Civil Engineering Reference
In-Depth Information
Fig. 2-27. Scanning-electron micrographs of hardened cement paste at (left) 500X, and (right) 1000X. (A7112, A7111)
Powers and Brownyard (1947)
,
Powers (1961)
, and
Taylor
(1997)
addressed the pore structure and chemistry of cement
paste. Fig. 2-28 shows estimates of the relative volumes of
the compounds in hydrated portland cement pastes.
A web-based computer model for hydration and
microstructure development can be found at
NIST (2001)
[
http://vcctl.cbt.nist.gov
]. The Concrete Microscopy
Library (Lange and Stutzman 1999) provides a collection of
micrographs of cement hydration and concrete at
http://www.cee.ce.uiuc.edu/lange/micro
.
The approximate percentage of each compound can be
calculated from a chemical oxide analysis (ASTM C 114 or
AASHTO T 105) of the unhydrated cement per ASTM C
150 (AASHTO M 85) (Bogue calculations). Due to the inac-
curacies of Bogue calculations, X-ray diffraction techniques
can be used to more accurately determine compound per-
centages (ASTM C 1365). Table 2-6 shows typical elemental
and compound composition and fineness for each of the
principal types of portland cement.
Although elements are reported as simple oxides for
standard consistency, they are usually not found in that
oxide form in the cement. For example, sulfur from the
gypsum is reported as SO
3
(sulfur trioxide), however,
cement does not contain any sulfur trioxide. The amount of
calcium, silica, and alumina establish the amounts of the
primary compounds in the cement and effectively the
properties of hydrated cement. Sulfate is present to control
capillary
porosity
100
Porosity
C-S-H
C-S-H
calcium
hydroxide
AFt and
AFm
calcium
sulfate
75
Ca(OH)
2
50
C
4
(A, F)H
13
C
4
AF
25
C
3
A
Ettringite
C
2
S
0
0
25
50
75
100
0
5
30
7
2
6
7
2
7
28
90
C
3
S
Degree of hydration, %
Age:
Hours
Days
Minutes
other
Fig. 2-28. Relative volumes of the major compounds in the microstructure of hydrating portland cement pastes (left) as a
function of time (adapted from
Locher, Richartz, and Sprung 1976
) and (right) as a function of the degree of hydration as
estimated by a computer model for a water to cement ratio of 0.50 (adapted from
Tennis and Jennings 2000
). Values are given
for an average Type I cement composition (
Gebhardt 1995
): C
3
S=55%, C
2
S=18%, C
3
A=10% and C
4
AF=8%. “AFt and AFm”
includes ettringite (AFt) and calcium monosulfoaluminate (AFm) and other hydrated calcium aluminate compounds. See Table
2-5 for compound transformations.
