Cements of the Portland type differ in kind from those of the
pozzuolanic class; they are not mechanical mixtures of lime and active
silica ready to unite under suitable conditions, but consist of definite
chemical compounds of lime and silica and lime and alumina, which, when
mixed with water, combine therewith, forming crystalline substances of
great mechanical strength, and capable of adhering firmly to clean inert
material, such as stone and sand. They are made by heating to a high
temperature an intimate mixture of a calcareous substance and an
argillaceous substance. The commonest of such substances in England are
chalk and clay, but where local conditions demand it, limestone, marl,
shale, slag or any similar material may be used, provided that the
correct proportions of lime, silica and alumina are maintained. The
earliest forms of cements of the Portland class were the hydraulic
limes. These are still largely used, and are prepared by burning
limestones containing clayey matter. Some of these naturally possess a
composition differing but little from that of the mixture of raw
materials artificially prepared for the manufacture of Portland cement
itself. Although hydraulic limes have been in use from the most ancient
times, their true nature and the reason of their resistance to water
have only become known since 1791. Next in antiquity to hydraulic lime
is Roman cement, prepared by heating an indurated marl occurring
naturally in nodules. Its name must not be taken to imply that it was
used by the ancients; in point of fact the manufacture of this substance
dates back only to 1796.
With the growth of engineering in the early part of the 19th century
arose a great demand for hydraulic cement. The supply of materials
containing naturally suitable proportions of calcium carbonate and clay
being limited, attempts were made to produce artificial mixtures which
would serve a similar end. Among those who experimented in this
direction was Joseph Aspdin, of Leeds, who added clay to finely ground
limestone, calcined the mixture, and ground the product, which he called
Portland cement. The only connexion between Portland cement and the
place Portland is that the cement when set somewhat resembles Portland
stone in colour. True, it is possible to manufacture Portland cement
from Portland stone (after adding a suitable quantity of clay), but this
is merely because Portland stone is substantially carbonate of lime; any
other limestone would serve equally well. Although Portland cement is
later in date than either Roman cement or hydraulic lime, yet on account
of its greater industrial importance, and of the fact that, being an
artificial product, it is of approximately uniform composition and
properties, it may conveniently be treated of first. The greater part of
the Portland cement made in England is manufactured on the Thames and
Medway. The materials are chalk and Medway mud; in a few works the
latter is replaced by gault.
The composition of typical samples of chalk and clay is shown in the
following analyses:--
+-----------------------------------+----------------------------------------------------------------------------+
| Chalk. | Clay. |
+-------------------------+---------+----------------------------+---------+------------------+-------+----------+
| |Per cent.| |Per cent.| | | |
|Silica (SiO2) | 0.92 | Insoluble silicious matter | 26.67 |Consisting of | | |
|Alumina+ferric oxide | | Silica (SiO2) | 31.24 | Quartz (SiO2) | 19.33 | |
| (Al2O2 + Fe2O3) | 0.24 | Alumina (Al2O3) | 16.60 | Silica (SiO2) | 5.19 |\ |
|Lime (CaO) | 55.00 | Ferric Oxide (Fe2O3) | 8.66 | Alumina (Al2O3) | 1.47 | >Feldspar|
|Magnesia (MgO) | 0.36 | Lime (CaO) | 0.25 | Magnesia (MgO) | 0.03 | | 7.34% |
|Carbonic anhydride (CO2) | 43.40 | Magnesia (MgO) | 1.91 | Soda (Na20) | 0.65 |/ |
| | ----- | Soda (Na2O) | 1.00 | | ----- | |
| | 99.92 | Potash (K2O) | 0.45 | | 26.67 | |
| | | Sodium Chloride (NaCl) | 1.86 | | | |
| | | Combined water, organic | | | | |
| | | matter, and loss | 11.36 | | | |
| | | | ------ | | | |
| | | | 100.00 | | | |
+-------------------------+---------+----------------------------+---------+------------------+-------+----------+
Mixing.
Loading the kiln.
[Illustration: FIG. 1]
These materials are mixed in the proportion of about 3:1 by weight so
that the dried mixture contains approximately 75% of calcium
carbonate, the balance being clay. The mixing may be effected in
several ways. The method once exclusively used consists in mixing the
raw materials with a large quantity of water in a wash mill, a machine
having radial horizontal arms driven from a central vertical spindle
and carrying harrows which stir up and intermix any soft material
placed in the pit in which the apparatus revolves. The raw materials
in the correct proportion are fed into this mill together with a large
quantity of water. The thin watery "slip" or slurry flows into large
settling tanks ("backs") where the solids in suspension are deposited;
the water is drawn off, leaving behind an intimate mixture of chalk
and clay in the form of a wet paste. This is dug out, and after being
dried on floors heated by flues is ready for burning. This process is
now almost obsolete. According to present practice the raw materials
are mixed in a wash mill with so much water that the resulting slurry
contains 40 to 50% of water. The slurry, which is wet enough to flow,
is ground between millstones so as to complete the process of
comminution begun in the wash mill. Thorough grinding and mixing are
of the utmost importance, as otherwise the cement ultimately produced
will be unsound and of inferior quality. The drying of the slurry is
generally effected by the waste heat of the kilns, so that while one
charge is burning another is drying ready for the next loading of the
kilns. The kilns commonly employed are "chamber kilns," circular
structures not unlike an ordinary running lime kiln, but having the
top closed and connected at the side with a wide flue in which the
slurry is exposed to the hot products of combustion from the kiln. The
farther ends of the flues of several such kilns are connected with a
chimney shaft. The slurry, in drying on the floor of the flue, forms a
fairly tough cake which cracks spontaneously in the process of drying
into rough blocks suitable for loading into the kiln. At the bottom of
the kiln is a grate of iron bars, and on this wood and coke are piled
to start the fire. A layer of dried slurry is loaded on this, then a
layer of coke, then a layer of slurry, and so on until the kiln is
filled with coke and slurry evenly distributed. Fresh slurry is run on
to the drying floors, and the kiln is started. The construction of an
ordinary chamber kiln may be gathered from the accompanying diagram
(fig. l). The operation of burning is a slow one. An ordinary kiln,
which will contain about 50 tons of slurry and 12 tons of coke, will
take two days to get fairly alight, and will be another two or three
days in burning out. Therefore, allowing adequate time for loading and
unloading, each kiln will require about one week for a complete run.
The output will be about 30 tons of "clinker" ready to be ground into
cement. The grinding of the hard rock-like masses of clinker is
effected between millstones, or in modern plants in ball-mills,
tube-mills and edge-runners. It is an important part of the
manufacture, because the finished cement should be as fine and
"floury" as possible. The foregoing description represents the
procedure in use in many English factories. There are various
modifications in practice according to local conditions: a few of
these may be described. In all cases, however, the main operations are
the same, viz. intimately mixing the raw materials, drying the
mixture, if necessary, and burning it at a clinkering temperature
(about 1500° C. =2732° F.). Thus when hard limestone is the form of
calcium carbonate locally available, it is ground dry and mixed with
the correct proportion of clay also dried and ground. The mixture is
slightly damped, moulded into rough bricks, dried and burned. A
possible alternative is to burn the limestone first and mix the
resulting lime with clay, the mixture being burned as before. By this
method grinding the hard limestone is avoided, but there is an extra
expenditure of fuel in the double burning.
Other kilns.
Many different forms of kiln are used for burning Portland cement.
Besides the chamber kilns which have been described, there are the
old-fashioned bottle kilns, which are similar to the chamber kilns,
but are bottle-shaped and open at the top; they do not dry the slurry
for their next charge. Their use is becoming obsolete. There are also
stage kilns of the Dietzsch type, which consist of two vertical
shafts, one above the other, but not in the same vertical line,
connected by a horizontal channel. At this middle portion and in the
upper part of the lower shaft the burning proper proceeds; the upper
shaft is full of unburnt raw material which is heated by the hot gases
coming from the burning zone, and the lower shaft contains clinker
already burned and hot enough to heat the incoming air which supplies
that necessary for combustion at the clinkering zone. A pair of
Dietzsch kilns, built back to back, are shown in fig. 2. There are
other forms of shaft kiln, such as the Schneider, in which there is a
burning zone, a heating and cooling zone as in the Dietzsch, but no
horizontal stage, the whole shaft being in the same vertical plane.
Another form is the Hoffmann or ring kiln, made up of a number of
compartments arranged in a ring and connected with a central chimney;
in these compartments rough brick-shaped masses of the raw materials
are stacked, and between these bricks fuel is sprinkled. At a given
moment one of these compartments is burning and at its full
temperature; the air for combustion is drawn in through one or more
compartments behind it which have just finished burning, and is
thereby strongly heated; the products of combustion pass away through
one or more compartments in front of it and heat their contents before
they are subjected to actual combustion. It will be seen that the
principle of the ring kiln is similar to that of the stage kiln. In
each case the clinker which has just been burned and is fully hot
serves to heat the air-supply to the compartment where combustion is
actually proceeding; in like manner the raw materials about to be
burned are well heated by the waste gases from the compartment in full
activity before they themselves are burned. (It may be noted that here
and generally in this article "burn" is used in the technical sense;
it is technically correct to speak of cement clinker being "burned",
although it is not a fuel; in accurate terms it is the fuel which is
burned, and it is the heat it generates which raises the clinker to a
high temperature, i.e. technically "burns" it.) By this device a great
part of the heat is regenerated and a saving of fuel is effected.
[Illustration: FIG. 2.]
Rotatory kilns.
The methods of burning cement described above are obsolescent. They
are being replaced by the rotatory process, so called because the
cement is burned in rotating cylinders instead of in fixed kilns.
These cylinders vary from 60 to 150 ft. in length, an ordinary length
in modern practice being 100 to 120 ft.; their diameter
correspondingly varies from 6 ft. to 7 ft. 6 in. The cylinders are
made of steel plate, lined with refractory bricks, are carried on
rollers at a slight angle with the horizontal, and are rotated by
power. At the upper end the raw material is fed in either as a dry
powder or as a slurry; at the lower end is a powerful burner. In the
early days of rotatory kilns producer gas was used as a fuel, but with
little success; about 1895 petroleum was used in the United States
with complete success, but at a relatively heavy cost. At the present
time, finely powdered coal injected by a blast of air is almost
universally employed, petroleum being used only where it is actually
cheaper than coal. In the working of this type of kiln the rotation
and slight inclination of the cylinder cause the raw material to
descend towards the lower end. At the upper end the raw material is
dried and heated moderately. As it descends it reaches a part of the
kiln where the temperature is higher; here the carbonic acid of the
carbonate of lime, and the combined water of the clay are driven off,
and the resulting lime begins to act chemically on the dehydrated
clay. The material is then in a partially burnt and slightly sintered
state, but it is not fully clinkered and would not make Portland
cement. The material continues to descend by the rotation of the kiln
and reaches the lower end nearest the burner where the temperature is
highest, and is there heated so highly that the union of the lime,
silica and alumina is complete, and fully burnt clinker falls out of
the kiln. It is extremely hot, and is cooled usually by being passed
down one or more rotating cylinders, similar to the first, but
smaller, and acting as coolers instead of kilns. On its way down the
cylinders the clinker meets a current of cold air and is cooled, the
air being correspondingly warmed and passing on to aid in the
combustion of the fuel used in heating the kiln. This regenerative
heating is similar in principle and effect to that obtained by means
of the shaft and ring kilns described above. The output of these kilns
varies from 200 to 400 tons per kiln per week according to their size
and the nature of the raw materials burned, as against 30 tons per
week for an ordinary chamber kiln. A large saving in labour is also
secured. The rotatory system presents many advantages and is rapidly
replacing the older methods of cement making. Fig. 3 represents
diagrammatically a rotatory cement plant on the Hurry & Seaman system,
which was one of the first to make cement by the rotatory process
successfully on a large scale, using powdered coal as fuel. Rotatory
kilns of various other makes are now in use, but the same principles
are embodied, namely, the employment of a rotating inclined cylinder
for burning the raw materials, a burner fed with powdered coal and a
blast of air, and some device such as a cooling cylinder or cooling
tower by which the clinker may be cooled and the air correspondingly
heated on its way to the burner.
[Illustration: FIG. 3.]
Another method of making Portland cement which has been proposed and
tried with some success consists in fusing the raw materials together
in an apparatus of the type of a blast furnace. The high temperature
necessary to fuse cement clinker makes this process difficult to
accomplish commercially, but it has many inherent merits and may be
the process of the future, displacing the rotatory method.
Cement clinker.
Portland cement clinker, however produced, is a hard, rock-like
substance of semi-vitrified appearance and very dark colour. The
product from a well-run rotatory kiln is all evenly burnt and properly
vitrified; that from an ordinary fixed kiln of whatever type is apt to
contain a certain amount (5 to 15%) of underburnt material, which is
yellowish and friable and is not properly clinkered. This material
must be picked out, as such underburnt stuff contains free lime or
unsaturated lime compounds. These may slake slowly in the finished
cement and cause such expansion as may destroy the work of which it
forms part. Well-burnt, well-picked clinker when ground yields good
Portland cement. Nothing is added during or after grinding save a
small amount (1 to 2%) of calcium sulphate in the form either of
gypsum or of plaster of Paris, which is sometimes needed to make the
cement slower-setting. For the same purpose a small quantity of water
(up to 2%) may be added either by moistening the clinker or by blowing
steam into the mills in which the clinker is ground. This small
addition for this specified purpose is recognized as legitimate, but
the employment of various cheap materials such as ragstone and
blast-furnace slag, sometimes added as diluents or make-weights, is
adulteration and therefore fraudulent.
Composition.
The composition of Portland cement varies within comparatively narrow
limits, and for given raw materials the variations are tending to
become smaller as regularity and skill in manufacture increase. The
following analysis may be taken as typical of cements made from chalk
and clay on the Thames and Medway:--
Per cent.
Silica (SiO2) 22.0
Insoluble residue 1.0
Alumina (Al2O3) 7.5
Ferric oxide (Fe2O3) 3.5
Lime (CaO) 62.0
Magnesia (MgO) 1.0
Sulphuric anhydride (SO3) 1.5
Carbonic anhydride (CO2) 0.5
Water (H2O) 0.5
Alkalis 0.5
-----
100.0
There may be variations from this composition according to the nature
of the raw materials employed. Thus the silica may range from 19 to
27%, the alumina and ferric oxide jointly from 7 to 14%, the lime from
60 to 67%. All such variations are permissible provided that the
quantity of silica and alumina is sufficient to saturate the whole of
the lime and to leave none of it in a "free" condition, likely to
cause the cement to expand after setting. Other things being equal,
the higher the percentage of lime within the limits indicated above
the stronger is the cement, but such highly limed cement is less easy
to burn than cement containing about 62% of lime; and unless the
burning is thorough and the raw materials are intimately mixed, the
cement is apt to be unsound. Although the ultimate composition of
cement, that is, the percentage of each base and acid present, can be
accurately determined by analysis, its proximate composition, i.e. the
nature and amount of the compounds formed from these acids and bases,
can only be ascertained indirectly and with difficulty. The
foundations of our knowledge on this subject were laid by H. le
Chatelier, whose work has since been supplemented by that of Spenser
B. Newberry, W.B. Newberry and Clifford Richardson. As the outcome of
these inquiries it has been established that tricalcium silicate
3CaO·SiO2 is the essential constituent of Portland cement. The
constituent of next importance is an aluminate, but whether this is
dicalcium aluminate, 2CaO·Al2O3, or tricalcium aluminate, 3CaO·Al2O3,
is still in doubt. In the following description it is assumed to be
the tricalcium aluminate. The remaining silicates and aluminates
present, and ferric oxide and magnesia, if existing in the moderate
quantities which are usual in Portland cement of good quality, are of
minor importance and may be regarded as little more than impurities.
The silicates and aluminates of which Portland cement is composed are
believed to exist not as individual units but as solid solutions of
each other, these solid solutions taking the form of minerals
recognizable as individuals. The two principal minerals are termed
alite and celite; according to the best opinion, alite consists of a
solid solution of tricalcium aluminate in tricalcium silicate, and
celite of a solid solution of dicalcium aluminate in dicalcium
silicate. Celite is little affected by water, and has but small
influence on the setting; alite is decomposed and hydrated, this
action constituting the main part of the setting of Portland cement.
Both the components of alite react, and for simplicity their reactions
may be stated in separate equations, thus:--
(1) 2(3CaO·SiO2) + 9H2O = 2(CaO·SiO2)·5H2O + 4Ca(OH)2
Tricalcium silicate. Hydrated mono- Calcium
calcium silicate. hydroxide.
(2) 3CaO·Al2O3 + 12H2O = 3CaO·Al2O3·12H2O
Tricalcium aluminate. Hydrated tricalcium
aluminate.
Since alite is a solid solution and, although an individual mineral,
is not a chemical unit, the proportion of tricalcium silicate to
tricalcium aluminate in a given specimen of alite will vary; but,
whatever the proportions, each of these substances will react in its
characteristic manner according to the equations given above.
The precise mechanism of the process of setting of Portland cement is
not known with certainty, but it is probably analogous to that of the
setting of plaster of Paris, consisting in the dissolution of the
compounds produced by hydration while they are in a more soluble form,
their transition to a less soluble form, the consequent
supersaturation of the solution, and the deposition of the surplus of
the dissolved substance in crystals which interlock and form a
coherent mass. This theory being accepted, it is evident that a small
quantity of water, by successive dissolution and deposition of a
substance capable of existing in a more soluble and in a less soluble
form, is able to bring about the crystallization of an indefinitely
large quantity of material. It is not necessary that there should be
present sufficient water to dissolve the whole of the reacting
substance at any one time; it is sufficient if there is enough for
hydration and a small surplus for the crystallization by successive
stages as above described. It is generally admitted that the aluminate
is the chief agent in the first setting of the cement, and that its
ultimate hardening and attainment of strength are due to the
tricalcium silicate.
As mentioned above, the constituents other than the tricalcium
silicate and tricalcium aluminate of which alite is composed, are of
minor importance. The function of the ferric oxide present in ordinary
cement is little more than that of a flux to aid the union of silica,
alumina and lime in the clinker; its role in the setting of the cement
is altogether secondary. In fact, excellent Portland cement can be
prepared from materials free from iron. Such cement, if free also from
manganese, is white, and its manufacture has been proposed for
exterior decorative use. Magnesia, if present in Portland cement in
quantity not exceeding 5%, appears to be inert, but there is evidence
that in larger proportion, e.g. 10-15%, it may hydrate and set after
the general setting of the cement, and may give rise to disruptive
strains causing the cement to "blow" and fail. In so-called natural
cement which is comparatively lightly burnt, the magnesia appears to
be inert, and as much as 20 to 30% may be present. Another constituent
of Portland cement which influences its setting time is calcium
sulphate, naturally formed from the sulphur in the raw materials or
fuel, or intentionally added to the finished cement as gypsum or
plaster of Paris. It has a remarkable retarding effect on the
hydration of the calcium aluminate, and consequently on the setting of
the cement; thus it is that a little gypsum is often added to convert
a naturally quick-setting cement into one which sets slowly. It will
be observed that in the hydration of tricalcium silicate, the main
constituent of Portland cement, a large portion of the lime appears as
calcium hydroxide, i.e. slaked lime. It is evident that this will form
a pozzuolanic cement if a suitable silicious material such as trass is
added to the cement. The ultimate product when set may be regarded as
a mixed Portland and pozzuolanic cement. The use of trass in this
manner as an adjunct to Portland cement has been advocated by W.
Michaelis, and undoubtedly increases the strength of the material, but
it has not become general.
Testing.
The quality of Portland cement is ascertained by its analysis and by
determining its specific gravity, fineness, mechanical strength and
soundness. A good sample will usually have a composition within the
limits cited above and approximating to the typical figures given
above. It will be ground so finely that not more than 3% will be left
on a sieve of 76 × 76 meshes per sq. in., the wires of the sieve being
0.005 in. in diameter. It will have, when freshly burned, a specific
gravity not lower than 3.15, and briquettes made from it and kept in
water will possess a tensile strength of 400-500 lb. per sq. in. seven
days after they are made, while briquettes made from a mixture of 3
parts by weight of sand and 1 of cement will give about 225 lb. per sq.
in. at twenty-eight days. Formerly the soundness of cement was
determined by keeping thin pats of the cement in cold water for
twenty-eight days, or in warm water (110°-120° F.) for twenty-four
hours, and examining for cracks or other signs of expansion. Modern
practice is to measure the expansion of a test piece of cement kept in
water at a temperature of 212° F. The simplest and most generally used
method is due to H.L. le Châtelier, and consists in measuring the
increase in circumference of a cylinder of cement 30 mm. in diameter
by means of a split ring encircling the cylinder, the motion of which
is magnified by two light rods extending radially. Another
quantitative test for soundness is that formulated by L. Deval, who
has shown that briquettes of 3 of sand and 1 of cement kept in water
for two days at 80° C. = 176° F. attain approximately the same
strength as similar briquettes attain at seven days in water at the
ordinary temperature. In like manner briquettes kept at 176° F. for
seven days are approximately equal in strength to those kept at the
ordinary temperature for twenty-eight days. A cement not perfectly
sound will give low results in the hot test, and a cement of
indifferent soundness will crack and go to pieces. The test is
admittedly severe, but can be passed without difficulty by cement made
with proper care and skill. There are many modifications and
elaborations of all the tests which have been mentioned. Cement for
all important work is submitted to a rigorous system of testing and
analysis before it is accepted and used.
_Hydraulic Lime_ is a cement of the Portland as distinct from the
pozzuolanic class. The most typical hydraulic lime is that known as
Chaux du Theil, made from a limestone found at Ardèche in France. This
limestone consists of calcium carbonate most intimately intermixed with
very finely divided silica. It contains but little alumina and oxide of
iron, which are the constituents generally necessary to bring about the
union of silica and lime to form a cement, but in spite of this the
silica is so finely divided and so well distributed that it unites
readily with the lime when the limestone is burned at a sufficiently
high temperature. English hydraulic limes are of a different class; they
contain a good deal of alumina and ferric oxide, and in composition
resemble somewhat irregular Portland cement.