Portland cements
are made by burning together limestone and
clay, or other chemically similar suitable raw materials, in a rotary
kiln
to form a clinker rich in calcium silicates. This clinker is ground
to a fine powder with a small proportion
of gypsum (calcium sulfate), which regulates the rate of setting when the cement is
mixed with water. Over the years several types of Portland cement
have
been developed. As well as cement for general use (which
used to be known as ordinary Portland cement), there are cements for rapid hardening,
for protection against attack from freezing and thawing or by chemicals, and white cement for architectural
finishes.
All Portland cements are produced to provide special
performance and properties that are of value in appropriate
applications. They all contain the same active compounds
– only the proportion of each is different. The main compounds
in Portland cements
are given in Table 1.
Table 1: Main compounds
of Portland cements (typical
percentage composition).
|
Compound
|
Rate of hardening
|
CEM I 42,5N
|
SRPC
|
Whit
e Portlan d cemen t
|
|
|
Tricalcium silicate
Dicalcium silicate Tricalcium aluminate
Tetracalcium aluminoferrite
|
C3S C2S C3A C4AF
|
Rapid Slow Rapid
Extremely
slow
|
56
16
8
9
|
64
10
2
14
|
65
22
5
1
|
|
NOTES
1. The abbreviated chemical notation given for the above compounds is based on C = CaO S = SiO2 A = AI2O3 F = Fe2O3
2. Only main compounds are listed; therefore they do not total 100%.
|
|||||
By incorporating other materials during manufacture, an even
wider
range of cements is produced, including air-entraining
cement and combinations of Portland
cement with mineral
additions. Cements are also made, for special purposes, from
materials other than those used for Portland cements,
but the use
of
these non-Portland cements
is outside the scope of this
publication.
The setting and hardening
of cement results
from a chemical reaction between cement and water, not from a drying process. This reaction is called hydration. It produces
heat and is irreversible.
Setting is the gradual
stiffening whereby the cement paste changes from a workable
to a
hardened state. Subsequently
the strength of the hardened
mass increases, rapidly at first but becoming progressively less rapid. This gain of strength will
continue indefinitely provided moisture is present.
All British cement manufacturers declare that their products conform to the appropriate British and European
Standard by
marking their cement test reports and either the bags or delivery notes with the name, number
and date of the relevant
Standard. In addition,
cement is currently manufactured and supplied
to the
nationally-recognized third-party product certification scheme - the BSI Kite mark
Scheme for Cement. In the course of replacing
British Standards for cement by harmonized European standards,
these principles of declaration and certification will be verified by
affixing the European CE marking.
Portland cement CEM I
The cement most commonly used was formerly
known as OPC in British Standards and, more recently, PC to BS 12. It
is now known
as Portland cement CEM I manufactured to conform to
BS EN 197-1 .
CEM I 42,5 and CEM I 52,5 strength
class products account for all
UK
CEM I cement production and their main active chemical compounds are proportioned so that they have medium to high
strength development and heat evolution. CEM I in bags is generally a 42,5N cement whereas CEM I for bulk supply tends to have a higher
strength classification such as 42,5R or 52,5N.
CEM I can include up to 5% of minor additional constituents such as limestone
fines.
Within a 30-year period from around 1960 to 1990, the 28-day
strength of this cement can be seen to have shifted from just below
the bottom of what is now termed strength class 42,5N
towards strength class 52,5N. This
was
a consequence of continued improvements in the production process and quality
control. In addition, early strength and the heat of hydration also increased as a result of the higher reactivity of the product.
Since around 1990, however,
the introduction of the strength classification system into British Standards has led to stability
in the strength of Portland cements. This stable situation
should continue into the long term, providing
users with a more predictable product regardless of their location
in the UK.
Cements are now classified in terms of both their standard
strength, derived from their performance at 28 days and at an early age, normally 2 days, using a specific laboratory test based on
a standard mortar prism. This is termed their strength
class; for example CEM I 42,5N where 42,5 denotes the standard strength
and N
indicates a normal early strength.
It is important to recognize that cement strength classes
do not limit the strength of concrete
that may be produced using these
cements: they simply represent a cement classification system
based on prisms of mortar tested in a laboratory.
The most common standard
strength classes for manufactured
cements are 42,5 and 52,5 with 32,5 used less often. These can
take
either N (normal) or R (rapid) identifiers depending on the early strength characteristics of the product. Another standard
strength class (22,5) also exists but this tends to be associated with particular special cements.
If cement clinker is ground more finely, the greater surface area of
the finer cement produces
a faster rate of early strength development. This is often used to advantage by precast concrete manufacturers in order to achieve a more rapid turn round of moulds, or on site where it may be desirable
to reduce the time for which formwork must remain in position.
Cements that have these
rapid-hardening properties, formerly known as rapid-hardening
Portland cement (RHPC), are now produced
in the UK within the 52,5 strength classes of CEM I. They generate more early heat than CEM I 42,5 and can often be useful in cold weather.
CEM I 52,5 can also be used as an alternative to CEM I 42.5 when high strength may be an advantage, particularly at early ages.
The term 'rapid-hardening' should not be confused
with 'quick- setting'.
Concrete made, for example, with CEM I 52,5N stiffens and initially hardens
at a
similar rate to
a CEM I 42,5N; it is after the initial hardening that the strength increases more rapidly.
A low early strength
identifier (L) also exists in British
Standards, but is reserved for blastfurnace cements
with
strength properties
outside the scope of BS EN 197-1 , and is not applied to CEM 1.
It is worth noting that the setting times specified
in standards relate to the performance of a cement paste of standard
consistence in a particular test under closely controlled
temperature and humidity conditions;
the stiffening and setting of
concrete on site are not directly related to these standard
setting regimes,
and are more dependent on workability, the cement content, any admixture
used, the temperature of the concrete
and ambient conditions.
Sulfate-resisting Portland
cement SRPC
Sulfate-resisting Portland cement (SRPC) is a form of Portland cement with a low tricalcium aluminates (C3A) content.
BS 4027, the British Standard
for SRPC, limits the C3A content to 3.5%. This limitation is achieved by adding iron oxide, thereby decreasing the proportion of alumina in the raw feed material;
this favors the
formation of calcium aluminoferrite (C4AF) over C3A in the cement
kiln.
This higher iron content tends to give SRPC a darker colour.
When concrete made with CEM I cement is exposed to sulfate
solutions that are found in some soils and groundwaters, a reaction may occur between
the sulfate and the hydrates
from the C3A in
the
cement, causing deterioration of the concrete. By limiting the
C3A content in SRPC, a cement with superior resistance to
conventional sulfate attack is produced.
However, resistance to
sulfate attack depends on the cement content and impermeability of the concrete as well as on the composition
of the cement.
Details of requirements can be found
in
BS 5328, BS EN 206-1 , BS
8500
and BRE Special Digest 1.
SRPC is normally
a low-alkali cement, but otherwise is similar
to other Portland cements in that it is not resistant to strong acids. Further details about durability and sulfate
resistance are given on page 21 under
Durability of concrete. The strength
properties of
SRPC are similar to those of CEM I 42,5N and it should be stored
and used in the same way. SRPC normally produces
slightly less early heat than CEM I 42,5N. This may be an advantage in massive
concrete and in thick sections.
It is not normal practice
to combine SRPC with pulverized-fuel ash
or
ground granulated blast furnace slag. See Cements and mixer combinations incorporating mineral constituents or additions (below) for further information on these additional
cementations materials.