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Materials for engineering, 3rd Edition - (Malestrom)

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154<br />

<strong>Materials</strong> <strong>for</strong> <strong>engineering</strong><br />

Up to 25% of microsilica may be added to cement. The ultrafine<br />

(~0.2 µm) particles act as a ‘space filler’ between particles, reducing porosity<br />

and, consequently, yielding a marked increase in strength. Sufficient time<br />

has to be allowed <strong>for</strong> the increased strength to be apparent, however, since<br />

the reactions involved are quite slow.<br />

The pozzolanic reaction has reduced heat evolution, thus making it suitable<br />

<strong>for</strong> use in concrete structures of large dimensions. Pozzolanic additions to<br />

OPC will thus reduce its tendency to cracking, due to a reduced thermal<br />

contraction in the mass and an increased ability to creep under load. Pozzolanas<br />

are added to mortars, cements and concretes to increase their durability and<br />

economy, and they also result in a cement that can set under water. They are<br />

used <strong>for</strong> concrete in marine, hydraulic and underground structures.<br />

Superplasticizers<br />

Superplasticizers are additions that allow a given degree of workability to be<br />

obtained at a reduced water/cement ratio. The ratio may be reduced by up to<br />

20% in the presence of, <strong>for</strong> example, sulphonated naphthalene <strong>for</strong>maldehyde<br />

condensate, which is absorbed at the solid/liquid interface. They have the<br />

effect of neutralizing the surface attractions between individual particles of<br />

the cement, causing them to <strong>for</strong>m a less open structure, which there<strong>for</strong>e<br />

requires less water to fill spaces and provide fluidity to the paste. By decreasing<br />

the water/cement ratio in this way, superplasticizers act to increase the strength<br />

of the cement product.<br />

By the addition of both superplasticizers and microsilica, cements with a<br />

six-fold increase in tensile strength have been achieved.<br />

Macrodefect-free (MDF) cement<br />

Recently a method <strong>for</strong> eliminating larger pores in cement has been used to<br />

produce MDF cement with a maximum pore size of ~15 µm. About 5% of a<br />

suitable polymer (e.g. polyacrylamide gel) is added to the cement paste,<br />

prepared from two size fractions of cement powder (~5 µm and 75–125 µm)<br />

to give denser packing. This is subjected to a dough-type mixing, followed<br />

by a compression at 5 MPa to eliminate the air. The resulting cement, with<br />

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