PORTLAND CEMENT - Materials for Civil and Construction Engineers - page 203

T ABLE 6.1 Main Compounds of Portland Cement.

Common Usual Range

Compound Chemical Formula Formula * by Weight (%)

3 CaO # SiO 2 Tricalcium Silicate C 3 S 45-60

2 CaO # SiO 2 Dicalcium Silicate C 2 S 15-30

3 CaO # Al 2 O 3 Tricalcium Aluminate C 3 A 6-12

4 CaO # Al 2 O 3 # Fe 2 O 3 Tetracalcium Aluminoferrite C 4 AF 6-8

* The cement industry commonly uses shorthand notation for chemical formulas:

and F

C = Calcium oxide,

S = silicon dioxide, A = Aluminum oxide, = Iron oxide.

complex chemical compounds. Calcination in the kiln restructures the
molecular composition, producing four main compounds, as shown in
Table 6.1.
and when hydrated, provide the desired characteristics of the
concrete. Alumina and iron, which produce and are included
with the other raw materials to reduce the temperature required to produce
from 2000 ° C to 1350 ° C (3500 ° F to 2500 ° F). This saves energy and re-
duces the cost of producing the portland cement.
In addition to these main compounds, there are minor compounds, such
as magnesium oxide, titanium oxide, manganese oxide, sodium oxide, and
potassium oxide. These minor compounds represent a few percent by
weight of cement. The term minor compounds refers to their quantity and
not to their importance. In fact, two of the minor compounds, sodium oxide
and potassium oxide are known as alkalis. These alkalis react
with some aggregates causing the disintegration of concrete and affecting the
rate of strength development, as discussed in Chapter 5.

C 3 S C 2 S,

C 3 A C 4 AF,

C 3 S

Na 2 O K 2 O ,

1 2 1 2

6.3

Fineness of Portland Cement

Fineness of cement particles is an important property that must be carefully
controlled. Since hydration starts at the surface of cement particles, the finer
the cement particles, the larger the surface area and the faster the hydration.
Therefore, finer material results in faster strength development and a greater
initial heat of hydration. Increasing fineness beyond the requirements for a
type of cement increases production costs and can be detrimental to the
quality of the concrete.
The maximum size of the cement particles is 0.09 mm (0.0035 in.); 85%
to 95% of the particles are smaller than 0.045 mm (0.0018 in.), and the av-
erage diameter is 0.01 mm (0.0004 in.). (For reference, a number 200 sieve