Civil Engineering Reference
In-Depth Information
In the presence of water, these compounds hydrate
(chemically combined with water) to form new compounds
that are the infrastructure of hardened cement paste in
concrete (Fig. 2-26). The calcium silicates, C
3
S and C
2
S,
hydrate to form the compounds calcium hydroxide and
calcium silicate hydrate (archaically called tobermorite gel).
Hydrated portland cement contains 15% to 25% calcium
hydroxide and about 50% calcium silicate hydrate by mass.
The strength and other properties of hydrated cement are
due primarily to calcium silicate hydrate (Fig. 2-27). C
3
A
reacts with water and calcium hydroxide to form tetracal-
cium aluminate hydrates. C
4
AF reacts with water to form
calcium aluminoferrite hydrates. C
3
A, sulfates (gypsum,
anhydrite, or other sulfate source), and water combine to
form ettringite (calcium trisulfoaluminate hydrate), calcium
monosulfoaluminate, and other related compounds. These
basic compound transformations are shown in Table 2-5.
Brunauer (1957)
,
Copeland and others (1960)
,
Lea (1971)
,
Fig. 2-26. Electron micrographs of (left) dicalcium silicate hydrate, (middle) tricalcium silicate hydrate, and (right) hydrated
normal portland cement. Note the fibrous nature of the calcium silicate hydrates. Broken fragments of angular calcium
hydroxide crystallites are also present (right). The aggregation of fibers and the adhesion of the hydration particles is
responsible for the strength development of portland cement paste. Reference (left and middle)
Brunauer 1962
and (right)
Copeland and Schulz 1962
. (69110, 69112, 69099)
Table 2-5. Portland Cement Compound Hydration Reactions (Oxide Notation)
2 (3CaO•SiO
2
)
+ 11 H
2
O
= 3CaO•2SiO
2
•8H
2
O
+ 3 (CaO•H
2
O)
Tricalcium silicate
Water
Calcium silicate
Calcium hydroxide
hydrate (C-S-H)
2 (2CaO•SiO
2
)
+ 9 H
2
O
= 3CaO•2SiO
2
•8H
2
O
+ CaO•H
2
O
Dicalcium silicate
Water
Calcium silicate
Calcium hydroxide
hydrate (C-S-H)
3CaO•Al
2
O
3
+ 3 (CaO•SO
3
•2H
2
O)
+ 26 H
2
O
= 6CaO•Al
2
O
3
•3SO
3
•32H
2
O
Tricalcium aluminate
Gypsum
Water
Ettringite
2 (3CaO•Al
2
O
3
)
+ 6CaO•Al
2
O
3
•3SO
3
•32H
2
O+ 4
2
O
= 3 (4CaO•Al
2
O
3
•SO
3
•12H
2
O)
Tricalcium aluminate
Ettringite
Water
Calcium monosulfoaluminate
3CaO•Al
2
O
3
+ CaO•H
2
O
+ 12 H
2
O
= 4CaO•Al
2
O
3
•13H
2
O
Tricalcium aluminate
Calcium hydroxide
Water
Tetracalcium aluminate hydrate
4CaO• Al
2
O
3
•Fe
2
O
3
+ 10 H
2
O
+ 2 (CaO•H
2
O)
= 6CaO•Al
2
O
3
•Fe
2
O
3
•12H
2
O
Tetracalcium aluminoferrite
Water
Calcium hydroxide
Calcium aluminoferrite hydrate
Note: This table illustrates only primary transformations and not several minor transformations. The composition of calcium silicate hydrate
(C-S-H) is not stoichiometric (
Tennis and Jennings 2000
).
