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Abstract: Water treatment plant (WTP) sludge is actively used in building materials production. The object of this research was modifying additives for ceramic bricks from WTP aluminium-containing sludge. The research aim of this study was to determine the suitability of a million-plus population city’s WTP sludge as a burning-out additive in the production of structural ceramics and to establish the optimal conditions for obtaining products with the best characteristics. The incoming water to the WTP water belongs to the hydrocarbonate class, the calcium group, and it is of low turbidity (1.5–40 mg/L kaolin). Sludge, sourced from WTP sedimentation tanks, was dewatered by adding lime or by using the freezing-thawing method. The spray-dried WTP sludge is introduced into the clay in amounts of 5% to 20% by weight. The addition of 20% reduces the sensitivity of the clay to drying, reduces the density of ceramic by 20% and simultaneously increases its compressive strength from 7.0 to 10.2 MPa. The use of WTP sludge as a modifying additive, pretreated by the freezing-thawing method, makes it possible to obtain ceramic bricks with improved properties. The results can be used for WTP sludge containing aluminium obtained by treating water of medium turbidity and medium colour. Keywords: ceramic bricks; ceramic sintering; air shrinkage; WTP sludge; water treatment plants
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materials
Article
Structural Ceramics Modified by Water Treatment
Plant Sludge
Alexander Orlov 1, Marina Belkanova 1and Nikolay Vatin 2, *
1Institute of Architecture and Construction, Russian Federation, South Ural State University,
454080 Chelyabinsk, Russia; orlovaa@susu.ru (A.O.); belkanovami@susu.ru (M.B.)
2Higher School of Industrial, Civil and Road Construction, Russian Federation,
Peter the Great St. Petersburg Polytechnic University, 195251 Petersburg, Russia
*Correspondence: vatin@mail.ru; Tel.: +7-921-964-3762
Received: 10 October 2020; Accepted: 16 November 2020; Published: 23 November 2020


Abstract:
Water treatment plant (WTP) sludge is actively used in building materials production.
The object of this research was modifying additives for ceramic bricks from WTP aluminium-containing
sludge. The research aim of this study was to determine the suitability of a million-plus population
city’s WTP sludge as a burning-out additive in the production of structural ceramics and to establish
the optimal conditions for obtaining products with the best characteristics. The raw water belongs to
water belongs to the hydrocarbonate class, the calcium group, and it is of low turbidity (1.5–40 mg/L
kaolin). Sludge, sourced from WTP sedimentation tanks, was dewatered by adding lime or by using
the freezing-thawing method. The spray-dried WTP sludge is introduced into the clay in amounts of
5% to 20% by weight. The addition of 20% reduces the sensitivity of the clay to drying, reduces the
density of ceramic by 20% and simultaneously increases its compressive strength from 7.0 to 10.2 MPa.
The use of WTP sludge as a modifying additive, pretreated by the freezing-thawing method, makes it
possible to obtain ceramic bricks with improved properties. The results can be used for WTP sludge
containing aluminium obtained by treating water of medium turbidity and medium colour.
Keywords: ceramic bricks; ceramic sintering; air shrinkage; WTP sludge; water treatment plants
1. Introduction
Water treatment plant (WTP) sludge is actively used in the production of building materials,
which reduces the consumption of natural resources, the environmental load and the cost of
building materials [
1
4
]. The use of WTP sludge in building materials could exclude expensive
and energy-intensive stages of sludge utilization. WTP sludge can be used in structural ceramic
production [
5
12
]. The volume of sludge to the volume of WTP treated water varies from 0.1% to 1%,
and in some cases, can reach 5% [
13
]. WTP sludge is formed when surface or ground water is clarified
in sedimentation and clarification tanks. Such sludge is a gelatinous mass, with a humidity of 95% to
99%, and contains mineral and organic substances, products of coagulant hydrolysis, phytoplankton
cells, and other components [14].
1.1. The Use of Water Treatment Plant Sludge in Building Materials Science
WTP sludge can be used in the production of building materials and products, such as ceramic bricks
and tiles, expanded clay, Portland cement clinker, and lightweight concrete aggregate
[2,3,6,7,1517]
.
For these purposes, the sludge is dewatered and dried to a constant weight aggregate [
2
,
3
,
6
,
7
,
15
17
].
The article [
16
] proposes introducing WTP sludge to the composition of Portland cement in the amount
of 4% to 7%. A further increase in the proportion of sludge sharply reduces the strength of the samples.
The author points out that, in the samples, there is no leaching of heavy metals that were initially
Materials 2020,13, 5293; doi:10.3390/ma13225293 www.mdpi.com/journal/materials
Materials 2020,13, 5293 2 of 11
present in the raw sludge, confirming the environmental friendliness of this method of sludge disposal.
According to [
16
], spray-dried sludge can be used in the amount of 12% in cement production, partially
replacing clay and limestone. A decrease in the compressive strength does not allow the use of WTP
sludge in the manufacture of structural concrete. More research is needed to assess the potential
negative impact of sulfate ions on the long-term eectiveness of cement materials made with WTP
sludge [
17
]. Sulfate ions lead to ettringite formation in concrete and, consequently, to a weakening of
the structural integrity and changes in mechanical properties over time [17].
There are some advantages in the use of WTP sludge for the production of structural ceramics.
First, it has a composition similar to clay in terms of inorganic oxides (oxides of aluminium, silicon,
iron). Secondly, potentially toxic elements are immobilized during heat treatment into forms that are
not prone to leaching [
17
,
18
]. Since WTP sludge contains a significant amount of finely dispersed
organic substances [
19
], it can be used as a burning-out additive for structural ceramics. Burning-out
additives increase the porosity and thermal resistance of ceramic bricks, the latter being one of their
most important characteristics. Studies [
20
,
21
] show that WTP sludge can be used as an additive to
reduce firing temperatures and as a red pigment. In [
22
], it was shown that the addition of WTP sludge
enhances clay sintering and leads to the formation of mullite at lower temperatures.
Recommendations for dosage of sludge in dierent publications dier significantly from each other.
On the one hand, the authors of [
18
] note a general decline of ceramic materials’ characteristics with
the addition of WTP sludge. Flexural strength, water absorption, linear firing shrinkage, and apparent
specific mass impair. Despite this, the authors recommend the addition of 10% WTP sludge for the
production of solid bricks that meet Brazilian national standards. In [
21
,
23
], it is also proposed to
add no more than 10% WTP sludge due to a decrease in the mechanical characteristics of products.
On the other hand, in [
23
] it is noted that the addition of 40% WTP sludge and 5% processed tea
waste allows to obtain fired clay bricks with improved thermal insulation properties and improved
compressive strength.
The use of sludge as a modifying additive requires its preliminary dewatering. WTP sludge is a
complex multicomponent system with a high content of coagulant hydrolysis products and a highly
developed surface. The humidity as a result of sludge pumping exceeds 95% [
18
,
24
]. Such sludge,
taking into account its hydroxide nature, cannot be filtered without preliminary treatment. The thawing
method is recommended to increase its water-carrying capacity, reagent treatment (lime, flocculant
and filler material) or freeze-thaw conditioning. After processing, the sludge is sent to filter presses for
mechanical dewatering.
The properties of WTP sludge must be consistent for using them as a raw material in the
construction industry. The quality of the water source is subject to seasonal fluctuations. Therefore,
the dose of coagulant and other reagents varies. This variation leads to a change in the composition
and the water-yielding capacity of WTP sludge. It is necessary to select an appropriate method to treat
the sludge before dewatering.
Dewatered WTP sludge can be used for the production of structural ceramics. It can reduce
the firing temperature and the density of the ceramics, i.e., work as a flux and burning-out additive.
However, there are no standard recommendations for the use of WTP sludge in the production of
structural ceramics in the literature.
1.2. Sludge Composition
Sludge obtained by the treatment of surface waters is classified according to the quality of the
water source [
13
]. To do this, the ratio of the water colour index to water turbidity (WCI/WT) is
determined. The higher this ratio, the more dicult it is to dewater the sludge, and additional treatment
is required before its dewatering. At a WCI/WT ratio of more than 30, the sludge is hardly dewatered.
Sludge contains free and bound water. The latter diers in its degree of boundedness: first, water
included in the composition of floccules; second, water bound to the particle surface by adsorbing and
adhesive forces; and, finally, chemically bound (hydrated) water [25,26].
Materials 2020,13, 5293 3 of 11
The technology of water treatment determines the composition and properties of WTP sludge, and
the reagents used [
14
,
17
,
27
]. In most cases, water is treated with coagulants, that is, hydrolyzable salts
of aluminium and iron. Therefore, it is common practice to distinguish sludge containing aluminium
and sludge containing iron [
14
,
27
29
]. Coagulant hydrolysis leads to a high aluminium or iron
hydroxide content. Due to its hydroxide nature, sludge has a low water-carrying capacity. The mineral
component of WTP sludge is close to the composition of clay [
15
,
16
,
30
33
] and includes compounds
of silicon, aluminium, iron, calcium and magnesium, sodium, and potassium [
15
]. In some cases,
manganese and titanium are present in the amount of less than 1%. There are also indications that
sludge contains zinc, cobalt, lead, cadmium, and nickel [34].
1.3. Aims and Objectives
There is no data in the publications on the influence of the preliminary preparation methods of
WTP sludge for dehydration on the properties of building ceramics.
Because of that, the study aims to evaluate the eect of the WTP sludge preparation method on
the properties of building ceramics. The research objectives are:
1. to choose the optimal method for preliminary sludge dewatering;
2. to establish the best method of sludge treatment on the properties of structural ceramics;
3. to determine the optimal amount of sludge introduced into the clay;
4.
to determine the most eective firing temperature and establish the properties of the resulting
ceramic bricks.
2. Materials and Methods
2.1. WTP Sludge Treatment
Aluminium-containing sludge from the WTP of Chelyabinsk, Russian Federation, was used in
experiments as a typical WTP sludge of a city with a million-plus population. The source of city
water supply is the Shershnevskoe Reservoir on the Miass River. In terms of its chemical composition,
the reservoir water belongs to the hydrocarbonate class, the calcium group, and it is of low turbidity
(1.5–40 mg/L kaolin). Depending on the season, it can be of medium and high colour (18–120 on Pt-Co
scale). The average WCI/WT ratio of the source water during the period of sludge formation is 3.3,
which makes it possible to classify the sludge by the quality of the source water as being of medium
turbidity and colour.
The choice of the optimal conditions for the sludge treatment was made according to the samples
obtained during the emptying of two-tier sedimentation tanks. If water treatment at the WTP uses
aluminium sulfate coagulants, aluminium oxychloride coagulants, and AN-905 flocculant based on
polyacrylamide, then the sludge classified as aluminium-containing.
Two methods were used to increase the water-yielding capacity of sludge. They are lime treatment
and freezing-thawing. Freezing was carried out at
16
±
2
C. Samples of sludge with a volume of
1–1.5 L were frozen for seven days. Thawing was carried out at 20 ±3C.
The lime treatment was carried out using powdered lime (PL) dosing and hydrated lime (HL)
dosing. PL dosing reduces the filtrate volume during sludge dewatering. Dry lime with 43% active
ingredient (CaO) was used. The lime was introduced in an amount of 15% to 40% of the dry sludge
matter and was mixed for 30 min.
An aqueous suspension of lime was prepared to treat water with HL slurry. The content of the
active part by CaO was determined. The hydrated lime was added at the rate of 10% to 15% of dry
sludge matter and was stirred for 10 min.
After treating the sludge with lime or by the freezing-thawing method, water was removed from
the sludge by vacuum pumping (67
±
5 kPa) in a laboratory installation (Figure 1). The cake was dried
to a constant weight at 105 ±2C in a drying oven.
Materials 2020,13, 5293 4 of 11
Materials 2020, 13, x FOR PEER REVIEW 4 of 12
Figure 1. Diagram of a laboratory installation for dewatering and determining the specific filtration
resistance. 1Buchner funnel; 2measuring cylinder; 3shut-off valve; 4receiver; 5vacuum
gauge; 6vacuum pump.
2.2. Experimental Testing
According to the Hartley method, two-factor experiment was carried out. Among varying
factors were the amount of additive as a percentage of the clay weight (D%), and the firing
temperature (TO, °C). The response indicators included plasticity number, shard density (ρ), air
shrinkage, and fire shrinkage. To obtain reliable results, it turned out that two repetitions of each
experiment were sufficient, while the experimental error was less than 5%. The properties of the clay
and ceramic shards were determined following the requirements of the regulatory documents listed
in Table 1.
Table 1. Testing methods.
Property
Regulatory Document
Plasticity number of clay mass
Russian State Standard GOST 21216-2014 [35] Clay raw materials.
Test methods, p. 5.3
Air shrinkage of clay sample
Russian State Standard GOST 21216-2014 [35] Clay raw materials.
Test methods, p. 5.26
Fire shrinkage
Russian State Standard GOST 21216-2014 [35] Clay raw materials.
Test methods, p. 5.27.4.3
Compressive strength of ceramic sample
Russian State Standard GOST 21216-2014 [35] Clay raw materials.
Test methods, p. 5.33.4
Sensitivity of clay sample to drying
Russian State Standard GOST 21216-2014 [35] Clay raw materials.
Test methods, p, п. 5.32
Density of ceramic shard
Russian State Standard GOST 7025-1991 [36] Ceramic and calcium
silicate bricks and stones. Methods for water absorption and
density determination and frost resistance control, p. 5
Figure 1.
Diagram of a laboratory installation for dewatering and determining the specific filtration
resistance. 1—Buchner funnel; 2—measuring cylinder; 3—shut-ovalve; 4—receiver; 5—vacuum
gauge; 6—vacuum pump.
2.2. Experimental Testing
According to the Hartley method, two-factor experiment was carried out. Among varying factors
were the amount of additive as a percentage of the clay weight (D%), and the firing temperature
(T
O
,
C). The response indicators included plasticity number, shard density (
ρ
), air shrinkage, and fire
shrinkage. To obtain reliable results, it turned out that two repetitions of each experiment were
sucient, while the experimental error was less than 5%. The properties of the clay and ceramic shards
were determined following the requirements of the regulatory documents listed in Table 1.
Table 1. Testing methods.
Property Regulatory Document
Plasticity number of clay mass Russian State Standard GOST 21216-2014 [35] Clay raw materials.
Test methods, p. 5.3
Air shrinkage of clay sample Russian State Standard GOST 21216-2014 [35] Clay raw materials.
Test methods, p. 5.26
Fire shrinkage Russian State Standard GOST 21216-2014 [35] Clay raw materials.
Test methods, p. 5.27.4.3
Compressive strength of
ceramic sample
Russian State Standard GOST 21216-2014 [35] Clay raw materials.
Test methods, p. 5.33.4
Sensitivity of clay sample to drying Russian State Standard GOST 21216-2014 [35] Clay raw materials.
Test methods, p, п. 5.32
Density of ceramic shard
Russian State Standard GOST 7025-1991 [36] Ceramic and calcium
silicate bricks and stones. Methods for water absorption and density
determination and frost resistance control, p. 5
The strength-density ratio (SDR) was calculated as the ratio of the sample density (
ρ
, g/cm
3
) to its
compressive strength (R, MPa):
SDR =ρ/R. (1)
Materials 2020,13, 5293 5 of 11
Preparation of specimens, air shrinkage and fire shrinkage tests were carried out in accordance
with the Russian State Standard GOST 21216-2014 [
35
]. Specimens with dimensions of
60 ×30 ×10 mm
were cut from the clay mass layer using a mould with a pusher. The specimens were marked at 50 mm
intervals. The specimens were dried at a temperature of 100
C for 24 h and were fired in a laboratory
electric oven.
The heating rate during the drying process was kept constant at 3
C/min. The samples were kept
at the final temperature for 30 min, in accordance with the requirements of the Russian State Standard
GOST 21216-2014 [35].
To determine the ultimate compressive strength, cube specimens with dimensions of
50 ×50 ×50 mm
were prepared and dried at a temperature of (105
±
5)
C to constant weight. The samples were fired
in a laboratory electric oven.
The heating rate during the drying process was kept constant at 3
C/min. The samples were
kept at the final temperature for 30 min, and the density was tested and determined by using a
hydraulic press.
The sensitivity to drying was determined in accordance with Russian State Standard GOST
21216-2014 [
35
] by the duration of exposure to a heat flux on a freshly formed sample until cracks
appeared on it.
The clay for the fabrication of samples was prepared at Kemma Ceramic Ware Plant,
Chelyabinsk, Russia.
3. Results
3.1. Sludge Treatment Methods
The traditional technology for the production of structural ceramics involves the introduction
of dry modifying additives. However, WTP sludge has a high content of bound water. The value
of the specific resistance to filtration (r, m/kg) characterizes the sludge’s water-carrying capacity.
Studies [
18
,
24
] have shown that the untreated sludge of Chelyabinsk WTPs has a specific resistance to
filtration between 1.5 ×1013 m/kg and 6.3 ×1013.
In the preliminary experiments, the use of powdered lime for reducing the specific resistance
to filtration requires up to 30% and 40% of lime and a mixing time of 20 to 30 min (see Table 2).
The resulting spray-dried sludge will contain up to 40% lime, which can have a significant eect on the
properties of ceramic products and requires additional research.
Table 2. Resistivity of sludges treated with powdered lime (PL) and hydrated lime (HL).
Treatment Method
Specific Resistance to Filtration, ×1013 m/kg
Lime% Sludge without Treatment
10 15 20 30 40
PL 2.34 1.16 0.77 8.40
HL 1.28 1.10 0.880 3.74
The use of HL makes it possible to distribute the reagent through the sludge more evenly and to
reduce the processing time to 10 min. Acceptable values of the specific filtration resistance are achieved
with the introduction of 20% lime.
FT showed the best results for improving the water-carrying capacity of sludge and can be
recommended as the best method for reducing the specific resistance to filtration of WTP sludge.
After thawing, the initial value of 3.74
×
10
13
m/kg decreased to 0.05
×
10
13
m/kg. The moisture content
of the sludge cake after thawing and vacuum pumping was 66.7%, while the cake without treatment
had a moisture content of 96.2%.
Materials 2020,13, 5293 6 of 11
3.2. Influence of the Sludge Treatment Method on the Properties of Ceramics
A comparative experiment was carried out to identify the influence of the sludge dewatering
method on the properties of the resulting ceramics. For clay mixtures with the addition of FT
treated WTP sludge (WTPS-FT) and HL treated WTP sludge (WTPS-HL), the plasticity number was
determined, and for ceramic samples, the density of the shard, and the air and the fire shrinkage were
determined. The sludge was introduced in the amount of 5% of the weight of the initial clay. According
to published data [
21
23
], WTP sludge was introduced into clay in an amount of 5% to 40%. In this
study, the minimum value for a preliminary assessment of the eectiveness of the WTP sludge was
chosen. The scatter of results for plasticity number does not exceed 0.1%; air shrinkage does not exceed
0.05%; fire shrinkage does not exceed 0.01%; the density of ceramic shard does not exceed 0.01 g/cm
3
.
Each test was repeated twice to obtain reliable results. Firing was carried out at 950
C. The results are
shown in Table 3.
Table 3. Comparison of clay properties.
Property Without
Additives
With WTPS-FT
Additive
With WTPS-HL
Additive
Mixing moisture content% 34.8 30.0 32.8
Plasticity number% 15.0 15.5 13.9
Air shrinkage% 9.7 8.0 8.0
Fire shrinkage% 1.4 1.6 1.4
Density of ceramic shard, g/cm31.67 1.53 1.48
Table 3shows both additives significantly reduce the air shrinkage of the sample from 9.7% to
8.0%. This positive eect arises due to the presence of a significant amount of natural organic matter in
the sludge [
34
]. The introduction of additives will reduce the tendency of the raw material to crack
and accelerate drying.
The fire shrinkage of the samples with WTPS-FT additive is 15% higher than the samples without
additives. This increase in shrinkage indicates an increase in the degree of sintering, which will increase
the strength and frost resistance of the samples. This eect is associated with the presence of fluxing
agents (FeO, K
2
O and MnO) in the WTPS-FT additive [
37
]. The presence of fluxing agents in the dry
matter of the sludge is 3% in total, which corresponds to 0.15% of the total weight. About 40% of
the mass is lime in the WTPS-HL additive, so the fluxing agent content decreases. The addition of
WTPS-HL does not improve ceramic sintering.
An increase in shrinkage indicates an increase in sintering, but the bulk density decreases.
This eect can be explained by the fact that WTPS-FT has a dual eect. On the one hand, WTPS-FT is a
burnout additive. Burnout of organic matter in the sediment creates additional porosity, which leads to
a decrease in density. On the other hand, the WTPS-FT additive has a fluxing eect due to the oxides
(FeO, K2O and MnO) present. With an increase in the amount of WTPS-FT additive in the clay batch,
the density of the ceramic decreases more than the shrinkage increases.
3.3. Selection of Optimal Conditions for Sludge Pretreatment
Both additives reduce the density of the ceramic shard due to the burnout of the organic component;
however, the WTPS-HL additive contains a significant amount of hydration in the Ca (OH)
2
lime
composition. Water vapour, evaporating during lime dehydration, contributes to the porosity of the
shard and a significant decrease in its density to 1.48 g/cm
3
in comparison with the samples with the
WTPS-FT additive.
The WTPS-FT additive has a complex positive eect, improving the properties of the clay,
raw material and ceramic sample. WTPS-FT has the properties of a plasticizer, a blowing agent, and a
flux. Therefore, further studies were carried out with FT pretreated sludge.
Materials 2020,13, 5293 7 of 11
3.4. The Influence of the Additive Dosage and Firing Temperature on the Structural Properties of the Ceramics
A two-factor experiment was carried out using the Hartley method to determine the eect of the
WTPS-FT additive on the properties of the clay and ceramic shard. The design matrix and the results
are presented in Table 4. The amount of WTPS-FT additive introduced varied from 5% to 10% of the
weight in increments of 2.5%. Each test was repeated twice to obtain reliable results. The scatter of
results for air shrinkage does not exceed 0.05%, for fire shrinkage does not exceed 0.01%, and for the
density of ceramic shard does not exceed 0.01 g/cm3.
Table 4. Design matrix and responses of the two-factor experiment.
First Factor, X Second Factor, Y Plasticity
Number%
Shrinkage Density of Ceramic
Shard, g/cm3
Code
Values D% Code
Values TO,CAir
Shrinkage%
Fire
Shrinkage%
1 5.0 1 950 15.5 8.0 1.60 1.53
0 7.5 1 950 16.8 7.6 1.70 1.47
1 10.0 1 950 16.9 7.5 1.90 1.42
1 5.0 0 1000 - - 1.86 1.53
0 7.5 0 1000 - - 2.02 1.47
1 10.0 0 1000 - - 2.15 1.45
1 5.0 1 1050 - - 2.43 1.57
0 7.5 1 1050 - - 2.46 1.51
1 10 1 1050 - - 2.69 1.49
An increase in the additive dose expands the range of permissible operating humidity, since the
plasticity number increases from 15.5% to 16.9%. In addition, an increase in the percentage of additive
from 5% to 7.5% leads to a decrease in air shrinkage from 8% to 7.6%. The isolines for the fire shrinkage
and the density of the ceramic shard are presented in Figures 2and 3. The equations describing the
graphs of dependencies are placed under the corresponding figures.
First Factor, X
Second Factor, Y
Plasticity
Number%
Shrinkage
Density of Ceramic
Shard, g/cm3
Code
Values
D%
Code
Values
TO, °C
Air Shrinkage%
Fire Shrinkage%
1
5.0
1
950
15.5
8.0
1.60
1.53
0
7.5
1
950
16.8
7.6
1.70
1.47
1
10.0
1
950
16.9
7.5
1.90
1.42
1
5.0
0
1000
-
-
1.86
1.53
0
7.5
0
1000
-
-
2.02
1.47
1
10.0
0
1000
-
-
2.15
1.45
1
5.0
1
1050
-
-
2.43
1.57
0
7.5
1
1050
-
-
2.46
1.51
1
10
1
1050
-
-
2.69
1.49
Figure 2.
Dependence of fire shrinkage (FS) on the amount of additive in the clay and the temperature
of raw firing. Equation of the dependency graph: FS =2+0.17x +0.43y +0.01x20.06xy +0.09y2.
Materials 2020,13, 5293 8 of 11
Materials 2020, 13, x FOR PEER REVIEW 8 of 12
The Fisher coefficient is 2.7, which is lower than the table value (4.3). This confirms the adequacy
of the model.
The nature of the dependence in Figure 2 indicates that an increase in the amount of additive
leads to an increase in fire shrinkage, that a larger amount of melt is formed, and that the ceramic is
sintered more efficiently. The effect of the additive is especially significant at lower firing
temperatures. The introduction of the additive allows a reduction in the firing temperature by 50 °C
without deteriorating the degree of sintering.
Figure 3. Dependence of the shard density on the amount of additive in the clay and the firing
temperature of raw material. Equation of the dependency graph: ρ = 1.47 0.05x + 0.02y + 0.02x2 +
0.01xy + 0.02y2.
The Fisher coefficient was 2.5, which is lower than the table value (4.3). Low value of the Fisher
coefficient confirms the adequacy of the model.
The addition of WTPS-FT reduces the density of the ceramic shard due to the burnout of organic
compounds in the sludge (Figure 3). The porous effect of the additive is most pronounced at lower
firing temperatures, since an increase in temperature contributes to a larger amount of melt.
The combination of the porous and fluxing action of the additive can significantly change the
quality of the ceramics. With a decrease in the density of ceramics, at the same time, an increase in
fire shrinkage is observed. The study hypothesized that due to the increase in fire shrinkage, the
strength of the sample would increase. Further experiments confirmed this hypothesis. In order to
confirm this assumption, an additional experiment was carried out in the laboratories of Kemma
Ceramic Ware Plant, Chelyabinsk, Russia. WTPS-FT was used as a modifying additive. The samples
were fired at a temperature of 1000 °C, since this temperature corresponds to the technology of the
enterprise. The following properties of the samples were controlled: air and fire shrinkage, sensitivity
to drying, density and the compressive strength of the ceramic shard. Each test was repeated three
times to obtain reliable results. The scatter of results for sensitivity to drying does not exceed 10 s; for
air shrinkage does not exceed 0.05%; for fire shrinkage does not exceed 0.05%, and for density of
ceramic shard does not exceed 0.03 g/cm3. The results of the experiment are presented in Table 5.
Dosages were agreed with Kemma Ceramic Ware Plant as a potential consumer of the additive being
developed. The upper limit (20% of the additive of the clay weight) is determined by the economic
feasibility and technical properties of the resulting ceramics.
Figure 3.
Dependence of the shard density on the amount of additive in the clay and the firing
temperature of raw material. Equation of the dependency graph:
ρ
=1.47
0.05x +0.02y +0.02x
2
+
0.01xy +0.02y2.
The Fisher coecient is 2.7, which is lower than the table value (4.3). This confirms the adequacy
of the model.
The nature of the dependence in Figure 2indicates that an increase in the amount of additive
leads to an increase in fire shrinkage, that a larger amount of melt is formed, and that the ceramic is
sintered more eciently. The eect of the additive is especially significant at lower firing temperatures.
The introduction of the additive allows a reduction in the firing temperature by 50
C without
deteriorating the degree of sintering.
The Fisher coecient was 2.5, which is lower than the table value (4.3). Low value of the Fisher
coecient confirms the adequacy of the model.
The addition of WTPS-FT reduces the density of the ceramic shard due to the burnout of organic
compounds in the sludge (Figure 3). The porous eect of the additive is most pronounced at lower
firing temperatures, since an increase in temperature contributes to a larger amount of melt.
The combination of the porous and fluxing action of the additive can significantly change the
quality of the ceramics. With a decrease in the density of ceramics, at the same time, an increase in fire
shrinkage is observed. The study hypothesized that due to the increase in fire shrinkage, the strength
of the sample would increase. Further experiments confirmed this hypothesis. In order to confirm
this assumption, an additional experiment was carried out in the laboratories of Kemma Ceramic
Ware Plant, Chelyabinsk, Russia. WTPS-FT was used as a modifying additive. The samples were fired
at a temperature of 1000
C, since this temperature corresponds to the technology of the enterprise.
The following properties of the samples were controlled: air and fire shrinkage, sensitivity to drying,
density and the compressive strength of the ceramic shard. Each test was repeated three times to obtain
reliable results. The scatter of results for sensitivity to drying does not exceed 10 s; for air shrinkage
does not exceed 0.05%; for fire shrinkage does not exceed 0.05%, and for density of ceramic shard does
not exceed 0.03 g/cm
3
. The results of the experiment are presented in Table 5. Dosages were agreed
with Kemma Ceramic Ware Plant as a potential consumer of the additive being developed. The upper
limit (20% of the additive of the clay weight) is determined by the economic feasibility and technical
properties of the resulting ceramics.
Materials 2020,13, 5293 9 of 11
Table 5. Strength of ceramic samples.
Sample Sensitivity to
Drying, Seconds
Air
Shrinkage%
Fire
Shrinkage%
Density of Ceramic
Shard, g/cm3
Compressive
Strength
Limit, MPa
Strength-Density
Ratio
Without
additives 100 6.2 1.5 1.57 7.0 4.2
With 10 % of
WTPS-FT
additive
>180 4.5 2.5 1.45 8.5 5.6
With 20 % of
WTPS-FT
additive
>180 4.2 3.5 1.33 10.2 7.7
The data presented in Table 5confirm previous experiments (see Tables 3and 4). The introduction
of WTPS-FT into the clay improves the properties of the ceramic samples. The amount of the additive
increases up to 20%, which leads to an increase of the ceramics’ strength and a decrease in the
density. On the one hand, the WTPS-FT additive has a fluxing eect and increases the strength of the
ceramic. This regularity is confirmed by an increase in fire shrinkage with an increase in the amount of
additive. On the other hand, WTP-FT porosizes ceramics, and therefore porosity increases. However,
the fluxing properties of WTPS-FT lead to the formation of denser pore walls, sintering of the bulk of
the ceramic body.
The high sensitivity of the clay to drying is a significant technological problem. Such clays require
longer drying and are prone to cracking. Table 5shows that the introduction of WTPS-FT into the
clay reduces the sensitivity of the raw material to drying, which means the clay is characterized as
insensitive (Russian State Standard GOST 21216-2014 [
35
]). The use of the WTPS-FT additive reduces
the drying time and the defect rate during production. These changes may be associated with the
presence of polyacrylamide in WTPS-FT, which is part of the flocculant used in the water treatment.
4. Conclusions
The article studies the applicability of WTP sludge in the production of structural ceramics.
A method for the preliminary treatment of sludge, making it possible to eectively dewater the sludge,
and use it to obtain high-quality structural ceramics, is proposed. The sludge was prepared using
powdered lime (WTPS-PL), hydrated lime (WTPS-HL) or the freezing-thawing method (WTPS-FT).
The eects of adding WTP sludge on the characteristics of ceramic products and the optimal amount of
additive are established. To select the most eective treatment method, the authors evaluated its eect
on the properties of the resulting ceramics. The most eective method is freezing-thawing. It should
be noted that the freezing-thawing method of sludge treatment before dewatering is implemented on
some WTPs [28].
A two-factor experiment varied the amount of sludge in the clay and the firing temperature.
The introduction of the WTPS-FT additive made it possible to reduce the density, and the firing
temperature of ceramics by 50
C, without loss of sintering. The testing was carried out at the
laboratory of Kemma Ceramic Ware Plant, Chelyabinsk, Russia.
The utilization of WTP sludge in the production of structural ceramics is possible. The introduction
of sludge helps to reduce the density of structural ceramics and increase its strength-density ratio.
A significant advantage of the WTPS-FT additive is a decrease in the sensitivity of the clay to drying,
leading to a reduced drying time and defect rate.
The results obtained are compatible with the following conclusions:
1. The utilization of WTP sludge in the production of ceramic bricks is promising.
2.
A 20% addition of WTP sludge reduces the sensitivity of the clay to drying, reduces the density
of ceramics by 20% and increases its compressive strength from 7.0 to 10.2 MPa.
3.
The use of WTP sludge as a modifying additive, pretreated by the freezing-thawing method,
makes it possible to obtain ceramic bricks with improved properties.
Materials 2020,13, 5293 10 of 11
4.
The results can be used for WTP sludge containing aluminum, obtained by treating water of
medium turbidity and medium colour.
5.
The results of the study confirmed the possibility of using WTP sludge on an industrial scale for
the production of high-quality ceramic bricks.
Author Contributions:
Methodology, A.O. and M.B.; validation, N.V.; writing—review and editing, A.O., M.B.
and N.V. All authors have read and agreed to the published version of the manuscript.
Funding:
This research work was supported by the Academic Excellence Project 5–100 proposed by Peter the
Great St. Petersburg Polytechnic University, St. Petersburg, Russian Federation, and by the Academic Excellence
Project 5–100 proposed by South Ural State University, Chelyabinsk, Russian Federation.
Conflicts of Interest: The authors declare no conflict of interest.
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