Materials for Concrete
CEMENTS
land
OST
CONCRETE is used for many purposes other than providing the strength required from the structural point of view, and its satisfactory performance for any particular use depends upon the correct selection of the type of cement and aggregate as well as on good practice on the site. There is now available a wide range of cements and a considerable choice of aggregates both natural and artifical. This Digest, published by permission of the Building Research Station in Great Britain, summarixes the main characteristics of these materials and of the different types of concrete made from them. It is slightly condensed from Building Research Station Digest No. 5 (Second Series).
M concrete is made with or- dinary or rapid-hardening Port- cement, but there are several other cements that will produce con- cretes having more specialized proper- ties. Most of these cements fall into one of the four main groups indicated! in the table below, which gives their more important properties. The follow- ing notes amplify the information given in the table.
term
Rapid-hardening cement. The 'rapid-hardening should not be con- fused with the term 'quick-setting.' The setting time of rapid-hardening_cement is similar to that of ordinary Portland cement; after setting. it develops strength more rapidly; this enables formwork to be struck earlier. This cement can be used to advantage in cold weather.
Low-heat cement. This cement is intended for use in large masses of concrete, as in dams, massive bridge abutments and retaining walls, to re- duce the temperature rise in the concrete
Main types of Portland cement
Ordinary
and the possibility of subsequent crack- ing. Its rate of strength development is lower than that of ordinary Portland cement, particularly at low tempera- tures, but its final strength is not lower It usually offers better resistance to attack by sulphate waters.
Sulphate-resisting cement. At present there is no British Standard for this
more
type of cement but it will generally comply with the physical test require- ments of B.S. 12:1958. It is resistant than ordinary Portland cement to the effect of sulphates, but it is not effective against all conditions of ex- posure to sulphates.
Extra-rapid-hardening cement. This cement is made by adding an accelerator such as calcium chloride to a rapid- hardening Portland cement. It is used
when
very high early strengths are required ог for concreting in cold weather. As well as the rate of harden- ing and the rate of heat evolution being increased, the setting process is acceler- ated; the mixing and placing operations should therefore be speeded up so that
British Standard No.
CEMENTS
Rate of strength development
Medium
Rate of heat
evolution
Medium
the concrete is placed and compacted within 30 minutes of mixing.
Waterproof and water-repellent ce- ments. Concrete made with these cements under site conditions is somewhat less permeable to water and to some oils than concrete made with ordinary Portland cement. Some types tend to entrain air during the mixing process, and this must be carefully controlled to avoid excessive air-entrainment with consequent reduction in strength. Low permeability depends here, as always, upon a good dense concrete being made.
Hydrophobic cement. Hydrophobic cement has been recently developed to prevent partial hydration of cement during storage in unavoidably humid conditions, which results in a reduction in strength and the formation of air-set lumps. Substances added during the grinding process form a water-repellent film round each grain of cement. cement can be stored under humid con- ditions for a long time without deterior- ation (it will even float on water). In the mixer, the protective film is rubbed off and hydration takes place normally.
Resistance to cracking
Resistance to sulphates
This
62
12
Medium
Rapid-hardening
12
High
Low-heat
1370
Low
High Low
Low
Low Low
High
Medium
Sulphate-resisting
Low to medium Low to medium
-
Medium
High
Other types of Portland cement
Extra-rapid-hardening
Waterproof and water-repellent
Hydrophobic
Fligh to very high High to very high Medium
Properties similar to those of ordinary Portland cement
Low
White
Coloured
Cements containing blast furnace slag
Portland blastfurnace
146
Super-sulphate
Medium Medium
Medium Low
Medium Medium
Medium
Very high
High-alumina cement
915
Very high
Very high
I ow
Very high
Pozzolanic cement
Low
Low to medium
High
High
THE HONG KONG & FAR EAST BUILDER--VOLUME 16, NUMBER 4
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