283757893275

346 Barry’s Advanced Construction of Buildings
Low-heat Portland cement
Low-heat Portland cement is used mainly for mass concrete works in dams and other
constructions where the heat developed by hydration of other cements would cause serious
shrinkage cracking. The heat developed by the hydration of cement in concrete in construction
works is dissipated to the surrounding air, whereas in large mass concrete works it
dissipates slowly. Control of the constituents of low-heat Portland causes it to harden more
slowly and therefore develop less rapidly than other cements. The slow rate of hardening
does not affect the ultimate strength of the cement yet allows the low heat of hydration to
dissipate through the mass of concrete to the surrounding air.
Portland blastfurnace cement
Portland blastfurnace cement is manufactured by grinding Portland cement clinker with
blastfurnace slag, the proportion of slag being up to 65% by weight and the percentage of
cement clinker no less than 35%. This cement develops heat more slowly than ordinary
cement and is used in mass concrete works as a low-heat cement. It has good resistance to
the destructive effects of sulphates and is commonly used in marine works.
Water-repellent cement
Water-repellent cement is made by mixing a metallic soap with ordinary or white Portland
cement. Concrete made with this cement is more water repellent and therefore absorbs less
rainwater than concrete made with other cements and is thus less liable to dirt staining.
This cement is used for cast concrete and cast stone for its water-repellent property.
High-alumina (aluminous) cement
High-alumina (aluminous) cement is not one of the Portland cements. It is manufactured
from bauxite and limestone or chalk in equal proportions. Bauxite is a mineral containing
a higher proportion of alumina (aluminium oxide) than the clays used in the manufacture
of Portland cements, hence the name given to this cement. The disadvantages of this cement
are that there is a serious falling off in strength in hot moist atmospheres, and it is attacked
by alkalis. This cement is little used for concrete in the UK.
Aggregates
Concrete is a mix of particles of hard material, the aggregate, bound with a paste of cement
and water with at least three-quarters of the volume of concrete being occupied by aggregate.
Aggregate for concrete should be hard, durable and contain no materials that are likely
to decompose or change in volume or affect reinforcement. Clay, coal or pyrites in aggregate
may soften, swell, decompose and cause stains in concrete. The aggregate should be clean
and free from organic impurities and coatings of dust or clay that would prevent the particles
of aggregate from being adequately coated with cement and so lower the strength of
the concrete.
Volume for volume, cement is generally more expensive than aggregate and it is advantageous,
therefore, to use as little cement as necessary to produce a dense, durable
concrete.
There is a direct relation between the density and strength of finished concrete and the
ease with which concrete can be compacted. The characteristics of the aggregate play a
considerable part in the ease with which concrete can be compacted. The measure of the
ease with which concrete can be compacted is described as the workability of the mix.