Effect of Chemical Composition of Clay on Physical-Mechanical Properties of Clay Paving Blocks

: Clay paving blocks made of natural, environmentally friendly materials have their own originality and colorful authenticity, but due to the complex technological production process, they are also relatively expensive products; therefore, their environmental resistance properties are strictly deﬁned and controlled by the standards. The physical and mechanical properties of clay paving blocks are the key factors aiming to ensure the longevity of products and their long-term success in the market. Therefore, ensuring high physical and mechanical properties of clay paving blocks have become a most crucial challenge for the manufactures. This article considers the parameters of the technological production process of clay paving blocks manufacturing and evaluates the inﬂuence of the chemical composition of three different types of clay on the physical and mechanical properties of the paving blocks. Water absorption, linear shrinkage, freeze/thaw, acid resistance, and transverse breaking load of clay paving blocks are investigated. This study reveals the importance of raw material selection in the production process of clay paving blocks and provides the concept of main quality factors of clay.


Introduction
Various clays have been used as a building material in the construction industry from ancient times to the present day due to their exceptional properties and usage possibilities, durability, economy, and environmental friendliness. Clay is one of the main ingredients applied in the production of clay paving blocks. Due to its functionality, authenticity of color range, and durability, clay paving blocks have been and are one of the most attractive building materials in the small architecture sector [1][2][3]. The industrial use of clay minerals is quite wide. The ceramic industry uses various types of clay for pottery making and construction ceramic (bricks, tiles, etc.) [4]. Clay bricks as well as clay paving blocks are commonly used outdoors and, therefore, are exposed to various environmental factors such as humidity, cold, heat, friction, and compression. Therefore, in order to ensure the resilience of these products to the effect of environment factors, the technological process of their production is constantly monitored and improved. The results of research confirm that great attention must be paid to the quality of the raw materials, that is why scientists and staff of manufacturing companies make daily efforts to determine the optimal chemical, mineralogical, granulometric, and other compositions of the raw materials. A number of existing studies are related to the properties of bricks using various additives such as fly ash [5][6][7], waste glass powder [8], sugarcane bagasse ashes [9,10], sawdust, tobacco residues [11], spent grains [12], valorized sludge [13], waste micro cellulosic fiber [14], industrial slags, such as steel slag, ferrochromium slag, or granulated blast furnace slag [15], charcoal [16], agricultural waste, such as hazelnut shells [17], waste rice husk [9,18,19], This research was directed towards investigating the performance of the manufacturing process of clay paving blocks made of different types of clay, evaluating the mechanical and physical parameters of the final product that would meet the requirements established by the EN 1344:2013/AC:2015 standard [43]. The parameters investigated included characteristics such as freeze/thaw resistance, acid resistance, water absorption, tensile bending strength, linear firing shrinkage after drying, and firing of the specimens.

Raw Materials
Necessity to investigate the chemical composition of clays and the influence of chemical composition of clays on physical and mechanical properties of clay paving blocks arose from the need to understand which clay is the most suitable for the production of them. All raw clay samples in this study were obtained from clay deposits located in different countries and extracted with bucket chain excavators and ordinary wheel loaders. Fist clay sample was obtained from a reservoir in eastern Germany, the clay sample being yellowish in color, further (S1). Another sample was obtained from the deposit in northeastern Estonia and was grey in color, further (S2). The third sample of clay was obtained from a clay deposit in western Lithuania and was dark brown and red in color, further (S3).
After the taking of clay samples, 100 g of each sample was separated, and the chemical composition was determined. To determine the chemical compositions of clays, all samples were dried in an electric oven at 60 °C temperature for about 12 h and after shredded to 2 mm particles. Further, the drying process of the clay samples was continued for another 6 h at the same temperature of 60 °C. Once the clay drying process was finished, the clay samples were milled to a powder and submitted for chemical analysis. The chemical composition of all clay samples was determined by applying energy dispersive X-ray spectroscopy method with an X-ray spectrometer "Spectro XLAB 2000 XRF". Samples of powdered clay before the determination of the chemical analysis are presented in Figure  1.

Figure 1.
Clay samples in powder form: S1-clay raw material from a deposit in Germany, S2clay raw material from a deposit in Estonia, S3-clay raw material from a deposit in Lithuania.
All clays were essentially composed of the same chemical oxides as Al2O3 (aluminum oxide), SiO2 (silicone oxide), Fe2O3 (iron III oxide), TiO2 (titanium oxide), MgO (magnesium oxide), and others; however, the amount of mentioned oxides in the clays was different. The X-ray spectroscopy results of clays S1, S2, and S3 are provided in Table 1.  Clay samples in powder form: S1-clay raw material from a deposit in Germany, S2-clay raw material from a deposit in Estonia, S3-clay raw material from a deposit in Lithuania.
All clays were essentially composed of the same chemical oxides as Al 2 O 3 (aluminum oxide), SiO 2 (silicone oxide), Fe 2 O 3 (iron III oxide), TiO 2 (titanium oxide), MgO (magnesium oxide), and others; however, the amount of mentioned oxides in the clays was different. The X-ray spectroscopy results of clays S1, S2, and S3 are provided in Table 1.

Experimental Methodology
The technological production process of clay paving blocks specimens was designed and developed in accordance with the requirements of the following standards: EN 17450-1:2011 requirements for production of masonry units [44]; EN 12620:2002+A1:2008 requirements for concrete aggregates [45]; SIST EN 1008:2003 requirements and test methods for water sampling and suitability [46]; ISO 3310-1:2016 requirement and test methods for sieves from metal wire mesh [47]. During the preparation of the raw material, water was added to clay mass twice: once of 12% of the clay mass before the clay maturation process and a second time during the final clay mass mixing, when the required plasticity of clay mass had to be ensured before the shaping and cutting operations. During the experiment, specimens of size 200 × 100 × 52 mm were manufactured.
All manufactured specimens of clay paving blocks were tested in accordance with requirements of EN 1344:2013/AC:2015 standard, which defines the physical and mechanical properties of clay paving blocks. According to the mentioned standard, the physical properties of the clay paving blocks were determined by performing freeze-thaw resistance and acid resistance tests. For determination of mechanical properties of specimens of clay paving blocks, the transverse braking load test was selected.

Linear Firing Shrinkage and Deviations
Dimensional deviations of specimens were measured by the procedures described in the standard EN 1344:2013/AC:2015. To find out the shape and size of the clay paving blocks, 10 specimens of blocks from each type of clay were randomly selected and their overall dimensions after firing were determined. The length, width, and thickness of the clay paving blocks were measured with an accuracy of 0.5 mm. Differences in the dimensions of the clay paving blocks of the same production batch must not exceed established norms.

Freeze/Thaw Resistance
Aiming to determine freeze/thaw resistance of clay paving blocks, selected specimens were placed in an oven and dried at 105 ± 5 • C temperature till constant mass. A constant mass of the specimens is considered when the mass of the specimens weighed over a period of 24 h does not differ by more than 0.2%. All specimens were cooled down at ambient temperature before weighing.
Once constant mass of the specimens was achieved, all specimens were immersed into the water at room temperature. Further, water temperature was raised till 80 ± 3 • C gradually, over a period of 2-5 h and then specimens remained in water for another 24 h. Later, heating of water was stopped, and the water temperature cooled down to room temperature gradually.
After determination of constant mass and water absorption, 10 specimens from each clay were assembled into the frames for freeze/thaw test. The temperature during the freezing periods was −15 ± 3 • C, the first freezing period took 6 h and other freezing periods lasted for 120 min. During the thawing process, the temperature was increased gradually from −15 ± 3 • C to 20 ± 3 • C between 15 and 20 min. After thawing, water was sprayed on the surface of specimens for 120 ± 10 s and the process was repeated.

Acid Resistance
The service life of the clay paving blocks may be reduced due to an influence of the aggressive media. One of such important threats to the blocks are the effects of acid. The sulfuric acid H 2 SO 4 , nitric acid HNO 3 , and deionized water were used as reagents in the acid exposure study. At the beginning of the test, five specimens were randomly collected. Specimens were crushed to a particle size of 10-12 mm, then grained and retained through 800 µm sieve. The same process was repeated, and the specimens sifted through 500 µm sieve, followed by washing with deionized water to a clean 500 µm fraction. Clean fraction of specimens was dried at 110 • C temperature to a constant mass. A constant mass was considered, when during the drying process, the loss of mass between the two determinations during the two subsequent weightings with an interval of 24 h did not exceed 0.1%. After drying, a specimen mass of 100 ± 5 grams' mass was weighed with the accuracy of 0.01 g. The mass of the specimen was transferred to laboratorial flask and charged with 75 mL of 10% sulfuric acid and 25 mL of 10% nitric acid. Using the reagents, the samples were boiled for 60 ± 2 min by immersing the flask in a bath of hot oil. After acid treatment and boiling, the whole mass of specimens was poured on a 150 µm sieve, washed with deionized water, and dried at 110 • C temperature to a constant mass. After the drying, the specimens were weighed with the accuracy of 0.01 g and the loss of mass was calculated and recorded.

Tensile Bending Strength
For the transverse braking load test, 10 specimens of clay paving blocks were selected and immersed into the water for 48 h at 20 ± 5 • C according to the standard [43]. Then the clay paving blocks were removed from the water and cleaned. Prior to applying external force on the clay paving blocks, they were measured as required and all measurements were recorded. Then the clay paving blocks were placed into the test apparatus. Two ends of clay paving block were supported, and an external force was applied in the middle of the block. The external force was applied to the specimen gradually, not exceeding 5 N/mm load per 5 s, until fracture occurrence and clay paving blocks broke in half.

Manufacturing of Specimens
In order to determine the effect of different raw materials on the physical and mechanical properties of clay paving blocks, the specimens of clay paving blocks were produced under the same production conditions and the same production parameters. The specimens' production process covered four main steps, which are presented in Figure 2.
considered, when during the drying process, the loss of mass between the two determinations during the two subsequent weightings with an interval of 24 h did not exceed 0.1%. After drying, a specimen mass of 100 ± 5 grams' mass was weighed with the accuracy of 0.01 g. The mass of the specimen was transferred to laboratorial flask and charged with 75 mL of 10% sulfuric acid and 25 mL of 10% nitric acid. Using the reagents, the samples were boiled for 60 ± 2 min by immersing the flask in a bath of hot oil. After acid treatment and boiling, the whole mass of specimens was poured on a 150 μm sieve, washed with deionized water, and dried at 110 °C temperature to a constant mass. After the drying, the specimens were weighed with the accuracy of 0.01 g and the loss of mass was calculated and recorded.

Tensile Bending Strength
For the transverse braking load test, 10 specimens of clay paving blocks were selected and immersed into the water for 48 h at 20 ± 5 °C according to the standard [43]. Then the clay paving blocks were removed from the water and cleaned. Prior to applying external force on the clay paving blocks, they were measured as required and all measurements were recorded. Then the clay paving blocks were placed into the test apparatus. Two ends of clay paving block were supported, and an external force was applied in the middle of the block. The external force was applied to the specimen gradually, not exceeding 5 N/mm load per 5 s, until fracture occurrence and clay paving blocks broke in half.

Manufacturing of Specimens
In order to determine the effect of different raw materials on the physical and mechanical properties of clay paving blocks, the specimens of clay paving blocks were produced under the same production conditions and the same production parameters. The specimens' production process covered four main steps, which are presented in Figure 2. The raw materials obtained from the quarries were not suitable for the production of clay paving block samples, as they were in the form of pieces of various sizes and had to be prepared according to the technological requirements.

Preparation of Clay Mass
All three different samples of clay, without mixing them together, were crushed and blended using the crushing machine. During the crushing process, additional additives such as water (12%), sand (10%), and barium carbonate (1%) were added to the clay mass. In order for the moisture to be evenly distributed, the clay raw material must be matured. During the maturing process, the clay raw material softens and acquires plasticity. The maturation time of clay depends on various factors; however, the most important factors are the type of clay and its granulometric composition. The raw materials obtained from the quarries were not suitable for the production of clay paving block samples, as they were in the form of pieces of various sizes and had to be prepared according to the technological requirements.

Preparation of Clay Mass
All three different samples of clay, without mixing them together, were crushed and blended using the crushing machine. During the crushing process, additional additives such as water (12%), sand (10%), and barium carbonate (1%) were added to the clay mass. In order for the moisture to be evenly distributed, the clay raw material must be matured. During the maturing process, the clay raw material softens and acquires plasticity. The maturation time of clay depends on various factors; however, the most important factors are the type of clay and its granulometric composition.

Extruding and Cutting of Specimens
The prepared clay mass was delivered to the clay shaping machine by belt conveyors for the shaping operation. During the shaping and extruding processes (Figure 3a,b), clay bars of size 200 × 100 mm were produced.

Extruding and Cutting of Specimens
The prepared clay mass was delivered to the clay shaping machine by belt conveyors for the shaping operation. During the shaping and extruding processes (Figure 3a,b), clay bars of size 200 × 100 mm were produced.
Specimens of clay paving blocks with a size of 200 × 100 × 52 mm made of each type of clay S1, S2, and S3 were cut by an automatic cutting machine.

Drying and Firing of Specimens
Specimens were dried in the drying chamber ( Figure 4) at temperature 90-110 °C for 68 h and then delivered to the firing site. If clay paving blocks are dried improperly, the cracks and splits may appear during the firing process, thus damaging the products. After clay paving block specimens were dried, they were placed into the tunnel firing chamber ( Figure 5). After preheating, the firing process was performed at 1100 °C temperature.  It was observed that before the clay maturation process, the same amount of water (12% by mass of the clay) that was added into the different types of clay mass, during the Specimens of clay paving blocks with a size of 200 × 100 × 52 mm made of each type of clay S1, S2, and S3 were cut by an automatic cutting machine.

Drying and Firing of Specimens
Specimens were dried in the drying chamber ( Figure 4) at temperature 90-110 • C for 68 h and then delivered to the firing site. If clay paving blocks are dried improperly, the cracks and splits may appear during the firing process, thus damaging the products. After clay paving block specimens were dried, they were placed into the tunnel firing chamber ( Figure 5). After preheating, the firing process was performed at 1100 • C temperature.

Extruding and Cutting of Specimens
The prepared clay mass was delivered to the clay shaping machine by belt co for the shaping operation. During the shaping and extruding processes (Figure 3a bars of size 200 × 100 mm were produced.
Specimens of clay paving blocks with a size of 200 × 100 × 52 mm made of ea of clay S1, S2, and S3 were cut by an automatic cutting machine.

Drying and Firing of Specimens
Specimens were dried in the drying chamber ( Figure 4) at temperature 90-11 68 h and then delivered to the firing site. If clay paving blocks are dried improp cracks and splits may appear during the firing process, thus damaging the produc clay paving block specimens were dried, they were placed into the tunnel firing c ( Figure 5). After preheating, the firing process was performed at 1100 °C tempera  It was observed that before the clay maturation process, the same amount o (12% by mass of the clay) that was added into the different types of clay mass, du

Extruding and Cutting of Specimens
The prepared clay mass was delivered to the clay shaping machine by belt conveyor for the shaping operation. During the shaping and extruding processes (Figure 3a,b), cla bars of size 200 × 100 mm were produced.
Specimens of clay paving blocks with a size of 200 × 100 × 52 mm made of each typ of clay S1, S2, and S3 were cut by an automatic cutting machine.

Drying and Firing of Specimens
Specimens were dried in the drying chamber ( Figure 4) at temperature 90-110 °C fo 68 h and then delivered to the firing site. If clay paving blocks are dried improperly, th cracks and splits may appear during the firing process, thus damaging the products. Afte clay paving block specimens were dried, they were placed into the tunnel firing chambe ( Figure 5). After preheating, the firing process was performed at 1100 °C temperature.  It was observed that before the clay maturation process, the same amount of wate (12% by mass of the clay) that was added into the different types of clay mass, during th It was observed that before the clay maturation process, the same amount of water (12% by mass of the clay) that was added into the different types of clay mass, during the maturation process was absorbed differently. This is shown by the fact that the clay raw material S2 had a higher plasticity compared to the S1 and S3 raw clay. The specimens of clay paving blocks produced are shown in Figure 6. ings 2022, 12, x FOR PEER REVIEW maturation process was absorbed differently. This is shown by t material S2 had a higher plasticity compared to the S1 and S3 raw clay paving blocks produced are shown in Figure 6. At the beginning of the laboratory tests to determine physic erties of clay paving block specimens, all manufactured specimen ally inspected and no fissures or external damages was founded. ing blocks after the firing process must meet standard dimension

Results and Discussion
The physical and mechanical properties of specimens were d laboratorial tests according to the requirements of EN 1344:2013 The aim of this experiment was not only to determine the influen tion of clay on physical and mechanical properties of clay paving the technological process of production and evaluate the impact o eters on product properties.

Results of Linear Firing Shrinkage and Deviations Tests
The linear firing shrinkage of the clay paving blocks is cau which results in close movement of solid particles relative to each is one of the most important properties of clay paving blocks du and expansion were determined by measuring the dimensions and after firing in accordance with standard EN 1344:2013/AC:2 results are presented in Table 2. At the beginning of the laboratory tests to determine physical and mechanical properties of clay paving block specimens, all manufactured specimens were thoroughly visually inspected and no fissures or external damages was founded. A good quality clay paving blocks after the firing process must meet standard dimensions.

Results and Discussion
The physical and mechanical properties of specimens were determined by executing laboratorial tests according to the requirements of EN 1344:2013/AC:2015 standard [43]. The aim of this experiment was not only to determine the influence of chemical composition of clay on physical and mechanical properties of clay paving blocks, but also to study the technological process of production and evaluate the impact of manufacturing parameters on product properties.

Results of Linear Firing Shrinkage and Deviations Tests
The linear firing shrinkage of the clay paving blocks is caused by the loss of H 2 O, which results in close movement of solid particles relative to each other. Linear shrinkage is one of the most important properties of clay paving blocks during sintering. Shrinkage and expansion were determined by measuring the dimensions of the specimens before and after firing in accordance with standard EN 1344:2013/AC:2015 [43]. The measuring results are presented in Table 2.
The results showed that all specimens are assigned to class R1 as its dimensional differences does not exceed established norms required by the standard [43]. However, despite the fact that all specimens of clay paving blocks were the same dimensions after shaping and cutting operations, it was notable that dimensions after the firing operation changed. Chart of dimensional deviations of all tested specimens is presented in Figure 7. Result: Assigned to class R1 shaping and cutting operations, it was notable that dimensions after the firing operation changed. Chart of dimensional deviations of all tested specimens is presented in Figure 7. The highest dimensional deviations were recorded in the specimens manufactured from clay type S3 and the most resistant to expansion and shrinkage were specimens from clay type S1.

Results of Freeze/Thaw Resistance Tests
Prior to freezing and thawing, the clay paving blocks were weighed, and the results obtained were recorded in order to determine water absorption. Determination of constant mass of specimens is presented in Table 3. The highest dimensional deviations were recorded in the specimens manufactured from clay type S3 and the most resistant to expansion and shrinkage were specimens from clay type S1.

Results of Freeze/Thaw Resistance Tests
Prior to freezing and thawing, the clay paving blocks were weighed, and the results obtained were recorded in order to determine water absorption. Determination of constant mass of specimens is presented in Table 3. When a constant mass of the specimens was reached, a water absorption test was started. The total immersion time of specimens into the water was 48 h. Measurements were performed in accordance with the requirements of standard [43]. Results are presented in Table 4. Water absorption directly affects the durability of clay paving blocks. As can be seen from the results of water absorption in Table 4, specimens manufactured from clay S3 can be characterized as the most moisture absorbing, because the average percentage value of difference between dry and moistened specimens was determined to be 7.52%, which is higher than maximum value of 6% established by the standard EN 1344:2013/AC:2015. Due to the determined moisture absorption value, the use of such clay paving blocks in outdoor conditions would not be recommended.
After 100 cycles of freezing and thawing, the specimens were analyzed with a special device. If defects were observed on the clay paving blocks, they were recorded. The results are presented in Table 5.

S1
Changes or damages 0 Assigned to class FP 100 Freeze/thaw resistant

S3
Changes or damages (units) Assigned to class From the results presented in Table 5, it should be noted that specimens of clay paving blocks made of clay S3 do not meet the minimum requirements of standard [43], which is 100 freezing and thawing cycles. First violations were observed after cycle 82. Later, specimens started to collapse, many cracks and fracture damages, flaking, and chipping were detected. Other specimens of clay paving blocks made from clay S1 and S2 met the requirements of the standard perfectly. Specimens made from clay S2 and S1 were assigned to class FP 100, which is resistant to freeze and thaw, and specimens made from clay S3 assigned to class FP 0 as not freeze and thaw resistant. A graphical chart of the results of freeze/thaw resistance test is presented in Figure 8.
Assigned to class From the results presented in Table 5, it should be noted that specimens o ing blocks made of clay S3 do not meet the minimum requirements of standard is 100 freezing and thawing cycles. First violations were observed after cycle specimens started to collapse, many cracks and fracture damages, flaking, an were detected. Other specimens of clay paving blocks made from clay S1 and requirements of the standard perfectly. Specimens made from clay S2 and S signed to class FP 100, which is resistant to freeze and thaw, and specimens clay S3 assigned to class FP 0 as not freeze and thaw resistant. A graphical c results of freeze/thaw resistance test is presented in Figure 8. As it was mentioned, resistance to freezing and thawing is one of the most properties of clay paving blocks as they are designed for outdoor usage.

Results of Acid Resistance Tests
Acid resistance, as well as freeze and thaw resistance, are attributed to th properties of clay paving blocks. The acid resistance of clay paving blocks portant when pavers are used outdoors during the winter, as most of the roads are treated with various salts or acids to melt ice or snow. Moreover, clay pav can be used near to the plants of chemical components or materials and som As it was mentioned, resistance to freezing and thawing is one of the most important properties of clay paving blocks as they are designed for outdoor usage.

Results of Acid Resistance Tests
Acid resistance, as well as freeze and thaw resistance, are attributed to the physical properties of clay paving blocks. The acid resistance of clay paving blocks is very important when pavers are used outdoors during the winter, as most of the roads and yards are treated with various salts or acids to melt ice or snow. Moreover, clay paving blocks can be used near to the plants of chemical components or materials and some of them reach the surface of the pavers and cause damage. For these reasons, clay paving blocks must meet the requirements of the standard for acid resistance properties. According to the standard [43], the loss in mass after acid treatment of the specimens must not exceed 7%. The results of acid resistance are presented in Figure 9. As it is visible from the results presented in Figure 9, all specimens ma S2, and S3 met the requirements of the relevant standard.

Results of Tensile Bending Strength Tests
The transverse breaking load property is very important because duri tation of clay paving blocks they are affected by various external forces and stand certain loads defined by the standard.
After determination of breaking loads of specimens, calculations of tran ing load and tensile bending strength were performed. The results are presen  As it is visible from the results presented in Figure 9, all specimens made of clay S1, S2, and S3 met the requirements of the relevant standard.

Results of Tensile Bending Strength Tests
The transverse breaking load property is very important because during the exploitation of clay paving blocks they are affected by various external forces and have to withstand certain loads defined by the standard.
After determination of breaking loads of specimens, calculations of transverse breaking load and tensile bending strength were performed. The results are presented in Table 6.  Average value: 43 5 Minimum value: 39 4 Assigned to class: T2 The results in Table 6 show that specimens of clay paving blocks manufactured from clay types S1 and S2 were assigned to class T4, as the calculated average value of the transverse breaking load is higher than 80 N/mm and the minimum value is higher than 65 N/mm. However, specimens of clay paving blocks made of clay type S3 were assigned to class T2, as the average value of the transverse breaking load is set at 43 N/mm and the minimum value is 39 N/mm. After identification of the values of breaking loads of the specimens, the transverse braking loads and tensile bending strength were calculated according to the standard [43]. The results show that insufficient silicon and aluminum oxides content reduces the strength of the blocks. A column chart of the results of specimens' transverse breaking loads values is presented in Figure 10.
From the results presented in the column chart, it is visible that all specimens of clay paving blocks are within the range of the requirements of the standard [43]. minimum value is 39 N/mm. After identification of the values of breaking loads of the specimens, the transverse braking loads and tensile bending strength were calculated according to the standard [43]. The results show that insufficient silicon and aluminum oxides content reduces the strength of the blocks. A column chart of the results of specimens' transverse breaking loads values is presented in Figure 10. From the results presented in the column chart, it is visible that all specimens of clay paving blocks are within the range of the requirements of the standard [43].

Summary of Results
The main goal of this experiment was to determine how different parameters of technological production process correlate with each other and how they affect the final physical and mechanical properties of clay paving blocks. As it is observed during the specimens' production process, there are many production parameters that affect the quality of the final products. As the experimental results have shown, one of the most important

Summary of Results
The main goal of this experiment was to determine how different parameters of technological production process correlate with each other and how they affect the final physical and mechanical properties of clay paving blocks. As it is observed during the specimens' production process, there are many production parameters that affect the quality of the final products. As the experimental results have shown, one of the most important factors is the raw material. The results in Table 7 show that the specimens manufactured from clay S1 and S2, which contained higher amounts of silicon oxides and aluminum oxides, had better physical and mechanical properties.
However, in order to confirm that lower values of the physical and mechanical properties of the specimens are always caused by the lower content of silicon oxides and aluminum oxides, additional studies are necessary, as this may be due to poor selection of other technological process parameters, e.g., incorrect selection of the compression force of the material during the extrusion-shaping operation, improper firing temperature and atmosphere in the firing chamber, improper plasticity of the raw material. When using different clays, the particle size, melting point, and plasticity of the clays differ, so for these reasons it is always necessary to determine the optimal parameters of technological production process. Based on the production methodology of clay paving blocks specimens and the obtained results of physical and mechanical properties of the specimens, it can be presumed that positive correlations of technological process parameters were found for S1 and S2 type clay specimens, as for those clay specimens, paving blocks had physical and mechanical properties, which fully comply with the requirements of the EN 1344:2013/AC:2015 standard. Specimens of clay paving blocks S1 and S2, manufactured according to the production methodology and under parameters of technological process presented in Figure 11, withstood 100 freeze and thaw cycles, were resistant to acids, and their transverse breaking load values and dimensional deviations did not exceed the limits specified in the relevant standard. factors is the raw material. The results in Table 7 show that the specimens manufactured from clay S1 and S2, which contained higher amounts of silicon oxides and aluminum oxides, had better physical and mechanical properties. However, in order to confirm that lower values of the physical and mechanical properties of the specimens are always caused by the lower content of silicon oxides and aluminum oxides, additional studies are necessary, as this may be due to poor selection of other technological process parameters, e.g., incorrect selection of the compression force of the material during the extrusion-shaping operation, improper firing temperature and atmosphere in the firing chamber, improper plasticity of the raw material. When using different clays, the particle size, melting point, and plasticity of the clays differ, so for these reasons it is always necessary to determine the optimal parameters of technological production process. Based on the production methodology of clay paving blocks specimens and the obtained results of physical and mechanical properties of the specimens, it can be presumed that positive correlations of technological process parameters were found for S1 and S2 type clay specimens, as for those clay specimens, paving blocks had physical and mechanical properties, which fully comply with the requirements of the EN 1344:2013/AC:2015 standard. Specimens of clay paving blocks S1 and S2, manufactured according to the production methodology and under parameters of technological process presented in Figure 11, withstood 100 freeze and thaw cycles, were resistant to acids, and their transverse breaking load values and dimensional deviations did not exceed the limits specified in the relevant standard.
The structure of clay raw materials often differs, so the same parameters of technological production process may not be the optimal solution. The suitability of the raw material must be determined before starting the production process. The structure of clay, the melting point, the particle size and plasticity of the clay, and the firing temperature are essential factors that directly affect the physical and mechanical properties of clay paving blocks.  Figure 11. Proposed technological production process of clay paving blocks.
The experiment analysis showed that accurate determination of the clay mass in the clay paving block production process is vital to calculate the additional amount of water that needs to be added to obtain optimal clay plasticity during the final clay mixing operation. As shown in Figure 11 (green square), it is recommended to install digitized clay mass scalers in the final mixing hopper to determine the clay mass. Based on the exact clay mass obtained, the computer software program based on programmed logarithms automatically calculates the required amount of water to be added to the clay mass before extruding.
The modernized technological production process of clay paving blocks covers all stages of clay pavers manufacturing and specifies all the necessary parameters of techno- The structure of clay raw materials often differs, so the same parameters of technological production process may not be the optimal solution. The suitability of the raw material must be determined before starting the production process. The structure of clay, the melting point, the particle size and plasticity of the clay, and the firing temperature are essential factors that directly affect the physical and mechanical properties of clay paving blocks.
The experiment analysis showed that accurate determination of the clay mass in the clay paving block production process is vital to calculate the additional amount of water that needs to be added to obtain optimal clay plasticity during the final clay mixing operation. As shown in Figure 11 (green square), it is recommended to install digitized clay mass scalers in the final mixing hopper to determine the clay mass. Based on the exact clay mass obtained, the computer software program based on programmed logarithms automatically calculates the required amount of water to be added to the clay mass before extruding.
The modernized technological production process of clay paving blocks covers all stages of clay pavers manufacturing and specifies all the necessary parameters of technological process required for the production of high-quality clay paving blocks.

Conclusions
The main conclusions have been drawn based on the research results. The X-ray spectroscopy results showed that all three clay samples in powder form consisted predominantly of silicone, aluminum, titanium, iron III, calcium, magnesium, potassium, and sodium oxides. From a mineralogical point of view, silicone and aluminum oxides are the main constituents, while other oxides are present as minor constituents.
Although the manufacturing fundamentals of clay paving blocks remain unchanged over time, some changes in the technological process have led to an overall improvement in product quality. The properties of the different clays resulted in different water absorption. It was determined that in order to provide required plasticity to some clays, it is necessary to add an additional amount of water just before the shaping operation, during the final clay mass mixing process. It was proven that specimens manufactured from clay type S3 were most water absorbing, as water absorption percentage was determined of 7.52%. Water absorption for specimens manufactured from clay types S1 and S2 was determined of 0.35% and 2.36%, respectively.
Based on the results of experiments, it can be stated that the performance of clay paving blocks can be improved using waste material additives such as silicon oxides and aluminum oxides. This could be a cost-effective solution to increase the durability of outdoor clay paving blocks. The results of the freeze and thaw tests showed that the specimens made of clay S1 and S2, which contain more chemical oxides SiO 2 and Al 2 O 3 , were 21.95% more resistant to freezing and thawing compared to the specimens made of clay S3. Specimens made of S1 and S2 types of clay withstood 100 freeze-thaw cycles; however, the violations on specimens made of clay type S3 were noted after 82 freeze/thaw cycles.
The acid resistance test showed that the mass loss of the clay paving blocks made of S1 type clay with the highest acid resistance properties after the acid resistance test was 3.45%. Specimens made from type S2 clay showed a loss in mass of 5.56% and type S3 clay a loss in mass of 6.86%. All the samples met the minimum requirement of EN 1344: 2013/AC: 2015, which is 7%.
The results of the transverse breaking load test confirmed that specimens made of clay type S1 could withstand higher loads compared to the specimens made of clay types S2 and S3 due to higher content of silicon and aluminum oxides and less of calcium oxides. The transverse braking load values of the S2 and S3 clay specimens were lower by 31.2% and 65.5% respectively, if compared to the S1 clay specimens.