Chap4 Discussion of Anhydrous Cement Preparation

8/2/2019 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.

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Chap4 Discussion of Anhydrous Cement Preparation