The and CaO.MgO.SiO2 (Monticellite) phases, physical and mechanical

The influence
of silica nanoparticles addition on the physical, mechanical, thermo-mechanical
as well as microstructure of Mag-Dol refractory composites

Gheisari Dehsheikh1, Salman Ghasemi-Kahrizsangi*2

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Department of Mechanical Engineering, Khomeinishahr
Branch, Islamic Azad University, Khomeinishahr, Isfahan, Iran

          2- Department of
Materials Science and Engineering, Sharif University of Technology, Tehran, Iran.

Author, Tel: +98 9137541686    , E-mail
address: [email protected]


high hydration potential of CaO and MgO phases restricted the application of Mag-Dol
refractory composites. In this study, the impact of nano-silica (SiO2) addition on the physical, mechanical,
thermo-mechanical as well as microstructure of Mag-Dol refractory composites is
investigated. Mag-Dol
compositions were prepared by using calcined dolomite and magnesite particles
(micron, 0-1, 1-3, 3-5, and 5-8 mm), liquid resin, and 0, 0.5, 1, 1.5, 2, and
2.5 wt% nano SiO2 as additives.
Specimens were heated up to 1650?C for the 3h soaking
period. Fired specimens were characterized by physical (apparent porosity, bulk
density, and hydration resistance), mechanical (cold crushing strength), and
thermo-mechanical (flexural strength at 1200?C) measurements. XRD and
SEM/EDS analysis were done to study phases and microstructure analysis of the
fired samples, respectively. Results showed that by adding up to 2.5 wt% nano-SiO2, due to the formation of CaO.MgO.2SiO2 (Diopside), 2CaO.MgO.2SiO2 (Akermanite),
and CaO.MgO.SiO2 (Monticellite) phases, physical and
mechanical properties were enhanced. But the highest flexural strength value is
achieved for 1 wt% nano-SiO2 containing sample.

Mag-Dol Refractory; Nano-SiO2;
Microstructural; XRD.





Mag-Dol (or Magnesite-Dolomite) refractory composites are widely
used in the industries such as secondary metallurgy
Argon Oxygen Decarburization
(AOD), Vacuum Oxygen Decarburization
(VOD, non-ferrous kilns (copper
converter), and also cement, lime, and glass making furnaces 1-4. High
attention to the application of Mag-Dol refractory composites is for
their desirable properties such as high corrosion and erosion resistance (in
alkaline environment), high melting point (Tm>2200), affordable (the low cost of the raw materials), low
thermal expansion, acceptable thermal shock resistance, and ability to generate clean steel melt1, 5-7. Conventionally,
Mag-Dol refractory composites are made of about 50-80 wt% magnesia, 2.5-5 wt. %
binder (resin, peck, tar, paraffin or other organic binders), which is utilized
in order to create a strong linkage among the matrix and aggregates, and
0.25-0.5 wt% hardener (hexamine) 2-4, 8-10. Various methods have been
mentioned to create Mag-Dol refractory composites. For example, some sources have been suggested using of fired or fused Co-clinker of Mg (CO3)2
and Mg.Ca (CO3)2 as a starting material for producing Mag-Dol refractory composites, which it
would result in more homogenous composite with high favorable properties. Other
suggested method is mixing Mg (CO3)2 and Mg.Ca (CO3)2
ores and calcined them at high temperature (more than 1550?C) that lead to generate in–situ Mag-Dol refractory composites 1-3, 11, and
12. Also, Mag-Dol refractory composites have
some benefits compared to magnesite and calcia-based refractory composites (Table 1). As well
as recently, Mag-Dol refractory composites are used as substitutes for
magnesia-chromite and magnesia-spinel refractory composites in the various industries
1-3. In spite of the high mentioned benefits
properties, these refractory composites have low hydration resistance in the
atmosphere. CaO and MgO phases quickly react with humidity in the atmosphere
and generate CaO (OH) 2 and Mg (OH) 2 phases (Equation. 1
and 2).  The volume expansion (?V=15-20%)
of the created phase CaO (OH) 2 and Mg
(OH) 2, lead to the destruction of these refractory composites (Fig.1) 1-7, 13-16.



CaO + H2O = Ca (OH) 2                           Eq 🙁 1)                       1, 2 and 5  
MgO+H2O = Mg (OH) 2                                   Eq 🙁 2)                              1, 2 and 5

For these reason, several research has been done to enhance the
hydration resistance of Mag-Dol refractory composites. For example, it
suggested that application of organic binders such as peck, tar, and etc. can improve
the hydration resistance of Mag-Dol refractory composites. This method is not
avowed as it lead to released mono-oxide carbon (CO)
and di-oxide carbon (CO2) gases into the atmosphere and polluting it
1-3, 5-9. Another suggested way is to
treading mag-dol refractory composites in a CO2 space or coating
their surface by phosphate, which leads to the formation of a dense
layer on the surface of CaO and protects CaO grain from hydration. This method
is also not economically desirable 1, 17. Another method that has recently been highly used by the
manufacturers of this refractory composites is the use of oxide compounds such
as TiO21, 2, Fe2O31, 3-7, Al2O33,
8, 9, Cr2O33,10, ZrO21,7, 11, 12, CuO13,
V2O514, FeTiO315, MgAl2O416,
ZrSiO417, and etc. Although use of the aforementioned oxide compounds have some
positive results, but application of them generated some restriction such as
high cost, decreasing refractoriness, and not available and etc 1, 3-8. Recently,
the use of nano-scale additives has attracted the attention of manufacturers in
many industries due to their excellent, singular, and unique properties 18-21.
 On the other hand, silica (SiO2) is used extensively in refractory
industry due to its high refractoriness (Tm>1700?C), availability and reasonable prices 1-3. According
to above mentioned, in this research study, the addition effect of nano-SiO2
as an inexpensive and affordable additive, on the physical, mechanical, thermo-mechanical as well as microstructure
of Mag-Dol refractory composites was evaluated.




Experimental procedure:

Materials (raw materials, binder, additive, and hardener)

dolomite and magnesite (extracted from Zefreh and
Birjand mines in Iran, respectively, Table 2)
with size range micron, 0-1, 1-3, 3-5, and 5-8 mm were used as the starting
materials. Also, Nano-SiO2 (supplier: Sigma-Aldrich, CAS Number 112945-52-5, Table 3, Fig.2), liquid resin (Table 4), and hexamine (supplier: Kanoria Chemicals & Industries Ltd.) were utilized in this research as an
additive, binder, and hardened, respectively.


2.1.2.   Compositions preparations:

batch compositions were formulated according to the following equation:

47 wt%
Magnesite + (53-X) wt% Dolomite + X wt. % Nano-SiO2                  Eq 🙁 3)

X = 0,
0.5, 1, 1.5, 2, 2.5 wt. %

As well
as batch codes, mixing times, mixing order, and other characterizes of
compositions preparation are presented in Table 5
and 6. Then the refractory compositions were uniaxial pressed (at
150 MPa, SACMI Model, Italy) in the shape of cylindrical whose dimensions were:
50mm* 50mm.


2.1.3.   Aging, tempering and firing processes:

composition were aged for 12 h at the air atmosphere then were thermal
treatment at 240?C for 12 h, and finally fired up to 1650 ?C
according to following diagram program (Fig. 3).


Characterizes measurement:

(bulk density, apparent porosity, and hydration resistance), mechanical (cold
crushing strength), and thermo-mechanical (flexural strength at 1200?C)
properties of the fired specimens were measured according to the following
standard methods. Also, presented values for each test are the average of 5
determinations for each refractory composite.



density and  apparent porosity: ASTM C-20

Bulk Density (g/cm3) = (M1-M2)/M3                                            Eq:


Apparent Porosity (%) = (M2-M3/M2-M1)*100                              Eq 🙁 5)

M1=initial weigh

M2= Saturation weight

M3= Immersion

Hydration resistance: ASTM C456 – 13 

Hydration Resistance (%) =

                                       Eq: (6)

W2= mass gain after hydration resistance test.

W1= initial mass gin before hydration resistance test.

Cold crushing strength: ASTM C133 – 97


strength at 1200: ASTM D790


Microstructure and phase analysis:

Scanning electron microscopy (model Philips XL30 TMP) with attached
energy dispersive analysis (EDS) analysis was performed for microstructure
evaluation of the fired samples. Also, for crystalline phases analysis of fired
samples, the X-ray diffraction (XRD analysis) using a Ni-filtered Cu Ka
radiation with a scanning speed of 28 (2u) per minute was used.







Results and Discussion:

Crystalline phases analysis:

Figs.4-5 show the X-ray diffraction patterns (XRD) of the MDS0,
MDS1 and MDS2.5 refractory compositions after
firing at 1650?C for
3h. Magnesia (MgO), calcia(CaO), and calcinum
hydroxide(Ca(OH)2 phases were detected for MDS0
composition. The presence of calcinum hydroxide (Ca(OH)2  phase indicates the high tendency of this
sample to be hydrated. Also, rather than magnesia, calcia; CaO.MgO.2SiO2
(Diopside), 2CaO.MgO.2SiO2
(Akermanite), and CaO.MgO.SiO2 (Monticellite) phases were the main identified phases for MDS1 and
MDS2.5 compositions. As it can be seen, by increasing the
nano-silica content, the peaks intensity for magnesia (MgO) and calcia (CaO)
are diminished, and also calcinum hydroxide (Ca(OH)2 peaks not
detected.  As well as the peaks intensity
of the CaO.MgO.2SiO2
(Diopside), 2CaO.MgO.2SiO2
(Akermanite), and CaO.MgO.SiO2 (Monticellite) phases are promoted. The melting point of the CaO.MgO.2SiO2 (Diopside),
2CaO.MgO.2SiO2 (Akermanite), and CaO.MgO.SiO2
(Monticellite) and phase are 1391?C, 1454?C and 1503?C respectively. Formation of the aforesaid phase at the firing temperature
(1650?C) leads to the covering grain, grain boundaries, and triple points and enhancing the firing process. Also,
raising additive content helps to the formation more liquid phases between the
main particles. Thus the wettability of CaO and MgO particles increases and leads
to grain growth via solution and precipitation.


Microstructure analysis(SEM/EDX)

Fig. 6a-c reveals the
microstructure images relating to the compositions without and with the
addition of nano-silica. In addition of 
porosities and voids, a light gray phase relating to CaO (calcia) and dark
gray phase relating to MgO (magnesia) particles were marked by energy
dispersive X-ray (EDX) (Fig.6a, Table 7) for MDS0 composition. With nano-silica
addition (Fig.6b-c.), the generation of phases with Si, Ca, and Mg elements
was observed. These phase have low melting point (lower than 1520?C).  As it can
see a homogeneous and dense microstructure with low apparent porosity were
generated by increasing nano-silica content. By using EDX analysis, generation
of CaO.MgO.2SiO2 (Diopside), 2CaO.MgO.2SiO2 (Akermanite),
and CaO.MgO.SiO2 (Monticellite) were confirmed (Table 7).


Densification parameters i.e. bulk density (BD) and apparent
porosity (AP) of the Mag-Dol refractory composites fired for 3 h at 1650 ?C with varying nano-SiO2 content are shown in Figs.7 and 8.  A gradual
enhancement is showed in density value with adding nano-SiO2 content
up to 1.5wt%. Also, for 2.5 wt% of nano-SiO2 content there is
diminished in density value, which it is related to the weak distribution of
SiO2 nano-particles and the formation of accumulating in the body.
Apparent porosity variation (see Fig.8) opposed to bulk density has
changed. The lowest and highest apparent
porosity values (7.54 and 5.34%) related to the specimens with 2.5 wt% MDS2.5
and MDS0 compositions, respectively. Factors such as a good compression of the
matrix on filling up of the pores among the calcia and magnesia
particles, (ii) more complete firing process of the compositions due to the
existence of   active nano-SiO2
particles, (iii) generation of low melting phase such as CaO.MgO.2SiO2, 2CaO.MgO.2SiO2, and CaO.MgO.SiO2
which leads to filling void and porosities in
the matrix and create a high strength connection between main constitution
particles i.e. magnesia and calcia. 


Cold crushing strength (CCS):

The results of the cold crushing strength test
of the fired compositions (at 1650?C for 3h) are shown in Fig. 9.  As can be seen, the cold crushing strength
changes trend of the samples has been progressed by increasing the amount of
SiO2 nano-particles. According to the previous reports, existence
of porosities, cavities and grain boundaries in the specimen’s matrix can leads
to the loss of strength. On the other
hand, the formation of the low melting point phase at the firing process
temperature can result in the filling of the porosities and cavities, the reduction of
grain boundaries, and also the growth of the main grain in the samples matrix.
Based on the phase’s analysis results (see Figs.4-5),
the formation of aforesaid low melting phases lead to the formation of a dense
body by removing porosities and cavities, as well as the growth of grains in
samples containing nano-silica. Ultimately, this has led to an increase in the
cold crushing strength of the samples.

Flexural strength at 1200?C:

The flexural strength (at 1200?C)
change trends of the fired (at1650 ?C for
3h) Mag-Dol refractories composites are depicted in Fig.10. As it can be observed, by increasing nano-SiO2
content (up to 1 wt %) the flexural strength values enhanced and the highest
value is related to MDS1composition (318kg/cm2).  But after it (from 1.5 to 2.5 wt. %), the
flexural strength gradually decreased (reached to284kg/cm2). The
main important factors that contribute to this initial increase are: (a) the
dense (compressed) created body by the removal of porosities and cavities in
the microstructure and (b) the grain growth of the main constituent grain of
the body and the reduction of the grain boundaries in the microstructure. Also,
the secondary reduction of the flexural strength is due to the: formation of more quantities of
low melting point phases at such as (CaO.MgO.2SiO2,
2CaO.MgO.2SiO2, and CaO.MgO.SiO2) the firing
temperature.  Generally, in order to
access the highest flexural strength value in Mag-Dol refractory composite,
selecting 1wt % nano-SiO2 can be the optimal content.


Hydration resistance:

Composite compositions include the MgO, and in
particular CaO phases, have highly susceptible to be hydrated (high tendency to
absorb moisture) and reaction with moisture in the atmosphere. In these compounds, CaO and MgO phases will be converted to the
lower density phases Ca (OH) 2 and Mg (OH) 2. Formation
of Ca (OH) 2 and Mg (OH) 2 phases lead to a
volume expansion about 15-20%. The created volume expansion results in the cracking and collapse of
the compositions containing these phases (Fig.11).
For this purpose, in order to enhance the hydration resistance of Mag-Dol
composite materials, coating of magnesia (MgO) and calcia (CaO) phases or their
conversion into higher hydration resistance phases can be effective.  On the other hand, porosities, holes, grain
boundaries, and in general all defects in the microstructure (due to higher
surface energy) tend to absorb moisture and perform hydration reactions. As
shown in Fig.12, with
increasing nano-silica content in Mag- Dol compositions the percentage of the gain
weight (tendency to moisture absorb) have decreased. It indicates the hydration
resistance improvement of the specimens. By comparing the SEM images of the
samples after firing at 1650?C for
3 h (Fig.6A-C), it can be seen that the
grain growth of the particles, porosities and cavities filling (the reduction
of the free specific surface (, as well as the loss of the grain boundaries (which all of them
are susceptible sites to hydration), occur more for MDS2.5 composition
in comparison with other samples. These created denser and more uniform
microstructure is prevents for easier hydration reactions, and ultimately
improved the hydration resistance of samples including nano-silica as compared
to the non-additive sample.


In the present research, the addition effect of Nano-silica on the
physical, mechanical, thermo-mechanical as well as microstructure of Mag-Dol
refractory composites was evaluated and the following results concluded:

helps in densification process by liquid phase generation at the firing sintering
temperature (1650?C). A
maximum density of 3.36 g/cm3 is achieved with addition 2 wt. %

-growth of CaO and MgO particles occurred by adding Nano-silica which finally
lead to hydration resistance enhancement of Mag-Dol refractory composites.

cold crushing strength values of Nano-silica containing composition is for the
development of a strong and continuous bonding and connection in the matrix.

1 wt
% nano-silica was selected as the optimum content for access to the high
flexural strength value due to increased fired density of Mag-Dol compositions
and limited liquid phase generation.

 And generally, in order to improving the
function of the Mag-Dol refractory composition, addition of nano-silica could
be effected by nanotechnology. As the unique properties of the all nanoparticles
such as significant surface effect, size effect and higher activity, addition
of nano-silica was more useful.









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