Core Raw Material—Cement - The Issues and Discussion of Modern Concrete Science - page 35

Table 3.1 Mineral component content and characteristics of cement clinker
Mineral name Chemical formula Code Content/

%

Main feature

Tricalcium
silicate

3CaO · SiO 2 C 3 S 52 - 63 Hydration rate: faster; hydra-

tion heat: higher; strength:
highest

Dicalcium
silicate

2CaO · SiO 2 C 2 S 13 25 Hydration rate: slowest;

hydration heat: lowest; highest
strength growth rate at later
age

-

Tricalcium
aluminate

3CaO · Al 2 O 3 C 3 A1 15 Hydration rate: fastest; hydra-

tion heat: highest; strength: not
high but with rapid develop-
ment; shrinkage: big

-

16 Hydration rate: faster; hydra-
tion heat and strength: moder-
ate; shrinkage: small,
beneficial to tensile strength
Note Mineral component contents in the table were counted basing on the PI, PII

Tetracalcium
aluminferrite

4CaO · Al 2 O 3 · Fe 2 O 3 C 4 AF 8

-

Besides, heat is emitted when hydrated reactions take place.
The main four mineral components in cement have different hydration rates, so
their effects on strength at age of 3, 7, 28, and 90 days are different. According to
reacting rate, tricalcium aluminate (abbreviated as C 3 A) is the fastest and has the
greatest in

uence on 3-day and 7-day strength; tetracalcium aluminferrite (abbre-
viated as C 4 AF) is slower than C 3 A; tricalcium silicate (abbreviated as C 3 S) is
slower than C 4 AF and has the greatest in uence on 28-day strength; and dicalcium

silicate (abbreviated as C 2 S) is the slowest and constitutes the main source for
concrete strength at later age, as shown in Table 3.1 .
PO and Portland cement for road were used by the author in recent years. There
may be certain deviation compared with the actual situation all over the country.
There are mainly three factors that affect cement hydration rate. Other than
mineral component, the others are cement

fineness and environment temperature.
As we can see from the above hydration reaction formulae, tricalcium silicate not
only generates calcium silicate hydrate but also calcium hydroxide (Ca(OH) 2 ) after
hydration. In concrete, Ca(OH) 2 belongs to white precipitate with layered structure
which is adverse to strength. However, it can continue reacting with acid material
and generate calcium silicate hydrate, playing a role in enhancing concrete strength.
The reaction formula is as follows:

Ca OH ð 2 þ SiO 2 þ H 2 O ! CaO SiO 2 H 2 O

Therefore, different degrees of acid or other admixtures are added to cement in
China except I-type Portland cement. Adding admixture is for the purpose of
reacting with Ca(OH) 2 sequentially and generating new cementitious material.