Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
Up to 25% of microsilica may be added to cement. The ultrafine
(~0.2 µm) particles act as a ‘space filler’ between particles, reducing porosity
and, consequently, yielding a marked increase in strength. Sufficient time
has to be allowed for the increased strength to be apparent, however, since
the reactions involved are quite slow.
The pozzolanic reaction has reduced heat evolution, thus making it suitable
for use in concrete structures of large dimensions. Pozzolanic additions to
OPC will thus reduce its tendency to cracking, due to a reduced thermal
contraction in the mass and an increased ability to creep under load. Pozzolanas
are added to mortars, cements and concretes to increase their durability and
economy, and they also result in a cement that can set under water. They are
used for concrete in marine, hydraulic and underground structures.
Superplasticizers
Superplasticizers are additions that allow a given degree of workability to be
obtained at a reduced water/cement ratio. The ratio may be reduced by up to
20% in the presence of, for example, sulphonated naphthalene formaldehyde
condensate, which is absorbed at the solid/liquid interface. They have the
effect of neutralizing the surface attractions between individual particles of
the cement, causing them to form a less open structure, which therefore
requires less water to fill spaces and provide fluidity to the paste. By decreasing
the water/cement ratio in this way, superplasticizers act to increase the strength
of the cement product.
By the addition of both superplasticizers and microsilica, cements with a
six-fold increase in tensile strength have been achieved.
Macrodefect-free (MDF) cement
Recently a method for eliminating larger pores in cement has been used to
produce MDF cement with a maximum pore size of ~15 µm. About 5% of a
suitable polymer (e.g. polyacrylamide gel) is added to the cement paste,
prepared from two size fractions of cement powder (~5 µm and 75–125 µm)
to give denser packing. This is subjected to a dough-type mixing, followed
by a compression at 5 MPa to eliminate the air. The resulting cement, with