A multi-site study of a new cement composite brick with partial cement substitutes and waste materials

The building industry is a major contributor to climate change since it emits massive amounts of greenhouse gases. As a result of this research, a cement composite brick with partial cement substitutes and waste material was developed. This novel building material holds a lot of promise for partly solving this problem. The fresh formula mixture and hardened concrete were tested for physical, mechanical, and thermal properties to ensure that the assumptions were true. In this study, 20 recipes that led to the composition of mentioned brick were presented as well as its final recipe. The results of the fresh mixture were assessed in terms of initial and final setting time, pH, and air content. The compressive strength of the brick developed with the modified formula increased by 22% compared to the reference sample. Also, higher mechanical values were obtained for the splitting strength and flexural strength, which increased by 14% and 32%, respectively, compared to plain concrete. The results of the SEM images showed an even distribution of the fibers in the composite matrix and a crack-free structure. Moreover, the results of the image analysis confirmed the failure character consistent with traditional concretes in the case of compression and the lack of split into two independent samples in the case of bending. Therefore, the new composite brick has great potential to be used as a primary replacement for traditional concrete masonry in civil engineering projects.


Introduction
The environmental damage caused by greenhouse gas emissions is on the point of becoming irreversible, according to recent developments in climate research.One of the greatest CO2 emitters responsible for that is the building industry.Because of that, this industry is looking for novel ways to move toward cleaner manufacturing.The main issue is that the most common material used in the building process of an object is concrete 1,2 .To emphasize the scale of the problem, it should be noted that it is the second most used material on the planet, after water.Concrete is made of fine and coarse aggregate bonded together with a cement paste that hardens over time and takes shape of the mold.Its formation freedom and great strength properties made concrete a highly demandable construction material, which left its mark on the environment.We are facing the end of an era, where the climate is changing invariably.It has become clear that humans have caused most of the past century's warming by releasing heat-trapping gases called greenhouse gases (GHG) 3 .
Carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and fluorinated gases all have risen and their levels are higher now than at any time in the last 800,000 years 4 .Fortunately, as global awareness of climate change raises, the topic of industrial development is more and more often taken up together with the topics of sustainable development and circular economy as indisputably related 5 .This way the building industry could at least partially offset the carbon footprint it leaves behind, helping to reduce the negative impact on the environment.However, as the raised problem is a complex issue, it will be rather solved by a set of targeted solutions than a magical panacea.
The first possible targeted solution is a partial reduction of cement consumption by using natural substitutes in the form of powdered minerals.Research shows that zeolites, which are microporous aluminosilicate minerals, have the potential to replace cement as a binder in concrete 6,7 .
Replacing cement with zeolite by a few percent already shows an improvement in the mechanical properties of concrete.Because natural zeolites are formed in places where ash layers and volcanic rocks react with alkaline groundwater, concretes containing them may also show differences in porosity, density, and even thermal properties compared to traditional concretes developed solely based on cement 8 .This is an additional field for their use, for example, in structures requiring increased thermal insulation or light structures.The second, equally promising mineral group for the construction industry, which can be a substitute for cement, are kaolinites such as micrometakaolinite and metakaolinite, differing in grain size 9 .Both materials are natural pozzolans produced by heating kaolin-containing clay to a temperature of about 600-800 • C, which then recrystallizes to transform into mullite 10,11 .Depending on the aluminum, three different chemicals can be found in micrometakaolinite and metakaolinite: mullite (Al6Si2O13), spinel (MgAl2O4), and amorphous silica 12 .The construction industry can find applications because they can be activated with silicates or hydroxides of sodium or potassium.In the environment of calcium hydroxide, which is formed during the hydration of Portland cement, micrometakaolinite and metakaolinite undergo a pozzolanic reaction, the main products of which are calcium-aluminum and silico-silicon hydrates.
The hardened material thus obtained contains less portlandite and the resulting concrete exhibits a higher compressive and flexural strength 13 .
Another promising partial offsetting of the construction industry's environmental impact from concrete production is the use of waste materials from other industries.Recent studies show that these materials can be used in various forms, such as powder, granule, shot, or fiber [14][15][16] .Especially the latter has a very high potential for use in construction due to the concrete's low tensile and bending strength.Thanks to the use of a small addition in the form of fibers, it is possible to obtain a material with better deformability, which is not as brittle as ordinary concrete.This type of admixture increases the tensile and bending strength and prevents the formation of cracks in the concrete 17 .As research shows, a properly selected material from which the fibers are made also increases the insulating properties of the final material 18 .Such use of properly processed waste can partially reduce the damage caused to the environment by solving another serious problem, which is residual waste.Each year, about 2 billion tons of solid waste are produced, and by 2050 their expected production may reach even 3.4 billion tons 19 .Partial incorporation of waste into the concrete matrix would therefore be an ideal example of the application of the principles of a circular economy.This research investigates the possibility of combining both indicated partial solutions and presents the results of experimental tests in a wide range, taking into account the material's physical, strength, and thermal characteristics.Both cement substitutes and recycled fibers are welldeveloping areas of research, however, there is still a lack of knowledge about the possibility of combining them in concrete.Due to this fact, in this work, we analyzed several possible combinations, and based on the interaction of concrete with a single additive (polypropylene, glass, and steel fibers) or a substitute (zeolite, metakaolinite, and micrometakaolinite), developed the final formula of concrete containing both cement substitutes and recycled waste in the form of fibers.
The decisive criterion was the strength criterion of previously developed partial recipes (containing only one modification) while maintaining the principle of sustainable development.We decided to combine several cement substitutes in one recipe.In addition, based on the final formula, a finished construction product was developed, i.e. a brick on which a failure analysis was performed using the Digital Image Correlation (DIC).This work presents a detailed analysis and confirms the possibility of combining cement substitutes and waste materials in the form of fibers.

Materials
To manufacture plain concrete (M0) four main components were used: Portland cement, granite aggregate, water, and superplasticizer.All other formulas from M1 to M19 also contained specified components and at least one extra additive (glass, polypropylene or steel fiber, zeolite, metakaolinite, micrometakaoline).The specific ratio of components of each formula is presented in Table 1.Also, based on tests done on M0 -M19 recipes, an M20 formula has been proposed and tested, and the composite brick with dimensions of 40x40x160 mm has been manufactured for further tests with the DIC apparatus.The manufacturing process of samples (Figure 1) started with a dry mix of powder components and aggregate, which were mixed in a concrete mixer (SIPE, Montichiari, Italy) for two minutes.After that time water with the superplasticizer was added and the wet mixing process occurred for three minutes.Next, if necessary for a formula, fibers were added evenly to each side of the concrete mixer and mixed for extra two minutes.The manufacturing process took place in laboratory conditions which were 50% humidity and 21 • C temperature.The fresh concrete mix was transferred to molds in layers of a maximum high of about 0.1 m and each of them was compacted for 30 s.After 24 hours, all samples were demolded and placed in a steel tank filled with water, where the curing process took place for 28 days, according to EN 12390-2:2019-07 20 .As the main binder in the concrete manufacturing process, Portland cement CEM I 42.5R NA was used 21 .Its parameters were specified according to two standards: EN 196-6:2019-01 22 and EN 196-1:2016-07 23 .The first one was used to determine physical and chemical parameters, and the second one was used to test its compressive strength, see Table 2. Wet components used in the manufacturing process were tap water and a superplasticizer.
The water was provided from the collector installed in the Military University of Technology and it came from the northern site of Warsaw where it was purified and filtrated.Any other extra treatment was not performed in the laboratory, and its chloride content of it was 28 mg/l.Also, to reduce the amount of water and lower the water/binder ratio (w/b = 0.31) a liquid chlorine-free, low-alkaline superplasticizer was used.It was based on melamine and silanes/siloxanes, which are both modified polycarboxylate ethers and helped to obtain the required workability of concrete mixtures.
Three substitutes of cement used as a partial binder were zeolite, metakaolinite, and micrometakaolinite.Their Scanning Electron Microscopy (SEM) photos show that all of the powders exhibit typically sharp-edged structures and create large agglomerates (Figure 3).This way they can be easily incorporated into a concrete matrix.Also, their maximum particle sizes suggest that they can be used as binders which stands for zeolite 46.9µm (59.8%), 44.9 µm metakaolinite (63.5%), and micrometakaolinite 4.2µm (67.3%).The analysis of cement substitutes indicates similar particle sizes of zeolite and metakaolinite, and about ten times less average particle size for micrometakaolinite compare to the previous two.Also, their morphology shows similarities as all partial binders contained mostly oxygen (O), silica (Si), and aluminum (Al).The exact results of the atomic composition of the cement substitutes are presented in Table 3.All fibers used in this study (Figure 4) were obtained in the recycling process.Polypropylene fiber was produced from plastic packaging using thermal and mechanical treatment.It also had an extra pattern on the outside surface made by the extruder.When it was on its way out of it, the fiber was but into regular length of about 31 mm and straight shape.Another fiber used was glass fiber, which also was produced from waste packaging, such as glass bottles.The recycling process involved melting the glass waste, the gravity fall of it, and, after the cooling, cutting it into a final fiber of about 49 mm in length and straight shape.Steel fiber on the other hand was the only one obtained just in the mechanical recycling process.No thermal influence was performed as it derived from old tires.Mostly cutting of the rubber was done and then the fibers were separated from the rubber.The final steel fiber was about 25 mm long.The strength parameters of each fiber used in this research are presented in Table 4.

Fresh concrete mixture
Before pouring fresh concrete into a mold, it was examined to determine its physical qualities, such as air content, initial and setting times, and pH value.These tests were carried out in accordance with EN 12350-7:2019-08 25 , EN 196-3:2017 26 , and PN-B-01810:1986 27 .The air content in the fresh concrete was measured in the calibrated instrument using the equalizing pressure method, such as a porosimeter (FORM TEST Prüffsysteme, Riedlingen, Germany).The concrete mixture was poured into the porosimeter base in two layers, each of which was compacted for 30 seconds using a vibrating table.The upper portion of the device was then placed on top of the base and sealed.A manual pump was then used to create a specific pressure in the chamber.The overflow valve was finally opened, and the pressure in the test container filled with fresh concrete was restored.The number of air voids in the material was determined by the pressure decrease in the test vessel.The average value was calculated after each test was repeated five times.The Vicat device (Merazet, Pozna, Poland) was used to measure the initial and final setting times in this research.To begin, the Vicat mold was packed tightly and filled with cement paste.The plunger was then removed from the Vicat apparatus to penetrate the paste.When the 1 mm diameter needle penetrated slightly above 5 mm from the bottom of the mold base, the initial time was recorded.The 1 mm diameter needle was then replaced by a needle with a 5 mm hollow cylindrical base.When this needle created an impression on the surface of the concrete but did not pierce it, the setting time was recorded.The average value was computed after each concrete formula was tested five times.A pH meter (Testo, Pruszków, Poland) was used to determine the pH value.The tests were conducted on a fresh mix with 1 ml of liquid phase as a 10% water solution of concrete.The pH value was recorded when the measurement stabilized for three minutes.
Five samples of each diluted concrete mixture were tested, with and without fibers and cement substitutes inclusion.

Hardened concrete samples
The samples were taken out of the water and dried after the concrete had matured for 28 days.To analyze the material's physics, the samples were set aside to dry completely for 24 hours.
A MEGA3-3000kN-100S testing machine (FORM TEST Prüffsysteme, Riedlingen, Germany) was used to evaluate the compressive strength of cubes with dimensions of 150x150x150mm.The machine was programmed to raise the force from 0.2 to 1.0 MPa/s, and the entire test was run at a steady speed with no dynamic disturbances 28 .The specimens were positioned perpendicular to the force action surface, with the troweled side perpendicular, allowing compression to be transmitted evenly over the sample surface.Figure 5a shows how the cube was centered on the active force's axis to correct for any non-linear compression anomalies.On the same testing machine as the compressive strength test, the tensile splitting test was performed.It did, however, have a different starter and jaws with a significantly smaller surface area.This setup also included unique guidelines to assist you in positioning the sample consistently, as seen in Figure 5b.A sample measuring 150 x 300 mm was inserted in the jaws and moved to the rear-mounted stop each time.
As a result, the sample was axially oriented concerning the applying load that increased between 0.04 and 0.06 MPa/s 29 .The flexural bending strength test was the most recent mechanical test done on the MEGA3-3000kN-100S testing equipment.It happened on a side attachment that had been set up and readied to represent the typical load pattern described in the standard 30 , as shown in Figure 5c.The 40x40x160mm concrete beams were positioned between the rollers and centered in the force axis to ensure that any flaws in the equipment were not reflected in the test results.The testing machine's results were represented in kN and as the maximum load that the beam could bear until it failed and calculated to MPa.All of the samples were compacted to failure, and onehalf of the sample from each recipe was examined under a microscope to investigate the inside structure of the concrete.Hardened samples' thermal properties such as thermal conductivity, thermal diffusivity, and specific heat were also examined to determine the characteristics of concretes.The ISOMET2114 analyzer (Applied Precision Ltd., Bratislava, Slovakia) was used for all measurements.The resistor heater of the analyzer probe, which had a 60 mm diameter, was in direct contact with the sample under test, allowing the temperature responses to heat flow pulses of the material to be evaluated.
To specify all thermal parameters of each concrete formula, 10 cubes with dimensions of 150 mm x 150 mm x 150 mm were investigated.

Testing methodology of final brick
In three-point bending and compression testing, the digital image correlation approach was

Material properties of fresh mixtures
The air content test results are presented in Table 5.The linear increase in air content was observed as the fibber addition increased.No visible changes, however, were noted between samples with only cement substitutes (M1 -M10) and plain concrete (M0).This trend was also reported by Markiv et al. 33 after replacing 10% of cement with zeolite.Obtained by them air content was 2.4% and remained constant compared to plain concrete.In our study, the highest air content of 2.8% was reported for M16 samples with a 1.5% addition of glass fibers, which showed about 47% greater value than M0 samples (1.9%).The final composite brick recipe also showed an increase in the air trapped inside the mixture and the rise was about 21%.Our findings are in line with Wang et al. 34 study as they showed an increase in air voids number after the use of polypropylene and steel fibers in the concrete mixture.pH value tests, on the other hand, proved that none of the modifications presented in this study resulted in pH value change, see Table 5.The acquired findings were almost within the measurement error range.As a result, no significant difference in the pH value of concrete with cement substitutes such as zeolite, metakaolinite, and micrometakaolinite, and/or fibers such as polypropylene, glass, and steel, in any combination was found when compared to conventional concrete (range of 12.83 -12.94).A similar trend was reported by Małek et al. 18,35 as they also showed no significant difference in the pH value after fiber additions into concrete mixtures.Furthermore, all modified with fibers mixes showed similar initial and final setting times.Only mixes with cement substitutes had extended initial and setting time, which was a result of partly replacing cement with the no-traditional binder (zeolite, metakaolinite, and micrometakaolinite).Małek et al. 18 and Zhao et al. 36 came to the same conclusions, reporting no effect on the initial and final setting times of concrete mortars after fiber modifications and extended time after zeolite modification.), showed a density of 2,320.3kg/m 3 .As the research showed, samples containing natural cement substitutes show a lower density by less than 1%, which is within the measurement error, hence it can be concluded that the amounts of cement substitutes used do not affect the density of hardened concrete.Similar differences were also shown in the samples modified with fibers, where the differences ranged from 0.4% to 2.7%.The smallest differences were observed for polypropylene fibers, i.e. 0.4% -1.2%, and it increased with the increase of the number of fibers used.A similar trend was also noted for samples modified with glass fiber (M14 -M16) and in this type, the increase in density was in the range of 0.7% -1.5%.The greatest differences were observed for samples that contained steel fibers from the recycling of old tires.There the differences for 0.5%, 1.0% and 1.5% of the additive used were respectively 1.8%, 2.1% and 2.7%.
The final brick developed based on a recipe containing 5.0% zeolite, 5.0% metakaolinite, and 5.0% micrometakaolinite as well as 1.5% steel fibers (M20), achieved an average density of 2.373.6 kg / m3, which is 1.5% higher value compared to the base concrete (M0).Reported findings are in line with Madhkhan and Katirai's 37 study, as they reported a 0.9% increase in density of hardened concrete with 1.5% glass fiber addition, which is in the range of our study.This way, it can be concluded that the low weight and small size of the fibers, that let them fit between the aggregates of the mixture, can generate such a minor change in all of the density test findings.
The obtained results for modulus of elasticity tests ranged from 36.2 GPa to 40.3 GPa (Figure 6).It can be observed that both cement substitutes and fibers influenced the final Young modulus values.They however showed opposite trends.With the increasing amount of cement substitutes, the Young modulus dropped, while when the fiber amount was greater the Young modulus was higher.The lowest value of Young modulus was reported for modification with 5% of each substitute (M10) and the highest value for M19 samples with 1.5% addition of steel fibers.As the trends were opposite for both modification groups, combining them resulted in 38.7 GPa Young modulus of final brick (M20).The presented results show similarities to Zhao et al. 36 study as they presented a drop in the Young modulus from 39.8 GPa to 37.0 GPa after adding 15% of zeolite to the concrete mix.

Destructive mechanical testing
Both cement substitutes and fibers increased the bond between aggregate particles surrounded by binder mortar, allowing the samples containing these additives to withstand larger loads, as presented in Figure 7.All of the samples' walls were damaged with cracks apparent only at the corners and base during the compressive strength test (Figures 8a and 8b).Modifications with cement substitutes reached similar values in the range of 91.Furthermore, combining all of these cement substitutes (5% of each) resulted in a 4.4% increase.
Also, fiber modification showed greater results and reported values were higher by 3.3 -8.6 %, 3.1 -9.0 %, and 7.7 -11.0 % respectively, for polypropylene, glass, and steel fibers incorporated into the concrete matrix.Tested cylindrical samples also showed a different mechanism of failure between each modification group, see Figures 8c and 8d.In the case of basic samples and samples containing cement substitutes, the sample cracked evenly in the middle and disintegrated into two independent parts.This phenomenon did not occur with samples containing recycled fibers.Their addition resulted in a crack of about 0.3 -0.5 mm, which appeared during the drop of the force in the machine that showed the end of the test.Those cylinders (M11 -M20) were compact and did not fall apart readily when the samples were taken out of the machine jaws.In the case of all three modifications with fibers and the final composite recipe, a practically undetectable crack did not run the length of the sample and was halted by the fibers.All of the specimens failed the threepoint bending strength test as predicted, with the fracture occurring in the center of the specimen span and hence at the location where the force was applied.Figures 8e and 8f show the example of the failure mechanism, where the reference sample (M0) and the samples modified with cement substitutes (M1 -M10) split in half, and the samples containing fibers (M11 -M20) did not break in the full length of the sample and both of their parts were held together by the fibers.
This analogy was also reported in split tensile tests and can prove the even distribution of fibers in a concrete matrix.The results of the flexural strength differed for the modifications done with cement substitutes.The values increased by 0.15 -0.8 MPa, and the lowest rise was reported for 5.0% substitution done with micrometakaolinite (M7).The highest increase, on the other hand, was achieved for samples with 15.0% of zeolite (M3).Also, samples M10 that had all cement substitutes incorporated showed greater flexural strength compared to plain concrete, which was equal to 8.6 MPa.A more visible rise was reported for fiber modifications where 0.5%, 1.0%, and 1.5% addition showed an increase of 8.2 -8.7%, 8.5 -9.2 %, and 9.1 -9.8 %, respectively, depending on the material type of fiber.The highest value of flexural strength was reported for the final composite brick formula (M20) -10.2 MPa.

Thermal properties of hardened concrete
Based on this study, the addition of both cement substitutes and fibers done with proper combination has the potential to increase the thermal insulation of building materials, such as concrete bricks.Tested plain concrete, without any modification, showed thermal conductivity of 1.68 W/mK, the specific heat of 1.73 MJ/m 3 K, and thermal diffusivity of 0.97 µm 2 /s.The thermal conductivity of tested hardened samples modified with cement substitutes and/or fibers was in the range of 1.54 -1.86 W/mK.The highest value was obtained for the sample M20, which was later used to manufacture the final composite brick.Furthermore, samples with glass fibers showed the highest drop in thermal conductivity compared to plain concrete.The greatest decrease in thermal conductivity was reported for samples with a 1.5% addition of glass fibers (M16) and it was about 8%.Also, polypropylene fibers showed the ability to lower the thermal conductivity of hardened samples, but the decreases were 4%, 5%, and 6%, respectively, for samples with 0.5%, 1.0%, and 1.5% addition of fiber.The only modification that increased thermal conductivity was steel fiber addition.The stated rise was between 6% and 9%, and the highest value of thermal conductivity was reported for samples with 1.5% of steel fiber.On the other hand, cement substitutes showed a random distribution of thermal conductivity as values fluctuated in the range of plain concrete.
This way it can be concluded that zeolite, metakaolinite, and micrometakaolinite do not influence the thermal conductivity of concrete, see Figure 9a, and fibers additions show linear influence, either drop or increase in values (Figure 9b).Reported findings are consistent with current papers as Małek et al. 18 , Algourdin et al. 38 and Messaadi et al. 39 made the same conclusions.Figures 9c   and 9d show the specific heat measured in this research.When compared to the reference sample (M0), the glass fiber-modified samples and the polypropylene fiber-modified samples had lower specific heat.The lowest specific heat was reported for M16 (1.51 MJ/m 3 K), which was nearly 13% lower than plain concrete.Other glass fiber modifications had a visible effect on specific heat as well as the decrease was 11% and 12% for 0.5% and 1.0% glass fiber addition, respectively.
Much lower decreases, however, were reported for polypropylene fiber modifications (2% -4% ).Similar to thermal conductivity, only steel fibers increased the specific heat of concretes.This thermal property rise to 1.78 MJ/m 3 K for samples with the highest steel fiber addition (1.5%) and showed an almost equal result to the final concrete recipe (M20 -1.79 MJ/m 3 K) that was used to manufacture composite bricks.M20 recipe had also a total of 15% substitution of cement done with zeolite, metakaolinite, and micrometakaolinite (5% each).This fact and almost the same results of specific heat of all the specimens modified with cement substitutes (samples M1 -M10) show that cement substitutes tested in this study do not affect the specific heat of concrete.The last tested thermal property of concrete samples was thermal diffusivity.As presented in Figure 9e, all cement substitute modifications showed no influence on this property as the distribution of values was rather random for zeolite, metakaolinite, and micrometakaolinite.No sustained upward or downward trend has been recorded with the increase in the number of substitutes.Also, samples containing a mix of substitutes (M10), i.e. 5% zeolite, 5% metakaolinite, and 5% micrometakaolinite showed a thermal diffusivity similar to ordinary concrete (0.99 µm 2 /s). Figure 9f, however, presents linear trends occurring in fiber-modified concretes.As the quantity of polypropylene and glass fiber increased the thermal diffusivity of concrete samples decreased, and the opposite for steel fiber modification.The lowest thermal diffusivity value (0.91 m2/s) was found in samples containing 1.5% of steel fiber addition (M19 and M20) and was about 6% lower than plain concrete.The thermal diffusivity of fiber-reinforced concrete was also reduced in the Fu and Chung study 40 .They found that adding 0.5% and 1.0% carbon fibers to concrete reduces its heat diffusivity by 11% and 30%, respectively.This disparity compared to our research is due to a different type of fiber but shows that any addition of it can significantly affect the thermal diffusivity of the final material.Reported trends of thermal properties changes are similar to the trends shown in the density of concrete samples, which proves close relations between them.Concrete modified with fibers and/or cement substitutes had nearly the same increase/decrease in thermal properties and density.
The mentioned analogy is due to the thermal inclusion of a heat-conducting substance incorporated in the concrete matrix and its weight.Also, in the case of steel fiber addition, there may be some small thermal bridges occurring that are responsible for higher thermal heat transfer between opposite concrete surfaces.

Examination of final composite brick
Three-point bending tests were carried out on the produced M20 concrete, reinforced with recycled steel fibers and containing natural cement substitutes.Tests carried out on the finished product showed the maximum value of the applied force which caused the fracture of the sample, amounting to 3.68 kN.This corresponds to the strength of 10.The displacement fields were also observed for samples made of concrete with recycled steel fiber and natural cement substitutes during compression using the digital image correlation method.The test was carried out on the breakthroughs of previously damaged specimens as a result of bending, which had a cross-section of 40x40mm.The analysis of the obtained images showed that the finished bricks made with the use of the M20 formula show the same crack pattern as the standard samples 150x150x150 mm tested during the compressive strength on the FORM TEST Prüffsysteme machine.In both cases, they were concentrated on the edges of the sample -at the base and perpendicular to it.Referring to Figure 12, it can be seen that the failure mechanism was gradual and multiphase.The lower edge fracture initially appeared due to the accumulated stress which gradually increased.Additionally, immediately before the decrease of the compressive force increase (Figure 10b), a radial fracture of the sample occurred, which progressed from the base to the edge of the concrete cube.The same mechanism was also found for cubes with dimensions of 150x150x150 mm and it differed from the traditional hourglass-like breakdown of concrete, see The structure of the brick breakthrough was also examined under a microscope at 15 times magnification.The recycled steel fiber showed high tensile strength and the concrete split around its surface before the fiber was drawn, which confirms its strong adhesion to the mortar.The structure of the hardened brick itself is not smooth and has pores, see Figure 14a.The amount of visible pores on the surface of ruptured samples is generally greater due to the presence of additional fibers (Figure 14b).This is because concrete tends to crack along the path of least resistance, and pores in concrete are the weakest link when it comes to durability.

Conclusion
The goal of this research was to see if cement substitutes and waste material in the form of fiber, added together to a concrete matrix affect its material, mechanical and thermal properties.The study's methodology included making samples with individual cement substitutes, their combinations, additives of various waste fibers, and combinations of fibers and cement substitutes.Based on the last, the final product was also developed, i.e. a composite brick with waste material and cement substitutes, which can be a perfect example of the implementation of circular economy ideas in the construction sector.The following conclusions could be drawn based on the findings of our study: 1. Cement substitutes extended the initial and final setting time of fresh concrete mixtures.
No change however was observed when fibers were added to the mixture.
2. No significant influence on the density and pH has been noted after adding cement substi-tutes and/or fiber to concrete.
3. Average ratio of air content in fresh formulas from M0 to M10 was about 1.95%.It increased after the addition of fiber to a maximum value of 2.80% for 1.5% glass fiber addition.
5. No visible change in thermal properties has been reported after the substitution of cement with zeolite, metakaolinite, micrometakaolinite, or all of them.
6. Composite brick has greater thermal diffusivity and specific heat, and lower specific heat compared to ordinary concrete.
7. DIC method proved the convergence of failure mechanisms between traditional concrete and composite brick in the case of three-point bending.In the case of compression tests, mechanisms slightly differed.
employed to measure deformations of the final brick (based on the M20 formula).The tests were performed on an Instron 8802 testing equipment (Instron, Norwood, MA, USA) with 120 mm apart supports.The three-point bending test used samples with dimensions of 40x40x160mm for the deformation field testing under bending and 40x40x40mm under compression.Also, the surface of the samples was painted with white flexible paint to prepare them for tests.The deformation field deformation tests in the three-point bending test were carried out on samples with dimensions of 40 mm x 40 mm x 160 mm.Measurements of deformation fields in the compression test were carried out on samples with dimensions of 40 mm x 40 mm x 40 mm.The surface of the sample subjected to DIC measurements was prepared for testing by painting it with white flexible paint.Dantec Dynamics (Dantec, Ulm, Germany) sensors and ISTRA 4D software were used as well in a scientific study on digital picture correlation.Using deformation field analysis, the program was able to graphically display the progress of cracking that occurred in the samples.

Figure 6 .
Figure 6.Young modulus results of tested concrete samples modified with (a) cement substitutes, and (b) fibers.

3 Figure 10 .Figure 11 .
Figure 10.Correlation between applied force and sample deformation during (a) three-point bending test and (b) compression of the composite brick.

Figure 14 .
Figure 14.The inside structure of composite brick with marked (a) pores (in blue), and (b) steel fibers (in orange).

Table 3 .
Chemical composition of cement substitutes.

Table 4 .
The strength parameters of fibers.

Table 5 .
Physical properties of concrete formulas.The performed density tests showed slight differences in the values of this parameter, falling within the measurement uncertainty.All tested samples were classified as normal concrete, with the reference concrete sample reaching the average density of 2,338.3kg/m 3 .The average density of zeolite modification was 2,323.0kg/m 3 , 2,321.1 kg/m 3 and 2,314.7 kg/m 3 , respectively, for 5.0%, 10.0% and 15.0% of the substitute used in place of cement.Similar values were also found for samples containing metakaolinite, i.e. 2,326.2kg/m 3 , 2,325.6 kg/m 3 and 2,320.3kg/m 3 , and micrometakaolinite, i.e. 2,336.3kg/m 3 , 2,331.1 kg/m 3 and 2,328.9kg/m 3 , respectively for 5.0%, 38a for 1.5% addition.The highest increase in values was obtained for steel fibers modification, which corresponds with Algourdin et al.38research, and that way this fiber type was chosen to be incorporated into the final composite brick (M20).For 3 -92.4MPafor 5.0% substitution, 94.8 -96.7 MPa for 10.0% substitution and 101.7 -103.3MPa for 15.0% substitution.The highest value of compressive strength for this modification type was acquired by samples containing 5.0% of zeolite, 5.0% of metakaolinite, and 5.0% of micrometakaolinite (M10) and it was equal to 102.5 MPa.Concretes modified with recycled fibers also obtained greater values of compressive strength as their results were in the range of 95.6 -97.2 MPa for 0.5% addition, 98.9 -101.3MPa for 1.0% addition, and 100.6 -104.7 and 5.0% micrometakaolinite obtained an even higher value equal to 5.2 MPa.The lowest split tensile strength showed plain concrete (4.55 MPa) about which about 1.1 -3.5%, 1.8 -3.1%, and 2.2 -4.8 % increase was reported for zeolite, metakaolinite, and micrometakaolinite, respectively.