Civil Engineering Reference
In-Depth Information
believed that additional bonds develop between the -OH or -CO-OH groups of the
polymer and ionic species at the cement grain surface (Drabik
et al.,
1992; Harsh
et al.,
1992; Hu, 1992). The bonding forces formed in this way are quite strong, and thus the
failure of MDF material tends to occur within the polymer phase or clinker grains rather
than at their interface (Hu, 1992).
On the micrometer scale the structure of MDF materials consists of densely packed
residual unreacted cement grains embedded in a polymeric matrix. An important role in
the development of strength is played by an interphase region that surrounds the
unreacted grains and is responsible for the high degree of bonding between cement
particles and the polymeric matrix. In this region crystallites of C
2
AH
8
(5-8 mm in size)
reside in an amorphous matrix (Lewis and Kriven, 1993; Lewis
et al.,
1994). The
following composition was estimated for a mature calcium aluminate cement-based MDF
material and the original mix (Lewis and Kriven, 1993; Lewis
et al.,
1994):
Component
Composit tion
(wt%)
(vol.%)
Original mix
High alumina cement
84.3
65.2
Polyvinylalcohol
5.9
12.3
Glycerol
0.6
1.4
Water
9.3
21.1
Mature MDF material
Unreacted cement (CA+CA
2
)
78.7
66.6
Binder phase
C
2
AH
8
10.1
13.2
Al(OH)
3
4.4
4.6
Plasticized PVA
6.8
15.6
Non-volatiles
Volatiles (in binder phase)
Plasticized PVA
6.8
15.6
H
2
O
5.5
14.0
The distribution of constituents of the binder phase between the interphase region and the
bulk matrix was estimated as follows (Lewis
et al.,
1994):
