Civil Engineering Reference
In-Depth Information
An autogenous shrinkage occurs in cement-slag and cement-silica
fume mixtures. In order to compare the rate of these shrinkages and also the
mechanisms involved, the heat effects were determined. It was concluded
that the shrinkage was due to the free water being consumed by the C-S-H
phase, and increased shrinkage in slag-cement pastes was attributed to the
formation of larger amounts of C-S-H.
[87]
Thermal methods have been applied to explain the causes leading
to the deterioration of various types of concrete. Normal concrete subjected
to calcium chloride deteriorates, and this can be reduced or prevented by
using supplementary cementing materials such as fly ash, slag, and silica
fume. Torii, et al.,
[88]
working with mortar mixes, found that with 10% silica
fume and 50% slag a good resistance to chloride was obtained, but with 30%
fly ash slight deterioration occurred. DSC-TG curves of mortars were able
to identify ettringite, Friedel's salt, CH and a complex chloroaluminate salt
in deteriorated specimens. In blends containing lower amounts of lime, the
deterioration due to chloride could be controlled.
Another form of deterioration in concrete involves the chemical
reaction between the products of cement hydration and carbon dioxide.
This reaction decreases the pH of the solution, and this may lead to the
corrosion of the embedded steel. Carbonation shrinkage is another phe-
nomenon that has been documented. Maslehuddin, et al.,
[89]
examined by
DTA/TG, the products formed in mortar specimens exposed to CO
2
at
55-75°C for 54 weeks. Some were contaminated with chloride and sulfate
ions. The amount of Ca(OH)
2
and CaCO
3
formed in several cement mixes
containing fly ash, silica fume, and slag was determined (Table 9). In
general, the amount of Ca(OH)
2
is lower in samples exposed to CO
2
, and
the amount of CaCO
3
in samples exposed to CO
2
is higher. There is also an
accelerated carbonation in contaminated specimens.
9.0
MISCELLANEOUS ADDITIVES
Many types of industrial by-products and chemicals have potential
for use in concrete. They are too numerous to mention. Experiments have
been carried out to assess the possibility of using them in practice. The
thermal techniques have been applied to monitor the hydration of mixes
containing these substances. These techniques also provide valuable data
on the durability aspects. In this section, a few of those that are not covered
in other sections are described.
