RETRACTED ARTICLE: Low-Temperature Synthesis of Porous Materials from Mortar Sands
Low-Temperature Synthesis of Porous Materials from Mortar Sands
0 Kazmina , Volland, and Dushkina
It is established that the eliminations of construction sand with the content of SiO2 about 70 wt.% and particle size less than 60 lm are suitable for the production of a foam-glass-crystal material on the basis of the low-temperature frit, which was synthesized at the temperature of 900 C. The obtained foam-glass-crystal material exceeds foam-glass (by 3.0 times) and clayite (by 1.5 times) by strength and is characterized by the low value of water absorption (0.1%).
The production of lightweight and efficient
building materials, in particular of lightweight
granulated materials, is a constant and modern
subject of scientific investigations. One of the
highefficiency heat-insulating materials that meet the
requirements of environmental safety is foam-glass.
Raw material for production of foamed glass can
serve various types of waste. Processing of
industrial waste is an important topic not only from the
point of view of reducing dangerous environmental
pollution but also in view of their potential
beneficial use as an alternative source of raw materials.
Therefore, as initial raw materials for producing
heat-insulating materials, the preference is given to
technogenic and substandard raw materials. The
most frequently used lightweight aggregates are
made from expanded clay, shale, perlite, vermiculite
and different kinds of sintered waste. The
publications Refs. 1 and 2 suggest the use of building
wastes in the production of aggregates for concrete.
The results of studies in which concrete aggregates
were obtained from ash waste are presented in
Refs. 3–9. The authors of Ref. 10 suggest the
possibility of using sludge from a decorative quartz
industry hot bituminous mixtures.
When the temperatures necessary to reach the
pyroplastical state of the melt and active formation of a
gas phase coincide, the mix foams and stabilizes with
the subsequent decrease of temperature. The high
dispersity of sand sludge allows the expectation of the
possibility of a low-temperature production of the frit.
The results of preliminary studies are encouraging,
since they confirm the possibility of using this type of
raw material for the production of lightweight
granulated material, that can be used as an aggregate for
concrete and as a heat insulation filler.
The aim of this study is to investigate the
possibility of the preparation of foam material similar to
foam-glass on the basis of frit synthesized from
quartz sands screenings. Industrial technology of
foam-glass is based on the use of glass cullet or frit
with the composition of sheet and container glasses.
The high dispersiveness of quartz sand wastes
allows one to suppose the possibility of
low-temperature synthesis of frit. Therefore, one of the
assigned tasks of this research was the development
of blend compositions suitable to obtain frits at
temperatures not exceeding 900 C, excluding the
traditional glass-melting process.
When one selects the chemical composition of
lowtemperature frit, it is necessary to meet the following
requirements. The first condition which defines the
component composition of a blend is provision
sufficient quantity of glass former (60–75 mass%) and the
alkali metal oxides (13–22%). It is also necessary to
take into account that foaming occurs at a viscosity of
105–107 Pa s, and therefore it is necessary to use the
glasses which reach such a viscosity at temperatures
of 750–900 C. It is possible to estimate the
preliminary viscosity properties of the glass by its
composition using a viscosity modulus, the value of which
can be in the range 1.6–1.8.11 The second selection
condition is the formation of not less than 70% of
melting at a temperature not exceeding 900 C, which
has been shown by the results of previous
experiments.12 The third condition is the hydrolytic
stability of glass (not below the 3rd class) and the
presence of an active oxidizing component in
sufficient quantity to carry out foaming reactions, for
example, the presence of SO3 in quantities not less
than 0.2%. One of the main requirements for all
blend components are their dispersiveness; particle
size must not exceed 100 lm.
Glass composition corresponding to the
abovementioned requirements, selected by the state
diagram of Na2O-CaO-SiO2, has the following content
of components: Na2O,14 mass%; CaO, 13 mass%;
and SiO2, 73 mass%. The composition of sand under
investigation according to the results of chemical
analysis is presented by a relatively low quantity of
the main glass-forming oxide (SiO2) and a
sufficiently high quantity of the alkali metal oxides,
aluminas, and iron oxides (Table I). According to
the data of x-ray phase analysis, sand contains,
together with quartz and illite, tiff, plagioclase, and
feldspar in about equal proportions. The high
dispersiveness of sand is confirmed by the results of
screen analysis according which 50% of sand is
presented by a fraction of particles less than 60 lm
which meet the necessary requirements.
As a foaming agent in the production of
lightweight aggregate, we used carbon black type 220
(ASTM D1765). This carbon black represents
technically highly active carbon with a high dispersity
and good structural properties, obtained from
thermal-oxidative decomposition of liquid hydrocarbon
raw materials. The specific surface of carbon black
is of the order of 1.14 9 105 m2/kg.
RESULTS AND DISCUSSION
To obtain frit of the selected composition we
accordingly prepared a blend containing the sand
under investigation and afluxing addition in the
form of calcined soda in quantities of 80 and 20
mass%. The calculated composition of the glass had
following oxide content (in mass%): SiO2, 66.3;
Na2O, 13.4; CaO, 6.5; Al2O3, 6.8; Fe2O3, 3.7; K2O,
1.8; MgO, 0,8; MnO, 0.1; and TiO2, 0.5. The value of
the viscosity module, calculated according to the
formula, was 1.8, which falls within the
MB ¼ ðMSiO2 þ 2MAl2O3 Þ=ð2MFe2O3 þ MCaO þ MMgO
where MB is the viscosity module, and MRmOn is the
quantity of the corresponding oxides in mass%.
Preparation of foam crystalline material has been
carried out by the two-stage technology proposed in
Ref. 11. The first stage includes synthesis of frit
consisting of glass (not less than 70%) and a
residual crystalline phase (not more than 30%), which is
provided by the chemical composition of a blend.
The second stage includes preparation of a
foamforming blend from frit powder to obtain the
finished foam material.
The results of the differential-thermal analysis
(DTA) showed that, at the heating of the blend of
the selected composition up to 1000 C, endo-effects
were observed corresponding to the removal of
hygroscopic and crystallization water of the blend
(82 C and 245 C), polymorphic transitions of quartz
(571 C), and the melting of forming double salts and
eutectics (733 C and 797 C). The main mass losses
occur in the temperature range of 500–800 C,
corresponding to silication reactions. At the
temperature of 800 C, the thermogravimetric curve is
horizontal, which shows the total binding of sodium
carbonate and the completion of the silication
reaction according to Eq. 2.
nSiO2 þ Na2CO3 ! Na2O
It is stated by the data of thermal analysis that
thermal treatment of the blend under investigation
at temperatures of 800–900 C secures total
Chemical composition, content of oxides, mass%
Na2O + K2O
Rutile (anatase) Ferric hydroxide
Mineralogical structure, mass%
Granulometric structure, mass%
Fig. 1. X-ray diffraction pattern for: (a) sand sludge, (b) frit after 900 C, and (c) lightweight aggregate: Qz – Quartz; Pl – Plagioclase; Ca – Calcite;
NCS – sodium calcium silicate Na2CaSi3O8.
completion of the silication processes. To confirm this
supposition, we carried out x-ray phase analysis of
the frit prepared at temperatures of 800–900 C. It is
seen from the obtained results (Fig. 1), that an
amorphous halo indicating the presence of a glass
phase and reflection maxima responsible for the
crystalline phase were observed on all x-ray images.
The results of quantitative x-ray phase analysis show
that the frit consists of a glass phase in the range from
66% to 72%, while the remaining crystalline phase
presented the residual quartz (d = 3.342 nm;
2H = 26.7 ), feldspar and silicate, Na2OÆCaO3ÆSiO2.
Differential-thermal analysis of the frit showed
no exothermic effects either at heating up to
1000 C, or at cooling to 200 C, which confirms the
absence of crystallization processes. DTA of
foamforming mixtures prepared on the basis of frits
prepared at various temperatures (from 800 C to
900 C) with the addition of soot (0.5%) showed the
presence of exo- and endo-effects corresponding to
the processes of oxidation of gas-forming agents and
the melting of the glassy phase. An increase of frit
preparation temperature biases the exo-effect
temperature in the higher field, so for an admixture
from the frit (800 C), the effect occurs at 463.7 C,
while for the frit (900 C), the value is 498.6 C,
which is conditioned by differences of the phase
composition of the frits (Fig. 2). According to
quantitative x-ray analysis, when the sintering
temperature of the frit increased through 800 C, 825 C,
850 C to 900 C, the content of the crystalline phase
decreased through 34%, 32%, 29% to 27%.
Bias of oxidation temperature in the higher field is
favorable for the foaming process since the probability
of early burning out of the gas-forming agent
decreases. Total mass losses for frit (800 C) is 5 times more in
comparison with the foam-forming admixture
prepared from frit (900 C). In addition, thermograms of
admixtures of low-temperature frits (800 C and
825 C) show some endo-effects, while thermograms of
frits with temperatures of 850 C and 900 C show one
endo-effect which points to a large inhomogenity of
low-temperature frits and the presence of some phases
differing with melting temperature.
Foaming of pellets (d = 10 mm), prepared from
foam-forming admixtures (specific surface
6000 cm2/g), has been performed in the temperature
range of 900–975 C (with steps of 25 C) with
exposure at maximal temperature in 20 min. It has been
stated that foam material with a relatively uniform
fine porous structure is obtained at a foaming
temperature of 950 C. The macrostructure of the
samples obtained at 900–925 C has a dense crust with a
thickness more than 3 mm, and the samples
obtained at 975 C are deformed in consequence of
vitrification and show a random structure with the
presence of large pores with sizes more than 5 mm.
It has been shown by x-ray analysis data that if the
foaming temperature increases from 900 C to
975 C, the quantity of the crystalline phase in the
finished foam material decreases from 23% to 8%.
There is a connection between the foam material
macrostructure and the temperature of the
preparation of the frit, from which the foam-forming
Fig. 2. TG, DTA and DTG graphs of raw mix (with expanding agent) after preliminary fritting at different temperatures: (a) 800 C, (b) 825 C, (c)
850 C, and (d) 900 C.
admixture is obtained. Photographs of the
macrostructures of samples prepared when foaming at
950 C from frit synthesized at 800 C, 825 C, 850 C,
and 900 C are presented on Fig. 3. The foam
material obtained from the frit at 900 C is the most
optimal from the point of view of pore size and the
uniformity of their distribution.
Values of the main physical–mechanical
properties of samples prepared from frit synthesized at
various temperatures (Fig. 4) allow one to note the
following laws in changes of properties. If the
temperature of frit synthesis increases from 800 C to
900 C, the density of pellets decreased on an
average by two times, the strength by four times, and
water absorption by 13 times.
Comparing the characteristics of the samples
prepared on the basis of mortar sand screenings
with other heat insulating materials shows that the
material occupies an intermediate position between
foam-glass and expanded clay (Table II). The value
of the strength coefficient of the foam crystalline
material, which is the ratio of strength to density,
exceeds that of foam-glass and expanded clay. The
material differs with its high strength, low water
absorption and relatively low heat conduction
coefficient that allows the recommending of it as a
heatinsulating and construction material.
The main objective of the investigations described
in this report was to analyze the feasibility of using
waste from the sand industry in lightweight
aggregate production. After analyzing the properties of the
lightweight aggregates produced in the course of the
present studies, the following conclusions can be
1. Screenings of mortar sands with a content of
SiO2 of about 70 mass% and the size of particles
less than 60 lm are suitable for the preparation
of a foam crystalline material from
low-temperature frit synthesized at a temperature of 900 C.
The glass phase of the frit is not crystallized in
the range of foaming temperature, and has
sufficient viscosity and temperature of its
pyroplastic state coinciding with the temperature of
oxidation of the gas-forming agent.
2. If the synthesis temperature increases from
800 C to 900 C, the quantity of the glass phase
in the frit increases from 66% to 73%. The
crystalline phase of the frit is represented by
residual quartz and feldspar as well as by the
appearance of a new phase in the form of silicate
Na2OÆCaOÆ3SiO2. The content of the crystalline
Fig. 3. Macrostructure of the foam material received from frit, synthesized at: (a) 800 C, (b) 825 C, (c) 850 C, and (d) 900 C.
bulk density, kg/m3
average density, kg/m3
water absorption, mass %
compression strength, MPa
Fig. 4. Properties of the foam material received from frit, synthesized at: 800 C, 825 C, 850 C, and 900 C.
Bulk density compression in the
(kg/m3) cylinder (MPa)
Heat conduction absorption Strength
coefficient (W/m K) volume (mass%) coefficient
phase in the finished foam material decreases
from 23% to 8% in dependence on the increase of
the foaming temperature from 900 C to 975 C.
3. Frit synthesized at 900 C and a foaming tem
perature 950 C is optimal for the preparation of
foam material with improved properties. The
prepared foam crystalline material has a higher
strength up to 5.5 MPa at a bulk density of
450 kg/m3 and low water absorption not
exceeding 1 mass%.
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