Diverse Applications of Surface and Colloid Chemistry in Science and Industry - Surface and Colloid Chemistry: Principles and Applications - page 221

have been used with cement that become part of a waste disposal process. The most
common of these are pulverized fly ash (PFA), a coal combustion product; blast
furnace slag (BFS) from iron making; and condensed silica fume (CSF) from the
ferrosilicon industry.
The hydration characteristics of cement (rate and extent) are of great importance
in the end product. Silica is known to react relatively fast in the cement system.
Hydration gives rise to effects on pore filling and the consequent enhancement of
mechanical performance (low-porosity pastes are stronger than high-porosity ones).
The first fast hydration step is followed by a relatively dormant period that may last
6 months or more, depending on temperature, particle size, and aqueous phase com-
position. In order to control the hydration step, alkyl sulfonate salts surface-active
substances (SAS) are used.
The durability of cement pastes is strongly influenced by (1) internal chemistry,
and (2) paste microstructure. The industrial by-product additives above all influence
the development of paste microstructures. Isolated pores are completely enclosed
by hydration products so that material transport into and out of the pore is limited.
Connected porosity is that through which a continuous pathway between regions of
the microstructure exists. Continuous or interconnected porosity often (although not
always) links the interior of the paste to the outside world so that aggressive chemi-
cal species can penetrate and degrade the paste internally, affecting paste durability.
The effect of blending agents identified previously on microstructure is to cause a
reduction in the degree of interconnected porosity. This is especially true in the
case of BFS-containing pastes. Although the overall porosity, as determined by neu-
tron scattering, is still significant, interconnected porosity, as measured by intrusion
methods (e.g., mercury intrusion porosimetry [MIP]) is low.
In addition, further oxidation and cathodic reactions lead to the production of
oxides and oxyhydroxides of Fe (III), which produces a low-permeability, passive
film that slows down the corrosion rate considerably. Where corrosion can continue
(by depassivation), the expansion of corrosion products at the cement-steel interface
and the subsequent spalling of cover concrete can occur. Many examples of this can
be seen in concrete structures.
Glass-fiber reinforcement corrosion : Unlike steel reinf orcement, glass fibers are
introduced in random orientation and throughout the paste. Typically, as filament
bundles (of around 50 filaments), the fiber will be of variable length (usually up to
2 cm).
Cement has been used in modern times to build houses and very large struc-
tures, such as dams. Some specifications for the Hoover Dam in the United States
Southwest are as follows:

Height 250 m

Weight 6.6 million tons

Volume of concrete used 118 million ft 3

Width 250 m

The surface and colloidal chemistry of cement is indeed very complicated, and the
Hoover Dam was among the leading engineering achievements of the 20th century.