal ash.

QUALITY CONTROL TOOLS

Silica Ratio
(S.R.) An indication of the ease of component combination. An increase normally indicates reduced liquid phase, higher temperatures for clinker burning, higher fuel requirements, & possibly higher free lime.

Burnability Index
(B.I.) Raw mixes with a low B.I. are normally easier to burn to a low free lime content. A high B.I. mix requires more fuel & longer sintering zones to form C3S & C2S compounds.

Lime Saturation Factor
(L.S.F.) This is an empirical formula based on the assumption that cement in its basic form, contains the compounds C3S, C2A, & C2F. Low values may result in lower strength cement & a high value usually indicates mixes may be difficult to burn.

Alumina or Iron Modules
(A.M.) An indication of the amount of melt that will be formed in a given mix. Higher AMs are harder to burn & give higher early strengths Holnam

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