Cement: - Cement is by far the most important
constituent of concrete, in that it forms the binding medium for the discrete ingredients.
Made out of naturally occurring raw materials and sometimes blended or
inter ground with industrial wastes, cements come in various types and chemical
compositions.
For general concrete constructions, following are the types of cement
a) Ordinary or low heat Portland cement
(OPC) conforming to IS: 269.
b) Rapid hardening Portland cement
conforming to IS: 8041.
c) Portland slag cement (PSC) conforming
to IS: 455.
d) Portland pozzolana cement (PPC)
conforming to IS: 1489.
e) High strength ordinary Portland cement
conforming to IS: 8112.
f) Hydrophobic cement conforming to IS: 8043.
g) high alumina cement conforming to IS: 6452
h) Super sulphate cement conforming to IS: 6909
Among the various types, ordinary
Portland cement conforming to IS : 269-19762 is perhaps the most common.
Further discussions relating to composition and hydration of cements,
generally pertain to ordinary Portland cement conforming to IS : 269. Ordinary
Portland cement is obtained by intimately mixing together a calcarious material
such as limestone or chalk, and an argillaceous material (i.e, silica, alumina
and iron oxide bearing material). For example, clay or shale, burning them at a
clinkering temperature of 1400 to 1450°C and grinding the resulting clinker
with gypsum. Since the raw materials consist mainly of lime, silica, alumina
and iron oxide, these form the major elements in cement also.
Depending upon the wide variety of raw
materials used in manufacture of cements, the typical ranges of these elements
in ordinary Portland cement may be expressed as below:
Table: 01
|
|
Maximum
% |
|
SiO2 |
19 - 24 |
|
Al2O3 |
03 - 06 |
|
Fe2O3 |
01 - 04 |
|
CaO |
59 - 64 |
|
MgO |
0.5 - 4 |
The compounds of these oxides present in the raw materials, however, interact with each other and form a series of more complex products during clinkering. The stage of chemical equilibrium reached during clinkering in the kiln may be disturbed somewhat during cooling. Assuming that cement has the same equilibrium as existing at the clinkering temperature, the basic compound composition of Portland cement along with their ranges may be as-shown in Table - 02.
Table: 02
COMPOUND COMPOSITION OF ORDINARY PORTLAND CEMENT,
|
COMPOUND |
FORMULA |
ABBREVIATION |
PERCENTAGE
BY MASS IN CEMENT |
|
Tricalcium silicate |
3 CaO. SiO2 |
C3S |
30-50 |
|
Dicalcium silicate |
2 CaO. SiO2 |
C2S |
20-45 |
|
Tricalcium aluminate |
3 CaO. Al2O3 |
C3A |
08 -12 |
|
Tetracalcium aluminoferrite |
4CaO.Al2O3 Fe2O3 |
C4AF |
06 – 10
|
As indicated in above Table-2, their relative proportions in the cement may vary
and indeed, the differences in the various types of ordinary Portland cement
are really due to the differences in the proportions of these major compounds
and fineness.
The two silicates, C3S and C2S which" together constitute about 70 to 75 percent of the cement, are more important from the considerations of strength giving properties.
Upon hydration, that is, reaction with
water, both C3S and C2S perhaps result in the same product - called calcium
silicate hydrate having approximate composition C3S2H3 and calcium hydroxide ".
Because of the similarity of their
structures with that of a naturally occurring mineral, the hydrates are called 'Tobermorite'. From approximate
stoichiometric calculations, C3S and C2S need approximately 24 and 21 percent water by weight,
respectively, for chemical reaction but C3S liberates nearly three times calcium hydroxide compared to
hydration of C2S.
The reaction of C3A with water is very quick and may lead to immediate stiffening of the paste - a phenomenon known as 'Flash set'.
The role of gypsum added in the
manufacture of cement is to prevent such fast reaction. The reaction of gypsum
and C3A with water first gives rise to an
insoluble compound called Calcium Sulphoaluminate (ettringite). But eventually the final product of hydration
is possibly cubic crystal of tricalcium aluminate hydrate (C3AH6).
Approximate stoichiometric calculation
shows that C3A reacts with 40 percent of water by
weight, which is more than that required for silicates; however, since the
amount of C3A in cement is comparatively small, the
net water required for the hydration of cement is not substantially affected.
The products of hydration of C4AF phase is not so well known. Neville!' states 'C4AF is believed to hydrate into tricalcium aluminate hydrate
and an amorphous phase, probably CaO. Fe2O3 aqueous. It is possible also that some
Fe2O3 is present in solid solution in the tricalcium aluminate
hydrate' .
The role of these four major compounds on the properties of cement can be summarized by the kinetics of reaction, development of strength and evolution of heat of hydration of these individual compounds.
The state of art can be summarised by Fig. I and II, and Table 3
From these it will be clear that C3A and C4AF are the
earliest to hydrate but their direct individual contribution
to overall strength development of the
cement is perhaps less significant than the silicates. In addition, C3A phase is responsible for the highest heat evolution both
during the initial period as well as in the long run. Among the silicates C3S has faster rate of reaction accompanied by greater heat
evolution and larger contribution to the initial strength than C2S phase; however, it is likely that both C3S and C2S phases
contribute equally to the long term strength of cement.
Apart from the chemical composition,
fineness of cement contributes to the kinetics of reaction and initial rate of
gain of strength. Generally greater
the fineness, greater is the rate of development of strength during the Initial
period (see Fig.
3) and larger is the heat evolution. This is possible because greater
fineness enables a larger surface of cement to come in contact with water
during the initial period, although the long term effect may not be different.
In addition, the particle sizes also
influence the hydration and strength at various ages. Particles below 5-micron
hydrate within 1 to 2 days and the hydration of 10-25 micron sizes may commence
after 7 days.
The different 'types' of cement are made by the adjustment in the relative proportion of chemical compounds and the fineness to suit the particular requirement.
A summary of the requirements for
physical characteristics and chemical composition of different Indian
cements" is reproduced in Table 4.
It will be seen that whenever a higher
rate of initial strength gain is required, this is achieved by grinding the
cement to greater fineness and the cement composition perhaps being richer in C3S and C3A phases,
but it may give rise to more heat of hydration.
2.1.1
HYDRATION OF CEMENTS –
The physical properties of concrete
depend to a large extent on the extent of hydration of cement and the resultant
microstructure of hydrated cement.
While the hydration products of individual compounds were described above, it must be realized that the hydration of cement is the collective hydration of each of the compounds present therein and there is no selective hydration of any of the compounds.
Nevertheless, the microstructure of
hydrated cement is more or less similar to those of the silicate phases. Upon
contact with water, the hydration of cement proceeds both inward and outward in
the sense that hydration products get deposited on the outer periphery and the
nucleus of unhydrated cement inside gets gradually diminished in volume.
At any stage of hydration the cement
paste (i.e. cement + water) consists of the product of hydration (which is called 'gel', because of
the large surface area), the remnant of unreacted cement, Ca(OH)2 and water, besides some other minor compounds.
Hexagonal prismatic crystals of ettringite are formed first on the tricalcium aluminate phases. Crystals of calcium hydroxide form about four hours after mixing. Thin acicular particles of calcium silicate hydrate start protruding from the surface' of cement grains after two hours". In matured pastes, particles of calcium silicate hydrate form an interlocking network and owing to the similarity with the naturally occurring mineral, tobermorite is called 'tobermorite gel'. This gel -is poorly crystalline, almost amorphous and appears as randomly oriented layers of thin sheets or buckled ribbon". The thickness of primary 'gel' particles is estimated to be 3.0 x (10)-9 to 4.0 x (10)-9 m.
The products of hydration as described
above form a random three dimensional network
gradually filling the space originally
occupied by water. Accordingly, the hardened cement paste has a porous
structure, the pore sizes varying from very small (4 x (10)-10 m) to much larger, and are called 'gel' pores and 'capillary'
pores.
The water present in these pores are
held with different degrees of affinity and the pore system inside the hardened
cement paste mayor may not be continuous. As hydration proceeds, the deposit of
hydration products on the original cement grain makes the diffusion of water to
the unhydrated nucleus more and more difficult, so the rate of hydration
decreases with time.
Each gram of cement of average composition needs about 0.253 g of water for chemical reaction. In addition, a characteristic amount of water is needed to fill the gel pores. The total amount of water thus needed for chemical reactions and to fill the gel pores is about 42%.
Since hydration can proceed only when
the gel pores are saturated, it has often been mistakenly held that
water-cement ratio less than 0.40 or so should not be permitted in concretes.
However, it must be emphasized that
even in presence of excess water, complete hydration of cement never takes
place because of the decreasing porosity of the hydration products, nor is it
necessary that cement should be fully hydrated". In fact, water-cement ratio less than 0.40 is
quite common in structural concretes, more so in high strength
concretes.
In concretes, the hardened cement paste
is thus a porous ensemble; the concentration of
solid products of hydration in the total space available (that is original water + hydrated cement) is an index of porosity. Like any other porous solid, the compressive strength of cement pastes (or concretes) is related to the parameter gel-space ratio" or hydratespace ratio"
2.1.1
HYDRATION OF CEMENTS –
The physical properties of concrete
depend to a large extent on the extent of hydration of cement and the resultant
microstructure of hydrated cement.
While the hydration products of individual compounds were described above, it must be realized that the hydration of cement is the collective hydration of each of the compounds present therein and there is no selective hydration of any of the compounds.
Nevertheless, the microstructure of
hydrated cement is more or less similar to those of the silicate phases. Upon
contact with water, the hydration of cement proceeds both inward and outward in
the sense that hydration products get deposited on the outer periphery and the
nucleus of unhydrated cement inside gets gradually diminished in volume.
At any stage of hydration the cement
paste (i.e. cement + water) consists of the product of hydration (which is called 'gel', because of
the large surface area), the remnant of unreacted cement, Ca(OH)2 and water, besides some other minor compounds.
Hexagonal prismatic crystals of ettringite are formed first on the tricalcium aluminate phases. Crystals of calcium hydroxide form about four hours after mixing. Thin acicular particles of calcium silicate hydrate start protruding from the surface' of cement grains after two hours". In matured pastes, particles of calcium silicate hydrate form an interlocking network and owing to the similarity with the naturally occurring mineral, tobermorite is called 'tobermorite gel'. This gel -is poorly crystalline, almost amorphous and appears as randomly oriented layers of thin sheets or buckled ribbon". The thickness of primary 'gel' particles is estimated to be 3.0 x (10)-9 to 4.0 x (10)-9 m.
The products of hydration as described
above form a random three dimensional network
gradually filling the space originally
occupied by water. Accordingly, the hardened cement paste has a porous
structure, the pore sizes varying from very small (4 x (10)-10 m) to much larger, and are called 'gel' pores and 'capillary'
pores.
The water present in these pores are
held with different degrees of affinity and the pore system inside the hardened
cement paste mayor may not be continuous. As hydration proceeds, the deposit of
hydration products on the original cementgrain makes the diffusion of water to
the unhydrated nucleus more and more difficult, so the rate of hydration
decreases with time.
Each gram of cement of average composition needs about 0.253 g of water for chemical reaction. In addition, a characteristic amount of water is needed to fill the gel pores. The total amount of water thus needed for chemical reactions and to fill the gel pores is about 42%.
Since hydration can proceed only when
the gel pores are saturated, it has often been mistakenly held that
water-cement ratio less than 0.40 or so should not be permitted in concretes.
However, it must be emphasized that
even in presence of excess water, complete hydration of cement never takes
place because of the decreasing porosity of the hydration products, nor is it
necessary that cement should be fully hydrated". In fact, water-cement ratio less than 0.40 is
quite common in structural concretes, more so in high strength
concretes.
In concretes, the hardened cement paste
is thus a porous ensemble; the concentration of
solid products of hydration in the
total space available (that is original water + hydrated cement) is an index of
porosity. Like any other porous solid, the compressive strength of cement pastes (or concretes) is related to the
parameter gel-space ratio" or hydratespace ratio"
The water-cement ratio, which is held as the most important parameter governing compressive strength, is really an expression of the concentration of hydration products in the total volume at a particular age for the resultant degree of hydration".
2.1.2. PORTLAND POZZOLANA AND SLAG CEMENTS –
Among cements of different types,
mention may be made of Portland pozzolana cement conforming to IS : 1489 and
Portland slag cement conforming to IS : 455 because of increased production of
these cements in the country mainly to offset the shortage of ordinary Portland
cement.
Portland slag cement is manufactured by inter grinding ordinary Portland cement clinker with granulated slags obtained as a by-product from the manufacture of steel.
The slags have more or less the same
constituents as in ordinary Portland cement in varying proportions, depending
upon the processes involved. Typical oxide compositions of Indian slags
suitable for the manufacture of Portland slag cement are as follows:
|
|
Maximum
% |
|
SiO2 |
27 - 32 |
|
Al2O3 |
17 - 31 |
|
FeO |
0 - 1 |
|
CaO |
30 - 40 |
|
MgO |
0 - 17 |
The slag should, however, be in glassy
form. IS : 455 permits the proportion of slag to being the range of 25 to 65
percent. The products of hydration of such Portland slag cement are believed to
be similar to that of ordinary Portland cement; Ca(OH)2 liberated by the hydration of ordinary Portland cement acts
as an activator for the reaction of slag", Since the hydration of slag
component depends initially upon liberation of Ca(OH)2 the rate of development of early strength may be somewhat
slower (see Fig. 7).
However, for all engineering purposes,
Portland slag cement may be held to be similar to ordinary Portland cement and
the requirements of physical characteristics are also identical in both cases.
Portland slag cements give lower heat of hydration and better sulphate
resistance". Portland pozzolana cement (see IS : 1489) is made by blending
or inter grinding reactive pozzolana (for example, fly ash, burnt clay,
diatomaceous earth, etc.) in proportions of 10 to 25% with ordinary Portland
cement. Pozzolanas as such do not possess cementitious property in themselves
but in combination with Ca(OH)2 liberated
from the hydration of ordinary Portland cement, give rise to cementitious
products at room ambient temperature and the ultimate products of hydration in
both cases are believed to be identical", The requirements of 7-day
strength of Portland pozzolana cement (Refer IS : 1489) are the same as that of
ordinary Portland cement (21.5 N/mm2). The use of Portland pozzolana cement is
recommended in IS : 456 as substitute for ordinary Portland cement for plain
and reinforced concrete work in general building construction.
In addition to 7 days compressive
strength, IS: 1489 specifies the minimum 28-day compressive strength of
Portland pozzolana cement. However, for the reasons cited, the rate of
development of early strength may be somewhat lower (see Fig. 8) and concrete made with
Portland pozzolana cement may need somewhat longer curing period under field
conditions, delayed removal of formwork, etc. Portland pozzolana cement also
has the advantage: of lower heat of hydration and better sulphate resistance;
in fact these properties led to its wide application in USA'.
2.1.3
TESTS ON CEMENTS -
The usual tests made on cement are: fineness, setting time, soundness, heat of hydration, compressive strength and chemical composition. All physical and chemical composition tests are carried out in accordance with the procedures described in IS: 4031 and IS : 4032.
The Blaines air permeability method is used for determining the fineness of cement. The method is based on the permeability to flow of air through a bed of the cement.
The fineness
is expressed as specific surface area per gram of cement.
The setting times are measured by Vicat apparatus, with different penetrating attachments. The term setting is used to describe the stiffening of the cement paste, and the terms 'initial set" and 'final set' are used to describe arbitrary chosen stages of setting.
The
soundness of cement is determined in an accelerated manner by Le-Chatelier
apparatus.
This
test detects unsoundness due to free lime only.
Unsoundness due to magnesia present in
the raw materials from which cement is manufactured can be determined by
autoclave test. This test is sensitive to both free magnesia and free lime!'.
In this test high pressure steam accelerates the hydration of both magnesia and
lime. The results of the autoclave test are affected by, in addition to the
compounds causing expansion, the C3A content. The test gives thus no more than a broad
indication of the long term expansion expected in service.
The heat of hydration is the amount of heat in calories per gram of unhydrated cement, evolved upon complete hydration at a liven temperature. The method of determinina the heat of hydration is by measuring the heats of solution _of unhydrated and hydrated cement in a mixture of nitric and hydrofluoric acids: the difference between the two values gives the heat of hydration.
The heat of hydration thus measured consists of the chemical heat of the reactions of hydration and the heat of absorption of water on the surface of the gel formed by the processes of hydration. The heat of hydration is required to be determined for low heat Portland cement, as specified in IS : 269.
The compressive strength of cement is
determined on 1:3 cement-sand mortar cube specimens with standard graded sand,
cast and cured under controlled conditions of temperature and humidity.
The water content is determined as {(P/4) + 3)} percent by 4 weight of cement and sand, where P is percentage of water required for standard consistency. In most of the cases, it corresponds to a water-cement ratio of 0.37 to 0.42.
The chemical analysis is carried out to
determine the oxide composition of cement. The percentages of main compound in
cement (i.e. C3S, C2S, C3A and C4AF) can be
calculated (rom oxide composition using Bogue's equation, which is applicable
to ordinary Portland cement only. In addition to the main compounds, two of the
minor compounds are of interest. They are alkalis – Na2O and K2O. The
insoluble residue determined by treating with hydrochloric acid, is a measure
of impurities in ordinary Portland cement; largely arising from impurities in
gypsum. The loss on ignition shows the extent of carbonation of free lime and
hydration due to the exposure of cement to the atmosphere.
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