Civil Engineering Reference
In-Depth Information
hydrogarnet phase, the specific surface area declines and the medium pore size increases
with prolonged curing under water. Along with this, the strength of the material declines
as well, though only moderately. These phenomena were attributed to a gradual
recrystallization of the existing hydrates associated with a coarsening of the texture of the
hardened paste (Hommertgen and Odler, 1991).
An addition of gypsum alters the hydration reaction, as ettringite is formed as the main
hydration product (see section 11.4). A simultaneous addition of gypsum and calcium
hydroxide is deleterious to the hydration process.
The development of glass cement is still in an experimental stage. Some compositions
that exhibit a very fast setting may be considered for highway repair works (MacDowell
and Chowdhury, 1990). The cement also exhibits very good resistance against diluted
acids and alkaline solutions regardless of the hydration products formed, and may be
applied in corrosive environments (MacDowell, 1991). Owing to the relatively low pH of
its pore solution, and the absence of calcium hydroxide among its hydration products,
glass cement may also be used as the matrix in glass fiber reinforced composites
(Hommertgen and Odler, 1991).
Another type of glass cement has been developed by Nakatsu
et al.
(1994). This binder
contains a CaO-Al
2
O
3
-SiO
2
glass with a CaO/SiO
2
molar ratio of 1.0 and an Al
2
O
3
content of 5 wt%. Such a glass may be produced by heating the appropriate blend of
starting materials to 1750°C and subsequent quenching. The binder is produced by
grinding the glass to a specific surface area of 700 m
2
/g (Blaine), and by adding to it
limited amounts of a Portland clinker powder to act as a hydration accelerator. Upon
mixing with water the cement sets and hardens, yielding a C-S-H phase as hydration
product. This cement exhibits a low heat of hydration: 72, 100, and 150 J/g after 7, 28,
and 90 days respectively.
S
uzuki
et al.
(1998) produced a glass corresponding to the composition 12CaO.7Al
2
O
3
that also contained Na
2
O or K
2
O. This was produced by fusion at 1500 °C and
quenching. After grinding and mixing with water the resulting cement yielded the phase
C
2
AH
8
as a product of hydration. The hydration reaction was intensified if Li
2
O, instead
of Na
2
O or K
2
O, was added to the system. At an addition of 3% of this oxide, lithium
aluminate hydrate was formed as a product of hydration in addition to C
3
AH
8
.
15.3
ALKALI SILICATE BINDERS
A number of crystalline alkali silicates exist in the alkali oxide-SiO
2
systems:
• Li
2
O.SiO
2
; Li
2
O.2SiO
2
• 2Na
2
O.SiO
2
; Na
2
O.SiO
2
; Na
2
O.2SiO
2
• K
2
O.SiO
2
; K
2
O.2SiO
2
; K
2
O.4SiO
2
All of them exhibit a high solubility in water and a low melting temperature. In addition
to their crystalline forms, alkali silicates may also exist in the form of alkali silicate
glasses with variable SiO
2
/alkali oxide ratios, which may be produced by a rapid cooling
of the pertinent melts. The term “water glass” designates concentrated solutions of alkali
