chap4 discussion of anhydrous cement preparation
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4.0 Discussion of anhydrous cement preparat ion
The production of ordinary Portland and calcium sulpho-aluminate cement by
rotary calcining kilns, as discussed in the introduction, is based on huge
volumes measured in tonnes of clinker per hour. Changing the composition of
the cement or raw meal during production may not only cause undesirable
variation in the performance of the cement, but can strip the lining out of a
cement kiln. These are just two of the many reasons cement manufacturers
go to extreme lengths to maintain the continuity of their production1.
To investigate the production of ettringite it was necessary to test the effects
of changes in the cement composition. To do this small laboratory samples of
cement with various compositions had to be prepared under repeatable and
controlled conditions.
4.1 Modificat ion of the Bogue equation
The Bogue equation, as introduced in the introduction, cannot be applied to
the formation of calcium sulpho-aluminate cement. To be able to generate the
different calcium sulpho-aluminate type cements required, a modification of
the Bogue equation needed to be produced. A number of different trial
cements were produced to ascertain the sequence of crystal formation when
cements are formed (Method 2.27 Results 3.6.1).
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A number of different cements were then prepared with excess levels of each
oxide. The mineral content of each was examined by XRPD to ascertain the
form the excess component occurred in (Results 3.6). The summation of this
information allowed the empirical equation (Figure 4.1) to be devised.
Figure 4.1 Bogue type equation for CSA
w/w %
C2S = 2.86 x SiO2
C4AF = 3.0375 x Fe2O3
C4A3 = 7.625 x SO3 - 1.2708 x Fe2O3
C12A7 = 1.9412 x Al2O3 - 2.33 x SO3 - 1.2375 x Fe2O3
FREE LIME = CaO - O.65 x C2S - 0.461 x C4AF-0.367 x C4A3 - 0.485 x C12A7
Assumption :- Free lime >= 0
Assumption :- The elements are fully dispersed through the system during
calcination.
The basic premise for the above equation was that phases crystallise
sequentially from the general mass as it cools. The equation does not take
into account the possibility of solid solutions between the different phases.
Crystallographic examination of the test cements prepared
(Results 3.1) showed there to be no deviation in the observed patterns from
the data standards, suggesting that there is no solid solution between C2S ,
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4.2.2 Preparation of a pure sample of CSA clinker using a Muffle
furnace
An attempt to produce a pure sample of calcium sulpho-aluminate cement by
literature29 methods was made (Method 2.10), using a muffle furnace. The
resultant clinker was loosely bound and contained a large quantity of
unsintered powder. A sample was examined by x-ray powder diffraction and
found to contain mainly calcium oxide, aluminium oxide and a small amount
of calcium aluminate. It was found that almost all the sulphates had been lost
during the preparation. A second problem with the method used was the
small size of the cement sample being produced. The investigation required
cement sample sizes in excess of 2000 g to be prepared. The method was
deemed unacceptable to produce samples of Kleins compound for the
investigation.
4.3.1 Preparation of cement samples in an I nductotherm corelessinduction furnace
D.Menetrier-Sorrentino, C.M.George & F.P.Sorrentino30 describe using a
fusion technique to produce alumina cements of varying phases. To reach
the high temperatures required by fusion techniques, a metallurgical furnace
was employed (Method 2.11). The smallest furnace available would normally
be capable of melting 10 kg of steel. This allowed about 3000 g of cement
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raw materials in the correct stochiometric ratio to be used in this technique.
The powders were dampened and compressed into blocks. The blocks were
then placed in an Inductotherm coreless induction furnace. The furnace
functions by inducing a high frequency current in the sample to be melted. As
the raw materials did not conduct electricity a graphite crucible was used. The
crucible was heated by induction and the sample heated by the crucible.
The fine control of the furnace proved difficult even with the help of the
experienced operator. The samples rapidly heated and melted at about
1500oC. Temperature measurement of the contents was impossible using
thermocouples while the furnace was running due to induced current in the
thermocouple. When the induction current was turned off it was found that
the induction coil cooling system rapidly cooled the crucible and a stable
temperature was impossible. An optical pyrometer was tested, however the
reading was dependant on the temperature of the cooler surface of the
cement. The pyrometer was found to be less accurate than the thermocouple.
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4.3.2 Production of mixed phase alumina cement in an induction
furnace.
A number of alumina cements were prepared using the induction furnace to
identify the sequence of mineral formation during cooling from a general melt.
The mineral content and the activity of the cements were compared to known
commercial cements. Sufficient materials to give 2000 g of each cement were
prepared by blending calcium carbonate, aluminium oxide and iron oxide in
the correct stochiometric ratio. The blend was dampened and compressed
into blocks. These were then placed in the furnace and heated until fully
molten. The molten liquid was then poured out into copper moulds to allow
fairly rapid cooling Fig 4.2
Figure 4.2 Pouring of molten HAC into copper moulds
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The cooled clinker formed was then broken up and ground in a ball mill until
the required surface area was achieved (Method 2.12). The surface area of
the cements was determined using a calibrated R and B instruments specific
surface area machine. The phases present in the clinker were identified by X-
ray powder diffraction. The oxide composition of the clinker was determined
by X-ray efflorescence (Method 2.24,Results 3.10). The method was then
tested by preparing calcium silicates. The high rate of cooling via quenching9
by poring the melt into cold copper moulds allowed the formation of-C2S.
The -C2S subsequently converted back to -C2S on exposure to the
atmosphere.
4.3.3 Preparat ion of a pure sample of CSA clinker using an induction
furnace
An attempt was made to prepare Kliens compound (yeelimite) in the
induction furnace as detailed above. The clinker produced was examined by x-
ray powder diffraction and found to contain mainly the minerals mayenite and
mono -calcium aluminate. The hot graphite caused highly reducing
conditions to exist within the crucible and as a result almost all of the sulphur
was lost to the atmosphere as sulphur dioxide. Although unsuitable for the
production of calcium sulpho-aluminate the method was deemed to be
acceptable for the production of alumina and calcium silicate cements.
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4.4 Construction of a bench scale rotary cement k iln
It was decided that the Methods 2.9 and 2.11 were unsuitable for the
production of calcium sulpho-aluminate cement. A purpose designed and
constructed furnace was prepared specifically to replicate conditions within
rotary cement kilns.
The furnace was constructed by adapting a Carbolite 960 mm long 1500 oC,
50 mm diameter, tube furnace. The furnace was fitted with a 1200 mm
Mullite worktube. The ends of the worktube protruded out either end of the
furnace. A finely controlled rotary drive mechanism was fitted to the extended
worktube. The insulation was cut away so that the work tube rotated freely.
An integrated 9000 series single zone temperature controller was fitted to
allow good temperature control to be exercised. Figures 4.3 and 4.4 are
diagrammatic representations of the furnace showing the various features of
the furnace. A photograph (Figure 4.5) shows the final working version.
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Figure 4.3
1435
D i a g r a m a t i c a l r e p r e s e n t a t i o n o f t h e b e n c h s c a l e f u r n a c e
Worktube
Drive mechanism
Drive controlTemperaturecontroller
Figure 4.4
1435
Temperaturecontroller
Variable speeddrive controller
Variable speeddrive
Re-crystallisedAlumina work tube
Silicon carbideelements
Power light
Load switch
Load light
Insulation
Bench scale rotary kiln (internal arrangement)
Calcining region
Raw Meal
Clinker
Variableinclination
Pre heat zoneQuench zone
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Figure 4.5 Photograph of the working furnace.
4.4.1 Init ial clinker production
The elemental content of an existing Chinese calcium sulpho-aluminate
cement was obtained by XRF (Results 3.10). A sample raw meal with the
same elemental analysis was prepared using Chinese bauxite, limestone, silica
fume and anhydrite. The ground powders were then suspended as a slurry in
water. The slurry was then heated to drive off most of the water. The sticky
paste was rolled into balls ready to be placed in the furnace. The furnace was
heated to 1200oC, the literature value for the formation of calcium sulpho-
aluminate clinker2,3,4. The balls were introduced into the pre-heater zone of
the furnace. The residence time in the hot zone was limited to 30 minutes.
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The progression of the reaction mixture was controlled by the inclination angle
and speed of rotation of the work tube.
The clinker produced was examined for texture. The free lime content was
crudely determined by measuring the temperature rise by thermocouple, of a
crushed clinker /water mixture. The clinker was found to show a reasonable
level of exothermic behaviour. The temperature of the furnace was increased
in two 50 oC increments until the clinker showed no exothermic behaviour.
The furnace temperature was then raised slowly in 3-4 oC increments until the
clinker became slightly tacky, partially adhering to the worktube walls. At a
temperature of 1338oC the clinker on a macroscopic scale closely resembled
the sample produced in China. The experimental clinker was ground and
examined by XRD and XRF to determine any changes that had occurred. The
results (Results 3.6.1) obtained indicated that clinker formation had been
successful.
4.4.2 Product ion of pure samples of C4A3
Analar calcium carbonate, calcium sulphate and aluminium hydroxide in the
correct stochiometric ratio were blended as a slurry in water. These were then
dried to a stiff gel in an oven. The furnace was heated to 1345oC. The raw
meal was lightly crushed to form angular chunks. The raw meal was then
added to the preheater zone of the worktube and allowed to pass through the
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furnace, residing in the hot zone for 30 minutes. The very pale sky blue
clinker was obtained. The clinker was examined by XRD to determine the
mineralogical composition. The composition of the clinker was determined to
be exclusively yeelimite(results 3.6.1). XRF and gravametric analysis indicated
that very little SO3 had been lost during calcination.
4.4.3 Observation of clinker formation
As the bench scale furnace was run it was noticed that several different
artefacts could be observed forming in the work tube. Five separate zones
could be observed. A thermocouple was introduced into the furnace to
ascertain the temperature at which these artefacts occurred.
Figure4.6 Cross sect ion through t he furnace work tube
zone 5 zone 3 zone 2 zone 1
201001345 1143
Temperature in Co
Worktube Cement particle deposits
zone 4
The different zones were examined and samples of clinker from the different
regions were obtained for XRF and XRD examination.
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Zone 1: The zone had little or no deposits of raw meal. However, it was
a condensation zone for the water used as a binder and a flux. The raw meal
was very wet at the extreme right hand (cold) side and quite dry at the (hot)
left hand side.
Zone 2: A soft loosely bound ring rapidly formed in this region. There
were no obvious chemical changes occurring in the raw meal. It is suggested
that this was the point at which the water of crystallisation of the gypsum lost
in zone 3 was absorbed by the incoming dried raw meal.
Zone 3: Considerable chemical changes were observed in this region.
Gypsum was converted to anhydrite and calcium carbonate was converted to
calcium oxide.
Zone 4: A second ring was observed to form in this region. On analysis
(Method 2.24,Results 3.10) it was found that the ring material contained a
disproportionate amount of sulphur when compared to the raw meal sulphur
content.
Zone 5: This zone contained clinker. The deposits on the surface of the
worktube were found to be indistinguishable from the clinker composition.
4.5 Effect of doping on t he stability of calcium silicate
When very high calcium silicate cements were prepared a sixth zone was
observed. The zone occurred at the cooling end of the furnace where the
conversion of beta dicalcium silicate to the gamma form occurred. The
temperature of this conversion was investigated and found to be 530 oC. Pure
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samples of dicalcium silicate were then prepared and placed in the furnace.
The temperature of the visual conversion was measured by thermocouple,
and found to be 823 oC
Samples of different bauxite were used to prepare a range of standard
calcium sulpho-aluminate compositions as predicted by the computer
composition model (Appendix 4). The cement was prepared using unusually
high levels of silica, when compared to Chinease CSA, within the system. It
was proposed to test the suggestion that high levels of sulphur present
preferentially direct,away from formation of C3S to the formation C2S. A
second phenomenon visually observed, was the secondary conversion of-
C2S to -C2S. It was found that only the calcium sulpho-aluminate produced
from Chinese bauxite gave an apparently stable clinker.
Close examination of the Chinese bauxite composition by XRF indicated that
the bauxite was heavily contaminated with a range of different
elements(Results 3.10).
It is suggested that high levels of sulphur preferentially directs the formation
of C2S as opposed to C3S. This effect is observed even in systems where
there is an excess of calcium and aluminium. It is further indicated that the
presence of the contaminant oxides help to stabilise higher energy
polymorphs of C2S.
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It was decided to try three different clinkers to separate the two effects
observed above. A test was performed where a sample of C2S was prepared
( Method 2.27) from GPR silica, CaCO3 and anhydrite. A second test was
prepared ( Method 2.27) using only GPR silica, CaCO3. The third test used
stoichiometricaly correct amounts of calcium oxide and silica for C2S, however
there was a 0.5% w/w vanadium pentoxide inclusion to act as a phase
stabiliser ( Method 2.27). The clinkers were calcined at 1593oC (maximum
temp obtainable in the furnace) and the stability noted.
The resultant clinker from the first test was sea green in colour and stable.
XRD examinations suggested (Results 3.6.1) that the clinker was calcium
silicate sulphate (Ca5(SiO4)2SO4) and excess anhydrite. This cement was
unreactive to hydration. The second clinker remained white, however, it
showed expansive disruption on cooling, occurring at approximately 800oC.
XRD examinations showed there to be both C3S and -C2S present (Results
3.6.1). The composition suggested that all the calcium was consumed. The
third clinker was white and showed some expansive disruption. On XRD
examination the clinker was shown to be a mixture of CaO, -C2S and -C2S
(Results 3.6.1).
A sample of bauxite was obtained from Madagascar via Ireland. This was used
to produce a calcium sulpho-aluminate similar to the Chinese calcium sulpho-
aluminate. Interestingly; the clinker disruptively expanded with the conversion
of-C2S to -C2S. The process was photographed (Figure 4.7) to show the
speed and degree of disruption.
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Figure 4.7 Clinker undergoing disrupt ive expansion.
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4.6 Preparat ion of an ideal cement for t he product ion of ett ringit e
The mechanism proposed in the discussion section 5.3, was used to design a
high reactivity cement. Ideally the cement would have a number of specific
characteristics.
1.The cement should have a high alumina content.
2.The cement should be friable.
3.The cement should contain calcium, aluminium and sulphate in the clinker.
4.The cement should be exothermic when mixed with water.
5.The cement should be capable of forming seed sites.
6.The cement should contain a high energy calcium silicate polymorph.
7.The cement should be capable of being formed at relatively low
temperatures
8.The inclusion of a slowly hydrating aluminate phase to allow continuous
ettringite formation would be advantageous.
It was decided that mayenite, would be a fair candidate for the aluminate
phase. However, mayenite is not friable nor does it contain all the target
elements for the formationn of ettringite. It was decided to dope the mayenite
with a small amount of yeelimite C4A3 . A sample was prepared and
calcined at 1404 oC (Method 2.27). The clinker produced was soft and easily
ground. It was found that the cement set within 5 mins without accelerators
26,30. On addition of lithium carbonate the cement set and exothermed
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sufficiently to melt a polystyrene cup within 3 mins at 0.5 w:p ratio. Hydrated
cement samples were left in the lab for 7 days. The hardened pastes showed
expansive cracking. The cement was tested in a simple grout (Method 2.4) to
estimate the cements ability to form ettringite.
The cement appeared to fulfil all the requirements 1-5 & 7, however , the
cement failed to fulfil item 6. An attempt (Method 2.27) was made to produce
a high silica version. Unfortunately; the cement did not form dicalcium silicate
but tricalcium silicate with some free alumina. It was decided that the
sulphate level was too low to direct the silicon to form dicalcium silicate.
A second attempt, (Method 2.27) combined high silicon, sulphate, calcium,
and aluminium in the preferred ratio to generate grout 2120. The clinker
generated was extremely hard and ceramic in nature. The hardness of the
clinker was such that it was quite resistant to grinding. This route was
abandoned.
It was concluded that the original Chinese calcium sulpho-aluminate cement
was extremely close to the required target cement. Any increase in the belite
content resulted in expansive disruption due to -C2S to -C2S conversion
occuring on cooling.