CiteSeerX — Thermal Properties of High Alumina Refractory Mortars Modified With

177

Thermal Properties ^of High ^Alumina Refractory ^Mortars Modified With

Lanthanide Chemical Admixtures

THOMAS NOVINSON Naval Civil

Engineering

^Lab

Port

Hueneme, California

HEINZ F. POPPENDIEK Geoscience Ltd Solana

Beach, California

(Received September ^{16, 1986)} (Revised ^November 17, 1986)

INTRODUCTION

I T

^{IS OF interest to}

^study

the thermal

insulating properties

of certain

chemically

modified high

^alumina

refractory

^mortars because of their

potential

^{use as heat}

resistant

insulating

liners for

portland

cement concrete structures

exposed

^{to heat}

in the range from 500 ^to 1000 ° F

Conventional alumina refractories are heat cured in

place,

ⁱⁿ

engineering

ap-

plications [1].

^{This is not a}

problem

^for

refractory

^liners

prepared by gunning

onto the interior steel surface of a furnace or a kiln. For

lining

pcc,

however,

^this

is a

problem

since flame heat

curing

^{of the}

refractory

^{liner is}

likely

^to

damage

^the

underlying

concrete meant to be

protected.

For this _reason, a

variety

of chemical

modifiers,

^{known as} &dquo;admixtures&dquo; have been

investigated

^{to lower the cure}

temperatures

of conventional refractories.

Among

the chemicals

reported

in this paper ^are various derivatives of the lanthanide

elements, lanthanum, cerium, neodymium

^and

praeseodymium, plus

the

closely

related but non-lanthanide element zirconium. The lanthanide com-

pounds

^were chosen for

potential

^flux

properties,

^that

is, ability

^{to lower the}

fusion

point

of alumina

[2].

The unmodified controls and modified

(chemical admixtures)

refractories were

heat cured and tested for cold

crushing strength following

the ASTM C133-82a

procedure [3].

^{Plots of}

strength

^vs.

temperature

^{were made to determine}

physical

behavior at elevated

temperatures.

Then thermal

properties, including

^thermal

conductivity, specific

^{heat and thermal}

diffusivity,

^were

measured,

^{to ensure that}

insulating properties

^{had not}

changed

for the modified materials.

Journal

of

^COMPOSITE

MATERIALS,

^Vol.

22 -February

¹⁹⁸⁸

0021-9983/88/02 0177-15 $4.50/0

178

EXPERIMENTAL

Crushing Strength

Measurements

Mortars were

prepared

from 1000 gm of

high

alumina calcium aluminate cement

containing

^bauxite

aggregate, plus

^{150 ml of water}

⁽¹⁵

^%

by weight)

^con-

taining

⁰

(control,

^no

admixture), 1 % , 5 %

^{and 10 %}

by weight

^of each chemical admixture. The

refractory

^{mortar was cast in the}

shape

^{of 2}

by

²

by

^{2 inch cubes.}

The cubes were moist cured 24 hours at

95 % humidity

and 75 ° R Then the molds

were

stripped

and the cubes were

slowly

^{baked in an oven at 220°F to remove}

water. The

cubes,

^{which were allowed to cool to room}

temperature,

^{were then} heated in sets of

three,

^to

temperatures

^of

500,

⁷⁵⁰ and 1000°F The cubes were

again

^{cooled to room}

temperature

^{and were crushed with a Tinnius Olsen test} machine. The cold

crushing strengths

^were then recorded. The results can be

seen in

Figures

^{1 and 2.}

Thermal

Conductivity

Determinations

The thermal

conductivity

is defined

by

the Fourier’s

equation,

where

q ⁼ heat flow rate k ⁼ thermal

conductivity

t ⁼

temperature

y ⁼ distance

along

^{heat flow}

path

A ⁼ heat transfer ^area normal to the heat flow

The

guarded

^hot

plate apparatus (ASTM C-177) [4]

^{was used to} make the ther- mal

conductivity

measurements.

Figure

3 illustrates the

major

elements of the

system.

^{Heat from a}

central,

^flat

plate

heater flows

through

^{two hot}

plates, through

^{two test slabs to two}

cooling plates.

^The

temperature

differences across

the two test

samples

^are determined from

thermocouples

^{that are} embedded in the hot and cold

plates

^{or in the test}

samples.

^{As shown in the}

figure,

the central heater is surrounded

by

^a

guard

heater. A

thermopile

^is

positioned

between the central hot

plates

^{and the}

guard

^hot

plates.

^At

steady

^state, when the central heater and

guard

heater have been

adjusted

^{so that the}

thermopile output

is very small

(yielding

unidirectional heat

flow),

^{a data set} is obtained. From ^a heat balance on the

system involving

heater power

inputs,

heat transfer _areas, tem-

perature

differences and

sample thicknesses,

the thermal

conductivity

^{can be}

determined.

Specific

^Heat Determinations

One convenient way of

measuring

^the

specific

heat of cementitious materials is

by utilizing

^a

gradient layer

calorimeter. The elements of such a

system

^{are shown}

179 Figure ^1. Comparison ^of crushing strengths of unmodified alumina refractory ^and

refractory

doped ^with 1 % lanthanide compounds.

180

Figure ^2. Comparison ^of crushing strength of alumina refractories with 1 % lanthanum

phosphate

and cerium oxide.

181

Figure

^3. Guarded hot plate (twin plate) ^thermal conductivity system.

in

Figure

^{4. A test}

sample

^is

positioned

inside of the heat flux

measuring envelope

that is housed in a fluid cooled heat sink. The heat sink in turn is sur-

rounded

by

^thermal insulation.

This calorimeter is based ^on the

principle

that all of the heat flow into ^or out

of the calorimeter must pass

through

its walls where the

temperature gradient

sensors are located.

Therefore,

the calorimeter

envelope integrates

the total heat flow in the

system

^{on an} instantaneous basis. The calorimeter walls consist of

special

thermoelectric heat flux transducers* that

yield

^{a DC}

voltage output

*A heat flux transducer is composed ^{of a} thermopile system that has &dquo;hot&dquo; junction ^{sets at one} depth ^{within the}

sensor and &dquo;cold&dquo; junction ^{sets at another} depth

182

signal.

^The calorimeter walls are thin so that low time constants are involved. As heat flows

through

^the

walls,

^{a small}

temperature

difference

directly proportional

to the heat flow is

developed.

^{In a}

properly designed calorimeter,

^the

output sig-

nal is affected

only by

^{the rate} of the heat flow.

The utilization of the calorimeter to measure the

specific

^{heat of a} material is done as follows: An unloaded calorimeter that has come to thermal

equilibrium

at

temperature level t1

^is

suddenly exposed

^{to a new}

temperature level,

t2. The heat flow trace as the transient process

proceeds

^from the initial

steady

^{state con-}

dition to the final

steady

^{state condition can} be obtained with a

recording poten-

tiometer. The area under this curve is

equal

^{to the heat added to or} extracted from the calorimeter and its liner for the

superposed temperature perturbation. Next,

the calorimeter is loaded with the

specimen

^{to be}

investigated

^and

again exposed

to the

original temperature tl

and allowed to

equilibrate.

Then the loaded calorimeter is

suddenly exposed

^{to the new}

temperature

datum t2, allowed ^to reach

equilibrium

^{and the}

corresponding

transient heat flow trace recorded. The

area under this curve is

equal

^to the heat added to or

given

up

by

^the

calorimeter,

its liner and the

specimen.

Subtraction of the areas under the two transient traces

yields

the desired heat flow for the

specimen

alone. The trace area is related to an energy flow per unit time

by

^{means of an accurate} resistance

heating

^calibra-

tion for the calorimeter. The

specific

heat for the

specimen

is obtained from the

Figure

^4. Elements of a gradient layer calorimeter system.

183

Figure 5. Thermal diffusivity measurement apparatus.

classical definition

(heat

transferred divided

by

^the

product

^{of the mass and tem-}

perature perturbation).

^The

defining equation

^{for the}

specific heat, _c,.,,

^is

where

q(O)

^{= time}

dependent

^{heat flow}

through

calorimeter when loaded with test

sample

q(6)e =

^time

dependent

^{heat flow}

through

calorimeter without the test

sample

0= time

0,=

^the

equilibrium

^time

period (no

further heat

flow)

ms = ^mass of the test

sample

tl = initial

temperature

^datum

t2 = final temperature

^datum Thermal

Diffusivity

Determinations

If it is desired to measure the thermal

diffusivity directly ^(rather

^than

by

^mak-

ing

the individual

property

measurements and

substituting

them into the

defining

184

property ratio),

^{it is} necessary ^to work with a

time-temperature

solution of a clas- sical

boundary

^value

problem

where transient heat transfer is in

operation [5].

A

typical example

^{of such a}

system

^{is a} slab that is

suddenly

^{cooled at both sur-}

faces

by

^forced convection

cooling. Symbolically,

^the functional

relationship

^of

the variable can be

expressed

^as,

where tc, to

and t,

^{are the}

centerline,

surface and initial

temperatures,

respec-

tively,

yc, is the

product

^{of the}

specific

heat and the

density

and h is the surface heat transfer conductance.

The surfaces and centerline

temperatures

^{of the slab are measured as a function} of time

during

^the

step

^function

cooling (see Figure 5).

From such data and the mathematical

time-temperature

^{solution for this}

boundary

^value

problem,

^the

thermal

diffusivity

^can be deduced.

Figure

^6. Refractory mortar at 100 x magnification. ^{F = flint}

clay,

^{M = matrix area,}

P ⁼ pore.

185

Figure ^7. Refractory ^{mortar with 1 %} cerium oxide at 100 x magnification. ^{F = flint} clay,

M ⁼ matrix area, P ⁼ casting pore, C ⁼ cerium oxide.

RESULTS AND CONCLUSIONS

Representative samples

of the control

(refractory

^without

admixture)

^{and 1%}

cerium oxide

doped

^{mortar cubes} were vacuum

impregnated

^{with an} epoxy

pot- ting compound.

The cured cubes were then machine

polished,

first with diamond

grit

^{down to} 1 ¡.tm, then with alumina

grit

^{down to 0.05 ¡.tm. The}

samples

^were

then viewed at several different

magnifications (40X, 100X,

^and

200X)

^{for the}

petrographic analysis. Photomicrographs

^were taken with a Polaroid camera at- tachment. Photos at 100X are included in this paper

(Figures

^{6 and}

7)

^{to illustrate}

major findings.

Samples

^{of 8}

refractory

^mortar

cubes, including

^{one control}

(no admixture)

and one

1 %

cerium oxide

doped

mortar, ^were examined under

high magnification using

^{an ISI}

(International

Scientific

Instruments) scanning

^electron

microscope (SEM).

^The

samples

^were

chipped

^{off the corners of the}

refractory

^mortar

cubes,

mounted on carbon blocks with a

special graphite adhesive,

^and

placed

^{in an}

analysis

chamber. The

samples

^were

sputtered

^with

gold

^for

high

resolution. The chamber was

placed

^{under vacuum and the}

samples

^were scanned with x-rays for the SEM

pictures (recorded

^{with a Polaroid camera}

attachment)

^{shown in}

Figures

8 and 9.

Samples

of the control

(without admixture)

^{and 1 %} cerium oxide

doped

^refrac-

tory

^{mortar cubes were}

pulverized

^{to -} 325 mesh and

chemically analyzed using

x-ray diffraction

equipment (Kevex).

^Chemical

analysis

of the mineral

phases

^of

186

Figure ^8. Refractory ^{mortar before} finng (75°F), at 500 ^x magnification.

Figure ^9. Refractory ^mortar after firing (to 700°F) ^and cooling, ^{at 500 x} magnification.

187 the mortars verified the existance of cristobalite

(Si02),

^mullite

(AI6Si2013)’

^and

gehlenite (Ca,A1,SiO,), plus

^some calcium aluminum

hydroxides

that could not

be identified. The mineral cerianite

(Ce02)

^was also identified in the 1 % cerium oxide

doped samples.

No other cerium

compounds

^were identified in the

doped refractory.

The cerium oxide did not

chemically

^{react with the}

components

^{of the} cements, ^since inclusions of cerianite were the

only

identifiable cerium com-

pounds

observed in the x-ray diffraction

experiments. ^However,

^the

petrographic analysis

^{at 100X}

magnification (Figures

^{6 and}

7) definitely

^{show a} difference in

casting density,

^that

is,

the cerium oxide

doped samples

^are denser than the

undoped

^control

refractory

^cubes.

Perhaps

cerium oxide acts as a

superior

^fine

aggregate

^to

replace part

of the aluminum oxide

(bauxite). Unfortunately,

^a

similar

study

^on the lanthanum oxide could not be

performed

^{due to time and}

funding

limitations.

Lanthanum

phosphate

may

actually undergo

chemical reaction with some of the other mortar

components, however,

^because

(a)

this salt is somewhat water

soluble

compared

^to the insoluble lanthanide oxides and

(b)

^{the curve for}

strength

vs.

temperature (Figure 1)

for lanthanum

phosphate

^{is much}

steeper (hence greater strength)

than that for the lanthanum oxide

doped refractory

^mortar.

Another reason for the observed

strength

increases may be due ^to

physical changes

^{in the}

refractory

microstructure caused

by

the lanthanide

compounds.

Figure

8 shows the surface of the alumina

refractory

^{at 500X}

magnification

before

firing.

^The surface appears like ^a series of

pottery

cups ^{or vases.} After fir-

ing

^{at 700}

degrees,

the surface

changes,

^as illustrated in

Figure 9,

^{and the}

frag-

ments appears ^to have melted or

annealed,

^thus

forming

^a denser material. The cerium oxide _may have aided in the densification of the material at this

tempera-

ture,

although

it is unclear as to

why

^the

strength

decreases between 700 and 1000

degrees.

^It should be noted that the

peak

ⁱⁿ

strength

^is

reproducible

^{and was}

observed in

laboratory experiments

^{both at NCEL and at National}

Refractory

^and

Minerals

Corp.

^This

strange

increase in

strength

^{at 700}

degrees

^{was also noted}

with

refractory

^cubes

containing

^{1 %} of certain

zinc,

barium and strontium salts

(results

^{to be}

published

^{at a later}

^date).

Refractory

^mortars

containing

^{1 %} cerium oxide exhibited the

greatest

^cold

crushing strength throughout

the range of 500 ^to

1000 ° F,

^{with an unusual}

peak

or increase in test

strength occurring

^at 750 ° F. Lanthanum

phosphate (1 % )

^also

produced

^a substantial increase in

strength

^{for the}

refractory

^mortar in the range of 500 to 1000 ° F The other lanthanide

oxides, neodymium

^and

praseodymium,

had little or no effect or

actually

decreased the

strength

of the heated mortar. The zirconium oxide showed

only slight

increases in

strength

when added in

1 % ,

^{5 %}

and 10 %

by weight

increments to the calcium aluminate mortars.

The thermal

conductivity, specific

heat and thermal

diffusivity

^{vs. densities are}

recorded in Tables

1,

2 and 3. In the case of thermal

conductivity,

^{the data}

points

shown in Table 1 indicate that this

property

^increases

slightly

^{with increases in}

weight

concentration of the mortar

additive,

^{as shown for}

ZrSiO3.

The thermal conductivities of solid materials are controlled

by

^two

primary

mechanisms, namely,

lattice vibrations

(atomic motions)

of the material and free

188

Table 1. Thermal

conductivity

measurements for mortar with various additives.

Table 2.

Specific

^heat measurements for mortar with various additives.

Table 3. Thermal

diffusivity

measurements for mortar with various additives.

189 electron flow or the electronic

component.

^{In the case} of cementitious

type materials,

^thermal

conductivity

^is

primarily

controlled

by

the lattice vibration mechanism.

Generally,

^{one cannot}

predict

thermal conductivities for such materials from first

principles.

^{It is}

possible,

^{however to}

predict

^the

equivalent

thermal

conductivity

^{of a} mixture of

components

if their thermal conductivities and volume ^or

weight

^{fractions are} known. The

following equations

^illustrate

how such

predictions

^{can be made.}

If the

components

^{of a mixture were}

arranged

in &dquo;n&dquo;

adjacent

slabs with heat flow

along

^the

lengths

^{of the}

slabs,

the classical

expression

for the effective or

equivalent

^thermal

conductivity, keq,

^is

where

~y ⁼

density

^{of the}

composite

^material

kn

^{= thermal}

conductivity

of the nth

component

wn ⁼

weight

fraction of the nth

component

yn ⁼

density

of the nth

component

If the

components

^{of such a mixture were}

arranged

ⁱⁿ

adjacent

slabs with heat flow

perpendicular

^{to the}

lengths

^{of the}

slabs,

the classical

expression

^{for the}

equivalent

^thermal

conductivity

^is

For the case of one

component (small particles) uniformily

distributed in a

second

component (the carrier)

^the

equivalent

^thermal

conductivity ^(as proposed by Eucken) is

where

k~

⁼ thermal

conductivity

of the carrier

kp

^{= thermal}

conductivity

^{of the}

particles

b ⁼ ratio of

particle

^{volume to} total volume

a ⁼

3/(2

⁺

kpkc)

The Eucken

equation

^is valid in the range 0 < b ^< 0.5.

190

On the basis of the small

percentages

of admixtures

present

^{and on the basis} that the thermal conductivities of the

components

^are all of the same

order,

^{it can}

readily

be shown from the three

equations

^{shown above that}

only

^small

changes

in thermal

conductivity

^occur

(as

^found

experimentally)

^{for the mortars contain-}

ing

the admixtures used in this

study.

The

specific

^heat measurements for the mortars

containing Zr02

^and

ZrSiO3

illustrate the fact that the

product

^{of the}

specific

heat and the

density

is approx-

imately

^{a constant.}

From the classical oscillator

theory,

^{it can} be shown that the

specific

^{heat of a}

material is relatable to the number of

degrees

of freedom of the molecule in- volved and the universal gas ^constant. Such an

expression

^{can be used to estimate}

the

specific

^{heat of some} materials.

Dulong

and Petit related

specific

^{heats of}

metal to their atomic

weights.

^More

complicated

^{materials like the} cementitious materials

being

considered in this

study

^{are more difficult to evaluate on first}

principle

bases. It is

possible, however,

^to describe the

equivalent specific ^heats,

c, , of such materials in terms of the

specific

heats of the individual

components

and

their

weight fractions, namely

where

w&dquo; ⁼ the

weight

fraction of the nth

component

C’n ^{= the}

specific

heat of the nth component

Calculations can be made for the

specific

^{heat of a mortar}

containing

^{an admix-}

ture,

using

the mixture

equation given

^above.

Again,

because of the small admix- ture

percentages involved,

^{one can} show that the

predicted changes

ⁱⁿ

specific

heat are small as a result of the admixture

additions,

^{as found}

experimentally.

The thermal diffusivities were found to be _very

nearly

^{a constant}

quantity

^for

all admixtures measured. This result is

apparently

^{related to}

corresponding changes

in thermal

conductivity

^{and the}

product

^of

density

^and

specific

^{heat as}

the

compositions change.

It has been shown that small

percentages

of admixtures which

significantly

influence the

strength

^of

refractory

^{mortars, did not}

significantly

affect the thermal

properties.

Analysis

of these data indicates that the refractories

doped

^with

^{1 %}

^lanthanide

oxides differ little from the unmodified

refractory

^mortar control. This informa- tion is

promising

in that it demonstrates that

insulating properties

of the modified mortars were not lost in those cases where the lanthanide oxides

imparted

^in-

creased

strength

^to the heated refractories.

ACKNOWLEDGEMENT

Funds for this work were

provided by

the Naval Facilities

Engineering

Command, Alexandria,

and the Office of Naval

Research, Arlington, Virginia.

191 The authors wish to thank the materials science

engineering

technicians at NCEL for assistance in

preparing

^{the mortar}

cubes, plates

^{and other}

specimens,

^the

ceramic

engineering

research staff of National Refractories and

Minerals, Pleasanton, California,

for modulus of

rupture (cold crushing) data,

^and

Geoscience staff members for their assistance in

making

the thermal

property

measurements.

REFERENCES

1. American Concrete Institute "Refractory Concrete," Special Publication SP-57, Detroit, ^Mich.

(1978).

2. "The Lanthanides," Chap 31, pp. 870-891 in Advanced Inorganic Chemistry, ^A Comprehensive Text, F. A. Cotton and G. Wilkinson, eds. New York, NY:Interscience Publishers (1962).

3. American Society ^for Testing ^and Materials, Philadelphia, ^PA (1982).

4. Annual Book of ASTM Standards, ^Part 18, Philadelphia, ^PA (1981).

5. "Heat Transfer Notes," ^{L. M. K.} Boelter, ^{V. H.} Cherry, H. A. Johnson and R. C. Martinelli, eds.

Chapter 1, University of California Press, Berkeley ^{and Los} Angeles (1946).