Civil Engineering Reference
In-Depth Information
• agents facilitating cement grinding;
• agents that improve handling and storage of the cement;
• modifiers of fresh concrete mix rheology;
• hydrophobic agents;
• accelerators and retarders of setting and hardening;
• air-entraining agents;
• expansive additives;
• agents inhibiting corrosion of the reinforcement in steel-reinforced concrete;
• agents that reduce the permeability of the hardened concrete;
• coloring agents.
In most instances these agents are intermixed with the fresh concrete mix as chemical
admixtures; however, it is also possible to interblend or intergrind some of them with the
cement in the course of its manufacture.
G
rinding aids
are agents employed to intensify the cement-grinding process so that a
high fineness of the material can be achieved. They are typically volatile organic
compounds, whose vapor, in the course of grinding, is adsorbed on the newly created
surface of the cement particles. In this way they prevent agglomeration of the particles
and enhance the progress of comminution. Triethanol amine and ethylene glycol are
examples of such agents. In addition to their effect on grinding, grinding aids also prevent
“packing” of the cement, which may take place during long-term storage under warm and
humid climatic conditions, and may impede the free flow and handling of the cement.
In a hydrophobic Portland cement (Bensted, 1992, 1993) the cement particles are
coated with a suitable
hydrophobic agent,
such as oleic, lauric or stearic acid or
pentachlorophenol, in amounts of about 0.2-0.3%. This is achieved by grinding together
Portland clinker, gypsum and the agent. Such a measure gives the cement a large degree
of protection against deterioration upon storage. Hydrophobic cement is favored in
situations where poor storage conditions, such as high air humidity, may impede cement
quality. Other benefits of hydrophobic Portland cement include improved plasticity of the
fresh concrete mix and water repellency of the hardened concrete. Lower grades of
hydrophobic Portland cement are designated water-repellent and waterproofed Portland
cements. A more extensive mixing of the concrete mix is required if hydrophobic rather
than ordinary Portland cement is employed. The setting time is usually extended
significantly.
An air-entrained Portland cement contains small amounts (between about 0.02% and
0.05%) of an
air-entraining agent
. These are organic compounds whose molecules
consist of a long non-polar chain coupled with a polar group on one end. Because of this
structural feature, upon dissolving in water they tend to concentrate at the liquid/air
interface, with the polar group in the liquid phase and the non-polar chain in air. At the
same time cations associated with the molecule dissociate and enter the liquid phase. In
the presence of such an “anionic surfactant,” air-filled, electrically charged, spherical
pores, with diameters of 10-250
µ
m, are produced in the cement paste in the course of
mixing. The presence of such pores improves the workability of the resultant fresh
concrete mix and increases significantly the frost resistance of the hardened concrete.
Cements with an added air-entraining agent are used mainly in road construction in
