White Portland cement is made from specially
selected raw
materials, usually pure chalk and white clay, containing only a very
small quantity of iron. In addition,
manufacturing processes are
modified so that discoloring materials are not included during
firing or grinding.
White Portland cement generally available in the UK is a 52,5 strength class product, which means it has a higher early strength
and higher standard 28-day strength than a CEM I 42,5N but with
similar setting properties.
It is made to satisfy the requirements of CEM I to BS EN 197-1 , so there is no separate British Standard. It is used for concrete where a white or light color finish is desired, often in conjunction with special aggregates. Extra care must be taken in handling white cement to avoid contamination, and in the batching, mixing and transportation of the concrete to ensure that all equipment is kept clean It is equally important to make sure that the finished concrete is protected, because it gets dirty very easily in the early stages of its life and is almost impossible to clean later Careful selection
is required of the type of release agent and, if used, the sprayed-on curing membrane The use of damp hessian
is
not recommended as it may stain the concrete
Cements and mixer combinations incorporating mineral constituents or additions CEM II and CII, CEM III and CIII, CEM IV and CIV
These are cements that are either interground or blended with
mineral materials at the cement factory or combined in the concrete mixer with additions The mineral materials and additions
most frequently used in the UK and to which British Standards apply are pulvenzed-fuel ash (pfa) to BS 3892, fly ash to BS EN 450, ground granulated blastfurnace slag (ggbs) to BS 6699
and
limestone fines to BS 7979
Other additions
include condensed silica fume, extracted during the smelting
process of ferrosilicon alloy, and metakaolin, produced from China clay (kaolin) These are intended for specialised uses of
concrete beyond the scope of this publication
The two methods of incorporating the mineral additions make little
or no difference to the properties of concrete
and, until recently, it was considered unnecessary
to
distinguish between them In 2000 a new notation system for cements was introduced
withnBS EN 197-1 and for mixer combinations with BS 8500 in 2002, giving a unique code identifying
both composition and method of production It is convenient to be able to identify cements by their notation and to consider
them
separately either as manufactured cement or mixer combinations
It should be emphasised, however, that the controlled ways by which mineral additions
have to be introduced
ensure that the quality of concrete
is unaffected by differences in
their production methods
Technical benefits
The incorporation of pfa, fly ash or ggbs with CEM I has been
particularly useful in massive
sections of concrete where they have been used primarily to reduce the temperature rise of the concrete, and thus to reduce temperature differentials and peak temperatures The risk of early thermal contraction cracking is
thereby also reduced
One of the options available for minimizing the risk of damage due
to alkali-silica reaction,
which can occur
with
certain aggregates,
and for increasing the resistance of concrete to sulfate attack, is to
use
additions with Portland cement or CEM II or CEM III cements
Most additions do not react very quickly at early ages at normal
temperature, and at reduced temperature the reaction
- particularly in the case of ggbs - can be considerably retarded and make little contribution to the early strength of concrete
Provided that the concrete is not allowed to dry out they can increase
the long-term strength and impermeability of concrete
|
Property
|
Pfa
|
Ggbs
|
Comment
|
|
Workability/
consistence
|
Increased for same w/c ratio.
Possible to keep same
consistence but reduce w/c ratio
|
Small differences
|
|
|
Setting times
|
Increased
|
Increased.
Substantially increased with
high ggbs content
|
Still within BS limits
|
|
Formwork pressures
|
Increased by about 10-2 0 kN/m2
|
May be increased
|
See Concrete Society Report CS030
and CIRIA Report
108
|
|
Bleeding
|
Generally reduced (some
exceptions)
|
Small differences
|
|
|
Quality of finish
|
Improved quality
with lean mixes.
Not much difference
with rich mixes.
Darker colour.
|
Much the same. May give temporary blue/green colour.
|
|
|
Time interval to
finishing
|
Increased
|
Substantial increase if ggbs
content is high and concrete temperature is low
|
Increased time until finishing can be a disadvantage in cold conditions and an advantage in hot weather
|
|
Plastic settlement
cracking
|
Generally reduced where bleeding is reduced
|
Greater risk, which increases as the ggbs content
increases
|
Re-vibration at
the
correct time will remove plastic settlement cracking. An alternative is to reduce bleeding
|
|
Plastic shrinkage
|
Increased
|
May be increased
|
Prompt curing will prevent
plastic shrinkage cracking
|
|
Early age strength a) Equal binder content
b) Equal 28-day strength
|
Reduced
Small reduction (about 10%)
|
Substantially reduced. Lower strengths with increasing % ggbs
Significantly reduced e.g. after 3 days at 20°C, a 40% ggbs mix will have about half the strength of
CEM I
|
Particular problems with ggbs based cements in thin sections
in cold weather
|
|
Formwork striking
times
a) Equal binder
content
b) Equal 28-day
strength
|
Increased
Small increase in thin sections;
much the same in large sections
|
Increased
Increased in thin and medium
sections
|
See CIRIA Report 1 36
Other methods such as pull-out
testing or temperature-matched
curing can be
used
|
|
Early-age thermal cracking
|
Risk reduced in sections between
500 mm and 2.5 m thick.
|
Using aggregate with low coefficients of thermal expansion is more effective
|
|
|
Curing
|
Increased sensitivity to poor curing but larger potential for
recovery. Longer curing
periods needed
|
Increased sensitivity to poor
curing. Longer curing
periods needed
|
Views differ on this subject.
See BS 8110 for curing periods
|
|
Air-entrainment
|
Considerable increase in admixture
dosage likely to be required
|
Small differences
|
Special admixture
may be required
where pfa is used
|