Microsilica in Refractory Castables- Surface Properties and ... - Elkem

MICROSILICA IN
REFRACTORY CASTABLES
SURFACE PROPERTIES AND SET
Presented at IREFCON 2010 at Kolkata, India in February 4-6, 2010.

MICROSILICA IN REFRACTORY CASTABLES
-SURFACE PROPERTIES AND SET
Bjørn Myhre,
Elkem Silicon Materials, P.O Box 8126 Vaagsbygd,
4675 Kristiansand, Norway.
bjorn.myhre@elkem.no
Abstract:
The surface of microsilica is covered by numerous
silanol groups that are able to dissociate in an
aqueous medium. This dissociation is responsible
for a number of effects and interactions that is seen
when a castable is made that contain both
microsilica and cement. In this paper some of the
effects of both deflocculants and gelling
mechanisms are explained, with relevance to the
use in low cement castables. Setting of such
castables is also reviewed and the effect of
microsilica on setting and the consequence of a too
early demoulding is explained
Surface of Microsilica
Although the overall composition of microsilica is
SiO 2 , the surface of a microsilica particle is not
plain siloxane bonds, but is also partially
hydroxylated and hydrated. Producers of fumed
silica (particularly Evonik Degussa) have been very
active in the characterisation and modification of
the silica surface in order to tailor specific
properties of precipitated and fumed silica.
In general, several connectivities of surface silicon
have been identified by means of FT-IR- and NMRspectroscopy,
as shown in Figure 1.
Figure 1. Surface Si connectivities idientified on silica.

Based on theoretical and experimental results 1 , it
seems that the maximum density of silanol (Si-OH)
groups on the surface of silica is approximately
4.6nm -2 .
The surface of microsilica is covered by these
silanol groups, although the presence of impurities
plays some role, measurements have indicated
silanol densities ranging from approximately 2 to
4.5nm -2 for microsilica from silicon production.
The good correlation between theoretical values
and those found for microsilica may indicate that
the impurities of the microsilica are not smothered
out on the surface of the microsilica particles but
rather as discrete particles or dissolved in the silica.
A combination is probable.
The presence of the silanol groups makes the
microsilica easy to disperse in aqueous systems.
Depending on the pH, a fraction of the silanol
groups dissociate resulting into a negatively
charged surface. The surface charge or zetapotential
can be measured and values higher than
approximately 25mV (positive or negative) are
often taken as an indication of stable suspensions.
Figure 2 shows a measurement of such a zeta
potential for microsilica 971, and we see that the
microsilica has a negative charge over the whole
range. Due to dissociation of the silanol groups, the
negative surface charge increases with pH up to
approximately pH 7. At higher pH than 7, the zeta
potential flattens out and at still higher pH the
microsilica starts to dissolve.
Zeta-potential (mV)
0
-5
-10
-15
-20
-25
-30
-35
1 2 3 4 5 6 7 8 9
Figure 2 Zeta potential of microsilica 971 as a
function of pH. A 10% microsilica slurry titrated
with HCl and with zeta potential being measured
using electro-acoustic techniques (Acoustosizer,
Collodial Dynamics)
pH
Microsilica and deflocculants
A refractory castable can be visualised as an
aggregate structure with voids filled by aqueous
slurry of microsilica, cement and other fines. The
negative charge of the microsilica makes it
vulnerable for “attack” by cations which causes
gelling or solidification of the system. The
concentrations necessary for gelling are depending
on type of cation but a quick calculation of the
number of sites (silanol groups) per gram
microsilica estimates that approximately
0.15mmol/g microsilica could be sufficient for a
full coverage. This quantity is for some less pure
microsilicas available as water soluble impurities in
the microsilica itself, and an autogenous gelling of
such a slurry (e.g. 50%) may occur. Although any
cation can be used for gelling microsilica slurry, the
characteristics of the resultant slurry are dependent
on the charge and size of the cation that attaches. In
the case of monovalent cations, the gelling action
has been attributed to a mechanism where it’s the
hydrated cations that link adjacent silica particles.
Such a gel normally feels quite “soft” and easy to
liquify by agitation, while that from polyvalent
cations tend to have a more “brittle” and “harder”
appearance. With polyvalent cations, it is easy to
imagine that the excess positive charge of the cation
links with another negative charge on an adjacent
microsilica particle. In this way three-dimensional
networks may form and this is what we normally
regard as gelled microsilica.
In castables, enough cations are generally available
from the cement and aggregates, and it is likely that
the initial setting of most castables is closely related
to gelling of the microsilica slurry. In the case of
ultralow and no-cement systems with microsilica, it
is probable that most of the setting is dependent of
this solidification although it is often triggered by
small amounts of additives that promote the gelling.
Such additives can e.g. be small quantities of
cement.
The absorption of polyvalent cations on the silanol
sites is most likely a dynamic process and requires
a certain concentration in the water phase before
absorption gets significant. The amounts of soluble
cations necessary for gelation are therefore
probably higher than just to attach to the silanol
sites, but it seems reasonable to expect that the
order of magnitude may be similar.
The tendency to react with cations to produce a
more or less rigid gel, creates problems if used in

densely packed castable systems where cement is
present. The calcium and aluminate cations from
the cement dissolve and attack the otherwise fluid
bond phase. To prevent this coagulation, the
calcium ions have to be prevented from absorbing
on the microsilica. This may either be done by
having a pH below 5, but as this may affect setting;
addition of a proper amount of a polyelectrolyte or
a surface active agent is a common stratagem. The
surface active agent, normally termed deflocculant
(in Portland cement based systems termed
plasticiser or superplasticiser), is believed to absorb
to the surfaces thus preventing the absorption of
calcium but it also creates an equal, normally
negative charge of the particles of the bond system.
As the particles then will repel each other, casting
can take place at reduced water additions.
Deflocculants commonly used are polyphosphates,
e.g. sodium hexametaphosphate (Calgon) at
approximately 0.2 wt% addition level, and
polyacrylates (Darvan 811D, 0.05 wt %).
The deflocculating mechanisms involved falls
normally into two main groups or combination
thereof:
1) Steric stabilisation, the deflocculant attaches to
the fines and the size of it prevents
agglomeration of particles
2) Electrostatic stabilisation, the deflocculant
creates an equal electrical charge on the surface
of the fines. Electrostatic repulsion prevents
particles from agglomeration.
3) Combination of 1) and 2):
The recent years a new group of deflocculants have
emerged on the market containing both steric
groups and electrical charges which gives a
“double” deflocculating effect (typically Castament
FS20 from BASF). Below, in Figure 3, the structure
of such a deflocculant is sketched. And in Figure 4
the deflocculating action is visualised.
Figure 3. Structure of a combined steric and electrostatic deflocculant. A typically polycarboxylate “backbone”
with polyether sidechains.
Figure 4.Visualisation of the deflocculating mechanism of a compound like the one presented in Figure 3.

The use of deflocculants enables us to maintain
flow of refractory castables for some time,
sufficient to place the castable before setting
(gelling) commences. Sometimes setting may get
adversely affected though, giving unacceptably
long or short set times. This has been explained
with several mechanisms. For long set time with
polycarboxylate ethers, one explanation is that the
surface of the cement get “shielded” by the side
chains and that longer side chains consequently
retard more than shorter 2 .
If deflocculants are added to microsilica slurry its
effect can be followed by measuring the resulting
Zeta-potential of the microsilica.
Castament FS 20 at 0.05% addition levels
(approximately 0.3 mg/m 2 ref Fig. 6 and 7)
Table 1 Standard LCC castable composition.
[wt%]
Microsilica (18 or 24 m 2 /g) 8
CA-14 [wt%] 6
White Fused Alumina. 3-5mm 10
White Fused Alumina. 0,5-3 mm 32
White Fused Alumina. 0-0,5mm 16
White Fused Alumina. -74 micron 12
Calcined alumina: CT 9FG 16
Deflocculant: Castament FS20 0,01-0,1*
Water [wt%] 4.15
* Standard dosage, 0.05%
Zeta-potential (mV)
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
Surfactant content (mg/m2)
FS20
Darvan 811D
Calgon
Figure 6. Zeta potential of microsilica 971 as a
function of deflocculant addition represented as
addition per surface area.
In Figure 6 the effect of deflocculant additions to a
10% microsilica (971) on zeta-potential is shown
for three different deflocculants. Calgon is a
sodium polyphosphate, Darvan is a sodium
polyacrylate and FS20 is polycarboxylate ether as
described above. For both Darvan 811D and
Calgon, additions to the microsilica slurry result in
an increased negative surface charge, zeta potential.
This increased zeta-potential strengthens repulsion
between particles and stabilise the slurry. For the
combined deflocculant FS20, the electrostatic effect
is less pronounced but this was expected as the
steric contribution is considerable in this type of
deflocculants. Based on the zeta potentials alone,
without taking the steric factor into consideration,
one could expect that the Darvan would give best
flow when used in a castable and that the
Castament would show much poorer performance.
This is not the case however; in real life
performance is quite similar for the Darvan and the
To check the effect of the Castament FS20 on flow
of a castable, we employed a standard LCC mix
(Table 1) used for testing in our (Elkem)
laboratories, where the amount of deflocculant was
taken as the variable. Two different microsilica
qualities with surface area of 18 and 24m 2 /g were
used. In Table 1, the composition of this mix is
given. In Figure 7, the resultant self-flow values are
shown as a function of deflocculant addition per
microsilica surface area (mg/m 2 ).
The two microsilica qualities gives very similar
results when expressed as a function of surface
area, only the ultimate self flow differs somewhat.
Although the lower self flow was obtained with the
microsilica with the higher surface area, and
conclusions in the direction of lower flow at higher
surface area would be tempting, we do not have
data proving that this actually is the case.
Self-flow (%)
100
90
80
70
60
50
40
30
20
10
0
24 m 2 /g
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
FS20 dosage (mg/m 2 )
18 m 2 /g
Figure 7. Self flow of white fused alumina based
LCC as a function of deflocculant addition.
Apart from the flow, the set is the other factor of
high significance when producing and using

efractory castables. As mentioned earlier in this
paper, set can be strongly influenced by additives,
typically by extending set time to unacceptable
levels. One example is Calgon, that according to
the zeta-potential measurements of Figure 6 should
be added at approximately 0.4mg/m 2 level (approx
0.064% in castable). In this case, flow gets
excellent, but the set time tends to change to a
castable with too long set time. Since set time
reduces by an increase in the Calgon content, it has
in our laboratories been found more practical to use
Calgon in levels of around 1 mg/m 2 (0.16%). In this
way the set-time gets reduced to acceptable levels.
The flow suffers so what we get is a castable
without the same high flow, but one that sets within
reasonable time.
Set of microsilica containing castables
and the heat evolution test. The heat evolution test
can, as opposed to the Vicat needle, also give
reliable results when applied on castables.
Time
TTM
TTS
START
Figure 8. Typical Strip-chart print out showing
temperature in a microsilica containing CA-cement
based refractory castable.
Up to the first half of the 1980’ies, the common
way of determining set time of refractory castables
was by assessing set with the use of a Vicat-needle
set up. This had a number of difficulties, of which
the labour and lack of precision when used on
castable systems was the most important.
In 1984, Fentiman, George and Montgomery 3 , from
the cement producer Lafarge did investigation
correlating results from Vicat measurements with
temperature development of castables.
Instead of using a Vicat needle (penetration of a
needle into fresh mortar) the temperature of the
castable (or mortar) is recorded. Adiabatic
calorimetric studies of aluminous cement in contact
with water have shown that there are two
exotherms. The first occurs immediately and is
associated with heat of wetting and rapid
dissolution. A dormant period then follows during
which hydrate nuclei form and develop. Once
critical nuclei have been formed, bulk precipitation
of hydrate occurs (unless nodule formation, see
above) giving rise to a second exotherm 4 . This
second exothermic reaction may be used to
determine when the setting starts by simply
embedding a thermocouple into a proper amount of
fresh castable or mortar. If the thermocouple is
connected to a strip-chart recorder, the exotherm
manifests as temperature increase. The time to
initial set may thus be measured. The time to
temperature increase correlates well with data
obtained by the Vicat needle. Figure 9 shows a
typical exotherm and the correlation between Vicat
Figure 9.Correlation between Vicat initial set and
results from heat evolution test for calcium
aluminate mortar.
For castables with both microsilica and CA-cement,
the castable normally shows an additional
exotherm, which has been associated with the
formation of a layer of CASH compounds on the
surface of the cement particles that dissolves and
enables setting to proceed, giving the final
exotherm 5 . Usually, the appearance of the first
exotherm (TTS) is associated with the end of
workability of the castable, and the maximum
temperature of the second exotherm (TTM) is
regarded as the time to final set. The temperature
method has been found easy and reliable, but is not
suited for castables with very low cement content
because of the small amount of heat liberated. The
minimum cement content for reliable

measurements may be around 3-5% depending on
sample size and apparatus.
At UNITECR 2005 in Orlando, Myhre 6 presented a
study in which some interesting correlations
between microsilica and set time were shown. The
system investigated was based on the standard
castable composition given in Table 1, with the
microsilica quality as the variable. All microsilica
samples came from the same source, but taken at
different occasions during a period of a few months
for two successive years. In this context it must
however be reminded that these results are taken to
show that unless microsilica production is closely
controlled, such quality variations should be
expected.
TTS [h:min]
36:00 12:00
30:00 6:00
24:00 0:00
18:00
12:00
2004
2005
and that is also the case for Parr et. al.’s data given
in Figure 12. There are indications however that the
duration of this temperature plateau is not as
perfectly stable as Figure 11 could indicate. Both
cement quality and type and dispersant system and
microsilica quality may have influence. It must be
admitted however that the major influence of
microsilica in setting seems to be on the time to
reach TTS. Any influence on the time between TTS
and TTM could in most cases be attributed to lack
of precision in the method.
TTM [h:min]
36:00 12:00
30:00 6:00
24:00 0:00
18:00
12:00
6:00
2004
2005
0:00
0:00 4:00 8:00 12:00 16:00 20:00 24:00 0:00 28:00 4:00 32:00 8:00 12:00 36:00
TTS [h:min]
6:00
0:00
24/1 3/2 13/2 23/2 4/3 14/3 24/3
Sampling date
Figure 11. Correlation between initial set (TTS) and
final set (TTM).
Figure 10. Initial set as a function of production
date for microsilica from one silicon furnace during
winter/spring 2004 and 2005.
From the presentation in 2005, two important
observations may be drawn: 1) set time is
notoriously variable unless the microsilica
production is steered, and 2) there seems to be a
close relation between initial (TTS) and final set
(TTM) at least in the present castable mix. Figure
10 shows the initial set as a function of production
date, and Figure 11 shows the correlation between
TTS and TTM for these castables.
Recently, a comprehensive paper on setting of
microsilica containing castables was presented
giving more information about this topic 7 .
In Figure 12, some of their measurements are
schematically shown, verifying that TTS indeed
can be regarded as initial set.
For the data as given in Figure 10 and 11, the time
between TTS and TTM is approximately 6 hours
Figure 12. Comparison of ultrasonic, exothermic
and conductivity curves for a microsilica containing
LCC mortar at 1.44 water/cement ratio (ref. 7)
Parr et al. attributed the working time (TTS) to be a
result of complexing of calcium with the sodium
Tri Poly-Phospate (TPP) and that silica played a
role as retarder for that complexing.
This mechanism may be true for the castables
investigated by Parr et. al. but the present castables
(Figure 10 and 11) have no phosphate additions.
Dependence on phosphate must therefore be on
phosphate as an impurity in the microsilica.

Microsilica often contains phosphate impurities in
the level 0.1wt% but as the amount of microsilica
rarely exceeds 5-10%, the contribution would be
approximately 10% of that from the phosphate
dispersant. As a consequence, in systems lacking
the phosphate dispersant, other mechanisms should
be sought.
After the initial set, strength develops, but it is not
until after the maximum temperature (TTM) at 8
hours that significant improvement in combined
water is found. In Figure 13, DTA curves of the
LCC mortar of Parr et al. is shown for samples
stopped at different curing durations. At 8 hours the
maximum temperature was reached, and
interestingly, no more combined water was found
than after initial set. It is between 8 and 24 hours
that combined water develops giving the hydraulic
phases that are responsible for strength.
after a given time. The samples were then placed in
a heating cabinet held at 110°C to stop setting by
evaporation of the water. Probably, the setting is
accelerated by the increased temperature during
drying, but it was considered closer to reality than
stopping reaction by other means.
CMOR[ %]
120.00
100.00
80.00
60.00
TTS
TTM
TTS(citric acid)
40.00
110°C
600°C
1000°C
20.00
110°C (0.02% Citric acid)
600°C (0.02% Citric acid)
1000°C (0.02% Citric acid)
0.00
0 5 10 15 20 25 30
Time
TTM (citric acid)
Figure 14. Strength(C-MOR) of LCC (ref Table 1)
as a function of curing time and firing (12hours at
temperature). Samples interrupted after various
curing times. Elkem Microsilica 971, lot 3046663.
0.02% citric acid was added as retarder for one
serie. TTS and TTM indicated by “ovals”.
Figure 13. DTA curves of LCC mortar during the
hardening phase (ref. 7)
In Figure 14, cold Modulus of rupture is shown for
two series of LCC castables as a function of curing
time (unpublished work). The resulting strength is
here expressed as percentage of the 24hours
strength for two series of castables based on the
same recipe with the same raw materials but one
series retarded with an addition of 0.02% citric
acid. This resulted in an increase of the setting as
expressed by TTS from 2 to 11 hours. The time
from TTS to TTM also increased from
approximately 6 to 9 hours. It should be noted
however that the precision of the readings of the
retarded castables is not as good as the reference,
primarily because the temperature response on
readout becomes diffuse at long set times. Despite
this uncertainty, this might be an indication of a
possible influence from citric acid. The sample
preparation was mixing and casting sample bars
(40x40x160mm) and demoulding was performed
The results presented in Figure 14 are interesting
due to at least two aspects. One is that one does not
get maximum strength unless TTM is reached, the
second is that firing at temperatures up to 1000°C,
does not change the general trend.
Conclusion.
The surface properties of microsilica and notably
the silanol groups have a many-facetted effect of
the properties of low cement castables. Particular in
flow and setting the use of additives becomes
important to prevent some of the effects induced by
the dissociation and reaction of the silanol groups
when microsilica is used in cement containing
refractory castables. Also the setting time is critical
as expressed ion the last figure, Figure 14.
Strength of a castable is related to the temperature
development and it is unlikely that optimal strength
will be gained unless the castable is allowed to
attain TTM. If the castable is demoulded and dried
before TTM is reached, the strength will be lower.

Or, expressed slightly differently, since the
temperature development is highly dependant on
the microsilica, use of improper microsilica may
result in unexpected castable strength values. This
is the consequence of a too early (before TTM)
demoulding, induced by an unexpectedly long set
time caused by improper microsilica quality.
References:
1 R.K. Iler, “The chemistry of silica”, John Wiley
& Sons, New York 1979.
2 H. Hommer and K. Wutz:” Recent Developments
in Deflocculants for Castables” in proc
UNITECR’05, Am. Ceram Soc. p.186-190
3 C.H. Fentiman et. al in " Advances in Ceramics
vol. 13, New Developments in Monolithic
Refractories" ed. by Robert E. Fisher, The Am.
Ceram. Soc. Inc. Columbus, Ohio 1985 p. 131-
135
4 L. S. Wells and E. T. Carlson; J. Res. Natl. Bur.
Stand., RP2723, 57, 335 (1956)
5 Steffen Möhmel: in “ Die Reaktionen von
Calciumaluminaten bei Hydratation und
thermischer Belastung – Beiträge zur Chemie der
Hochtonerdezemente” dr. ing thesis Tech. Univ.
Freiberg Germany 2003.
6 B.Myhre, ” Microsilica in Refractory Castables-
How does Microsilica Quality influence
performance” in proc. UNITECR 2005 Orlando
USA Nov. 9, 2005
7 C. Parr et al. ”Investigations Into the Interactions
of Deflocculated Castables Which Control
Placing and Hardening Properties, Part 1: Placing
Properties” Ref. Appl. Trans. Vol 4 [2] 2-8