White cement. To make а white cement the amount of iron oxide in Portland cement must be considerably reduced, by selection of the raw materi- als and by special manufacturing pro- White Portland cements should comply in all respects with the require- 12:1958 although they ments of B.S. are usually not quite so strong cidinary Portland cement.
cesses.
as
Coloured cements. These are basically white or ordinary Portland cements to which between 5 and 10% of pigment has been added, usually at the grinding stage. The pigment, being inert, dilutes the cement, and the strength of the concrete will be slightly reduced; it necessary, this can be compensated for by adding about 10% more cement to the concrete mix.
Portland blastfurnace cement. This cement is made by grinding a mixture of granulated blastfurnace slag and ordinary Portland cement clinker. Is is very similar to ordinary Portland cement but its strength development is somewhat slower; it therefore requires longer curing periods, particularly in cold weather. Portland blastfurnace cement is useful when concreting large masses in warm weather and it is also rather more resistant to chemical attack by sulphates or by sea water.
Super-sulphate cement. This cement differs from those discussed so far, in that Portland cement is not one of its major constituents. Its Tesistance to chemical attack by sulphate waters and weak acids is greater than that of sul- phate-resisting cement. Its strength is lower at very early stages but after 3 days it develops strength more rapidly
than
ordinary Portland cement. Its heat of hydration is lower and its strength development is more retarded by cold weather than is that of ordinary Portland cement. It deteriorates readily if stored under damp conditions. liete based on super-sulphate cement must be carefully cured by keeping its surface moist to prevent the formation of a powdery surface skin.
Con-
A
High-alumina cement. This cement is manufactured in a different way from Portland cements and has different characteristics. Its main advantages lie in the speed with which concrete can be brought into use, and in its resistance to chemical attack. Concrete made with high-alumina cement develops very high early strength which is particularly advantageous for working in cold weather. Heat evolution is very rapid, and the temperature rise may need to be checked by cooling; if, at any time, the temperature of the con- crete exceeds about 85° F under moist conditions, the concrete will lose substantial proportion of its strength. Although there is this loss of strength at higher temperatures, high-alumina cement when mixed with a suitable aggregate, such as crushed firebrick, makes a refractory concrete which will
a
withstand temperatures up to 1300° C, suitable for furnace linings Or kiln floors; with special aggregates, even higher temperatures can be withstood.
Pozzolanic cements. These cements mixtures of a Portland cement and a natural or artificial pozzolanic material, i.e. a material that combines with lime (either present as such, or set free by the setting of cement in the presence of moisture) to form a hard mass. Hitherto, they have been little used in Great Bri- tain, although they have had consider- able use on the Continent and in the U.S.A. Their main advantage lies in their increased resistance to chemical attack, but the rate of strength develop- ment is reduced. They have a lower rate of heat evolution and can be used to advantage in mass concrete.
The use of different cements
From the above comments it can be seen that if a cement having a special property is required, then other pro- perties of this cement may restrict its use. Heat of hydration, for example, is linked with the rate of strength develop- ment of Portland type cements; if a high early strength is obtained the user must be aware that heat will be evolved quickly, and this may not be desirable in some circumstances.
When an un- familiar type of cement is used for the first time, the user should be aware of how it differs from ordinary Portland cement, and of any special precautions that should be taken in its use.
Mixing different types of cement is not recommended. Although different varieties of Portland cements can be mixed together there is little advantage in doing so; the properties of the mixed cement would be somewhat uncertain, and in any case the range of Portland cements is large enough to cover most requirements. Neither high-alumina cement nor supersulphate cement must be mixed with any other type of cement, and all mixers, plant and tools must be thoroughly cleaned before and after using these cements to avoid inter. action with other types.
AGGREGATES
Aggregate makes up the major part of the bulk of any concrete, and its properties are important. Aggregates in general should be stable in air and in water, and free from organic impurities or other constituents capable of reacting harmfully with the cement. The me- chanical properties of the aggregate
must be such as to endow the concrete with the strength required. Also, the changes in volume of the aggregate on wetting and drying should
be excessive; such changes increase the risk of cracking in the concrete.
not
Concrete is normally used to obtain one or more of three requirements: strength, fire protection, thermal insula- tion. Both cement and aggregate are
THE HONG KONG & FAR EAST BUILDER-VOLUME 16, NUMBER 4
important in relation to strength and durability, but fire protection and thermal insulation depend almost clusively on the type of aggregate used.
ex-
It is convenient to classify aggregates as lightweight or dense, according to whether their bulk density is less than or greater than 70 lb/cu.ft; this figure is quite arbitrary, and has no significance.
Lightweight aggregates
exact
The following are the types of light- weight aggregate most commonly used:
Clinker (B.S. 1165)
Foamed slag (B.S. 877)
Expanded clay, shale or slate Sintered pulverized-fuel ash Exfoliated vermiculite Expanded perlite
Only the first two of these aggregates are covered by a British Standard, but there is ample experience to indicate the value of the others in appropriate circumstances.
A recent development is the use of lightweight concrete as а structural load-carrying material. For this pur- pose richer mixes than those usually
associated with lightweight concrete are used, and the mix proportions are selected so as to obtain a workable mix that can be fully compacted into a dense
concrete.
Dense aggregates
Most aggregates fall into this category; the main functional requirement of concrete made with them is to provide strength and durability. Con- crete made with these aggregate may also be required to provide fire protec- tion and the amount of protection given will depend on the aggregate used, as discussed later.
Air-cooled blastfurnace slag. This is a by-product from the manufacture of pig iron. Near blastfurnaces,
it may be the cheapest aggregate available; it also has good fire-protection properties. Concrete made with slag satisfying the requirements of B.S. 1047 will have pro- perties similar to those of a concrete made with a natural aggregate comply- ing with B.S. 882.
Crushed brick. This aggregate is used for its better fire-protection properties and also to obtain a variety of colours in surface-finishing slabs by exposing the aggregate. It should consist of clean brushed bricks and should not be crush- ed brick rubble containing mortar and plaster. Bricks usually contain soluble sulphates; if they are used as a con- crete aggregate.
the sulphate shoull not exceed 1 per cent expressed as sulphuric anhydride (SO3).
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* Its use for radiation shielding, nuclear reactors and X-ray installations, is not discussed here
in
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