Research on the Effect of Fire Thermal Energy on the Microstructure and Properties Mechanical of Fiber-Reinforced Cement Mortars

Abstract

Cement mortar is made of a combination of cement, sand, and water, mixed in the right proportions. It is ideal for erecting walls and masonry structures, including those that must bear heavy loads. In addition, it is used in places that are exposed to higher humidity and in facilities located below ground level. The potential uses of a mortar are determined by material modification. The aim of the experimental studies was to evaluate the effect of high temperature on the microstructure and mechanical properties of cement mortars modified with polypropylene fiber. The novelty of this study is an attempt to compare the use of different types of polypropylene fibers in mortars heated at different temperatures. Cement mortars based on Portland cement CEM I 42.5 R with a constant content of three types of fibers in the amount of 0.9 kg/m³ were designed^. The samples were cured and then heated in an oven at 300, 500 and 700 °C. The functional properties of cement mortars, i.e., density, flexural and compressive strength after 28 and 56 days of maturation, as well as flexural and compressive strength at elevated temperature, were determined using samples of 40 × 40 × 160 mm. By modifying cement mortars with fibers, it is possible to obtain a cement composite with good strength parameters which is, at the same time, resistant to high temperature.

1. Introduction

In accordance with the adopted principles of sustainable development, low-emission and energy-intensive, environmentally friendly technologies and materials should be used, and waste should be managed. According to Plastics Europe (Association of Plastics Producers in Europe) and EPRO (European Association of Plastics Recycling and Recovery Organizations), in 2018, the global production of plastics and plastic products was 2.5% higher than in the previous year and exceeded 368 million tons. 61.8 million tons, then decreased in 2019 to 57.9 million tons, i.e., by 6.3%. The demand of the European market for polymers in 2019 amounted to a total of 50.7 million tons, which is slightly lower than in 2018 (51.4 million tons). Although the role of the EU in the global plastics market is decreasing due to the strengthening of the position of Asian countries, led by China (China’s share in the global production of polymers in 2019 reached 31%, i.e., 114 million t), Europe still has a significant share in the global market of 16% (59 million tons). The strength of the European market is mainly determined by the demand for polymers and plastic products reported by six countries—with Germany playing the dominant role with 24% market share. Poland, with a demand of 3.55 million tons (7% share in the European market), is ranked sixth—behind Germany, Italy, France, Spain and Great Britain [1].

Cement composites are commonly used construction materials all over the world due to their many undeniable advantages, primarily high compressive strength, fire resistance, low cost of operation and speed of construction. The functional properties of cement mortars are based on the specific proportions of the binder, fine aggregate and water. Their high compressive strength means that they are used as a binder in the joints of two structural elements (brick with brick). Their limitations, however, result from brittleness and easy cracking due to low tensile strength, shrinkage and poor impact resistance [2]. This material is unsuitable for some industrial construction projects that require greater adhesion and strength. Despite these disadvantages, the demand of users is growing, which forces the continuous development of technology and production of mortars [3]. A common method of modifying a cement mortar is the use of a polymer additive in the form of water colloidal suspensions—Styrene-butadiene rubber (SBR) and Styrene-acrylic ester copolymer latex (SAE). Mortars modified in this way are used to repair mortars or renovate floor tiles. Another common way of strengthening cement mortar is to add various fibers to their masses. The main purpose of using the fibers is to control cracks during the loading stage. Compared to steel and carbon-glass fibers, polymer fibers are attractive due to their high ductility, which affects the flexural strength of cement-based materials.

Cement mortar is a material similar to concrete that allows for waste disposal [4,5,6,7,8,9,10,11,12,13]. This is important in the case of hardly degradable or non-biodegradable materials, which include polymers. Polymer materials have been used in concrete technology as a substitute for cement [14,15,16] and aggregates [17,18] as additives in the form of polypropylene fibers [19,20,21].

The superior binding properties of fiber-reinforced cement mortars make them suitable for repairs of existing structures, but also for newly built elements [22,23,24].

The fibers used to strengthen mortars are most often made of coal, glass, basalt steel and polypropylene [25,26,27,28].

Polyamide (PA) and polypropylene (PP) fibers are successfully used in cement composites to improve hardness and impact resistance, control shrinkage cracking and significantly increase the energy absorption capacity of the material [29,30,31].

Habib et al. [32] conducted research to determine the effect of synthetic fibers (glass, nylon and polypropylene) on the mechanical properties of mortars. Such industrial plastic fibers can offer better mechanical performance than recycled plastics.

Szwabowski et al. [33] prepared a standard cement mortar with the addition of the following fibers: fibrillated polypropylene, single polypropylene, steel, glass and pitch-type carbon fibres. As a result of the tests carried out on a fresh cement mortar, it was found that mortars with the addition of fibers, compared to the reference mortar, showed worse workability. The deterioration of workability in the case of single, carbon and glass polypropylene fibers is caused primarily by the increase in the volume fraction of fibers in the mortar. Mortars with the addition of fibrillated steel and polypropylene fibers have the least impact on the deterioration of mortar workability.

Ezerskiy et al. [34] used cements of various classes to produce cement mortars. It was found that samples made of cement with a higher strength class significantly increased the strength of the mortar by up to 30%. Therefore, a question arises regarding the economics of using fibers, perhaps it will be more economical and effective to use cement with an increased strength class than using additives in the form of fibers [34].

The subject of the influence of high temperature on strength was discussed by Drzymała et al. [35], who found that polypropylene fibers cause an increase in bending strength in the case of high temperature; an increase in temperature to about 300 °C reduces the amount of moisture, resulting in an increase in bending strength. The exposure to temperatures of 500–600 °C destroys the reference samples, while samples with the addition of polypropylene fibers retain residual strength.

A literature analysis shows that polypropylene fibers are widely used in fiber-reinforced concrete and HPC concrete. The fibers bear the tensile stress, reduce the shrinkage deformation of the cement matrix and prevent the formation of cracks and fissures. In the case of concrete, the fibers also increase the strength of the concrete. Research carried out to date has shown that the use of polypropylene fibers (3) in concrete can reduce the phenomenon of explosive spalling of concrete linings, also known as spalling. In the event of a fire, the polypropylene fibers melt, leaving an open pore network that allows water vapor to escape, reducing its pressure, and the internal stress does not exceed the limit. However, all presented studies dealt with concrete. It is not clear from the literature on this topic what effect polypropylene fibers have on thermally stressed cement mortars and how the type of fibers used affects the behavior of these mortars.

The aim of the research work was to determine the effect of elevated temperatures on the strength properties of cement mortars reinforced with polypropylene fiber or polymer fiber. The annealing temperatures of the samples were selected to correspond to the conditions prevailing at different stages of fire in buildings. The temperatures selected for the tests are characterized by characteristic points where structural changes in mortars may occur. The tests were aimed at determining the effect of the fibers used in mortars on their flexural and compressive strength in normal conditions and after initial thermal loading. The annealing of the samples was carried out in accordance with the adopted temperature distribution curve, similar to the so-called standard curve used to test the fire resistance of objects.

2. Materials and Methods

2.1. Cement Mortar

The cement mortar samples for testing were prepared on the basis of Portland cement CEM I 42.5 with high early strength R (Cement Ożarów SA, Ożarów, Poland), which meets the requirements of EN 197-1:2012 [36] and PN-B-19707:2023-05 [37], CEN standard sand with a grain size of 0–2 mm (Kwarcmix, Tomaszów Mazowiecki, Poland), water and an additive in the form of polypropylene fibers and polymer fibers (Steel Poland Factory SC, Jaworzyna Śląska, Poland). The parameters of Portland cement are presented in Table 1. These values are average values guaranteed by the manufacturer for 2022. CEN sand has a certificate of compliance with PN EN 196-1:2016-07 [38]. The sand grains are rounded, the mass content of silicon dioxide (SiO₂) is 98%. The size distribution of standard sand was carried out by sieve analysis on a sample weighing 1350 ± g—Table 2. The temperature of the batches during the measurements was 20 ± 2 °C. The mortars were not modified in terms of the w/c ratio, which was constant and amounted to 0.5 [36,37,38].

As part of the existing experimental work, two types of samples were prepared to determine the effect of the fiber additive on the flexural and wall strength:

• CEM I, CEM II (without additive): reference mortar;

• CEM II 1, CEM II 2, CEM II 3 (with additive): mortar with the addition of fibers in the amount of 0.5% of the weight of the cement.

The recipes had the same composition of cement, aggregate and water, they differed only in the type of fibers introduced—Table 3.

Samples with dimensions of 40 × 40 × 160 mm were prepared for testing. The samples were cured under standard conditions for 24 h, and after removal from the mold, they were marked and stored in water at 20 ± 2 °C.

2.2. Fibers

CH₂(CH)₃, bundled and cut—Figure 1, compliant with EN 14889-2:2007 (Bautech Sp. z o.o., Piaseczno near Warsaw, Poland) were used for the tests [41]. In the mortar, they form a three-dimensional load-bearing network, resistant to gravity, thanks to which the mortar maintains a constant level, cracks are prevented and plastic shrinkage and the formation of shrinkage cracks are reduced. An additional advantage of the fibers is the ability to mitigate the explosive tendencies of the mortar during fire—the fibers melting lower the pressure of the volatile substances. The sinusoidal shape of macro-synthetic fibers allows for strong anchoring in the mortar, provides protection against cracks, eliminating the risk of corrosion, and provides adhesion.

2.3. Methodology of Testing

The prepared cement mortar was tested for consistency according to EN 1015-3:2000 [43] and bulk density according to PN-EN 1015-6:2000 [44]. To assess the influence of the type of fibers on the properties of cement mortars produced with their participation, the bending strength (f_cf) and compressive strength (f_c) were tested in accordance with the EN 196-1:2016-07 standard [36,43,44,45]. The Controls Advantest 9 hydraulic press (Controls, Warsaw, Poland) was used for the tests. The test was carried out on samples of 40 × 40 × 160 mm, with a sample size of 5 samples for each series. The beams were subjected to bending with increasing load until failure. A single-point load scheme was adopted—Figure 2. The beam halves formed after the test were used to perform the compressive strength test.

The resistance of cement mortars to high temperature was carried out in a PK 1100/5 furnace (Termolab SC, Warsaw, Poland) with electrically powered heating sections. The test was carried out in accordance with PN-EN 1363-1:2020-07 [46] at the temperatures of 20, 300, 500 and 700 °C—Figure 3. During the tests, the aim was to ensure that the temperature distribution over time was close to the conditions corresponding to a real fire. All prepared samples were dried to a constant weight at 105 ± 5 °C before starting the annealing. The signal from the thermocouples was recorded by a computer equipped with dedicated software [47].

The analysis and assessment of the effect of high temperature on the cement mortars was carried out omitting the cooling phase in a specific period. After reaching the appropriate temperature, i.e., 20, 300, 500 and 700 °C, the samples were heated for 30 min. This was to equalize the temperature in the entire volume of the element, and then it was freely cooled to ambient temperature. The total times of loading the composite with the temperatures of 300, 500 and 700 °C were 60, 120 and 180 min, respectively, for individual mortars.

2.4. Statistical Methods

After collecting the data for statistical analysis, statistical analysis was performed using Statistica 14 software (TIBCO Software Inc., Palo Alto, CA, USA) to identify selected characteristics of the variables. For the analyzed variables (strength/temperature), selected descriptive statistics were calculated—Table 4, including mean results, standard deviation, asymmetry and kurtosis coefficient (with appropriate standard errors) for all cases.

The arithmetic mean is an estimator of the expected value in the population, for which the level of variable temperature for the study group is similar for bending and compression at 20 and 300 °C and is approximately 7 and 40 MPa, respectively. We observe a weak concentration of results around the mean for compression (kurtosis is below 0). There are many extreme results that are far from the average; the value of kurtosis for bending at 20 and 300 °C is above 0, which means that the number of such observations is decreasing.

3. Results and Discussion

3.1. Consistency

Based on the tests carried out on fresh cement mortar, it was found that the fibers affect its parameters—consistency and density. The reference cement mortar based on CEM I obtained a spread of 14.3 cm. The largest spreading diameter equal to 15.2 cm was obtained by the CEM II mortar, the smallest equal to 8.0 cm by the CEM II 3 mortar. The results for the CEM II 1 and CEM II 2 mortars are 15.0 cm and 11.0 cm, respectively. The highest bulk density equal to 2.28 g/cm³ was obtained by the mortars CEM II 1 and CEM II 2, the lowest equal to 2.15 g/cm³ by the reference mortar. Fiber parameters such as type, length, shape and their content in the cement matrix significantly affect the consistency, but its density to a lesser extent—Figure 4 and Figure 5.

3.2. Compressive and Bending Strength

The flexural strength was checked in two periods of maturation after 28 and 56 days on the trabeculae. The samples were subjected to a bending moment in the three-point bending scheme—Figure 6. During the test, the samples were set in such a way that their formulation surface was perpendicular to the direction of load application. The cement mortar halves formed during the test were used to determine the average compressive strength—Figure 7.

After the tests, a positive effect of polypropylene (1) and polymer (2) fibers on the compressive strength of cement mortars was observed. The highest strength in the first period of the measurement was 51.6 MPa for the CEM II 1 mortar, the smallest was 40.0 MPa for the CEM II 3 mortar. Samples of mortars made based on CEM II cement without additives obtained a strength higher by 10% than the standard one, while the CEM II 2 samples showed a strength higher by only 3% compared to CEM I. In the second period of curing, after 56 days, the highest compressive strength was 58.0 MPa, obtained by mortar CEM II 1, while the smallest amount was 41.8 MPa for mortar CEM II 3—similar to the first measurement. Compared to the reference mortar, the increase was 17%, while the decrease was 15%.

The increase in flexural strength in selected cases was also confirmed in the latest studies [48]. The strength of samples containing 0.15% of the volume increased by as much as about 40% in relation to the control samples. With a higher fiber content, however, a decrease in strength was observed. Regarding the compressive strength after 28 days, slight decreases were found, which increase with the fiber content. With an amount of 0.25% by volume, the compressive strength drops by as much as 36% compared to control samples. Additionally, an earlier study by Habib et al. [32] and Chen et al. [49] showed that adding PPF to cement mortar can increase the compressive strength. However, Feng et al. [50] concluded that adding too much PPF may reduce the compressive strength, but in general, the availability of polymers in a cement mortar plays an important role in increasing both its compressive and flexural strength. In these studies, attention was also paid to the length of polypropylene fibers. By stating that much better strength results are obtained in the case of long fibers dosed in the amount of 0.6 kg/m³.

3.3. Influence of High Temperature on Strength

Figure 8 shows the results of bending strength tests. The results are the arithmetic mean of five measurements. The bar halves obtained after the flexural strength test were used to determine the average compressive strength of 10 measurements. The results are shown in Figure 9. Error bars with a value of 5% are marked on the bar graphs.

The analysis of the results of the tests on the impact of CEM I cement substitutes and fiber additive on the flexural strength showed differences between the tested variants. Mortars based on CEM II were characterized by an increase in flexural strength for the temperature of 300 and 700 °C, by 11% and 30%, respectively. By introducing polypropylene 1 and polymer 2 fibers into individual mortars, an increase in strength was observed for each firing temperature. Different results were observed for mortars with polymer fibers 3; the strength for each variant is lower than the strength of the reference mortar.

The analysis of the test results of the effect of CEM I cement substitutes and the addition of fibers on the flexural strength showed differences between the tested variants. Mortars based on CEM II were characterized by an increase in flexural strength for the temperature of 300 and 700 °C, by 11% and 30%, respectively. By introducing polypropylene 1 and polymer 2 fibers into individual mortars, an increase in strength was observed for each firing temperature. Different results were observed for mortars with polymer fibers 3; the strength for each variant is lower than the strength of the reference mortar.

It was observed that the use of CEM II cement and fibers had a positive effect on the increase in compressive strength for the CEM II, CEM II 1 and CEM II 2 samples. Comparing the obtained results of mortars with the addition of standard mortar, the best results were recorded for the mortar made with CEM II cement and polymer fibers 2. Mortar with polymer fibers showed a decrease in compressive strength.

Importantly, during the tests of samples with fibers, their explosive destruction (spalling) did not occur, thus avoiding the phenomenon of the coupled thermo-hydro-chemo-mechanical mechanism described, among others, in [51]. The confirmation of the beneficial effect of PPF fibers on the phenomenon of spalling was confirmed in several publications [52,53]. Polypropylene fibers, in addition to strengthening the structure of the cement matrix, are also used to limit the explosive behavior of concrete because of high temperature. These fibers melt at a temperature of about 170 °C, i.e., at a temperature lower than the maximum water vapor pressure in concrete, which is about 220 °C. After melting, polypropylene is partially or completely absorbed by the cement matrix, which results in the formation of evenly distributed voids throughout the structure, constituting the internal volume for the increasing water vapor pressure. At 350 °C, polypropylene burns into CO₂ and H₂O.

Polypropylene fibers are also used to increase the high temperature resistance of concrete (300–400 °C). The fibers prevent the phenomenon of peeling (breaking) from occurring. Concrete fragments rapidly disintegrate (sink) at high temperatures (in the event of a fire), exposing the reinforcement and thus causing a loss of the load-bearing capacity of the reinforced concrete member.

The beneficial properties of using PPF fibers in mortars subjected to thermal loads have also been demonstrated in [54]. In tests carried out at temperatures up to 1000 °C with a fiber content of 0.2%, during the compressive strength tests, a residual strength greater by 19% was obtained in relation to the control mortar without fibers. However, in [55], it was shown that the increase in residual strength can be obtained by hybrid use of polypropylene and basalt fibers in amounts of 0.1 and 0.2%. When the samples are loaded with a temperature of 200 °C, the residual compressive and bending strength increases by about 20%.

3.4. Test Results of the Structure and Composition of the Samples

The highest compressive strengths were obtained in a series of samples containing polypropylene fibers (CEM II 1 series) and this series of samples was selected for the structure tests. Polypropylene fibers 6 mm long and 20 μm in diameter were used (Figure 10).

Samples with the largest fiber dimensions (CEM II 3 series) were selected for the study of composition differences. The series contained polymer fibers with a length of 39 mm and a diameter of 780 μm. To determine the differences in the elemental composition of concrete samples (especially in the binder and places where the fiber and binder meet) caused by the temperature of their annealing (for temperatures of 20, 300, 500 and 700 °C), scanning electron microscopy SEM (FEI Quanta 3D FEG scanning electron microscope) with additional equipment was used m in the form of an EDS microanalyzer. The software allows for the automatic determination of the elemental composition of samples and can create profiles that determine the oxide content of elements.

Figure 11, Figure 12, Figure 13 and Figure 14 shows a SEM photograph containing breakthroughs of the base sample and samples annealed at 300, 500 and 700 °C (CEM II 1 sample series).

In the SEM photographs, the cracks appeared only in the samples heated at 500 °C. Heating at a temperature of 700 °C causes greater degradation of the samples and an increase in the opening of cracks. The obtained results confirm the known patterns of concrete destruction, in which the process of degradation of its structure begins at a temperature of over 200 °C.

Below are the photographs taken with a scanning microscope showing the points where the EDS analyses of the tested samples were carried out. Analyses were performed at two points on each sample. The first point was where the fiber was previously located, and the second point was the binder. The following figures are EDS microanalyses of the points marked in the photograph.

Table 5 presents the results of the EDS analyses, i.e., the oxide compositions of four concrete samples. The results were limited to four main components: CaO, SiO₂, Al₂O₅ and Fe₂O₃.

The obtained result of the EDS analyses (oxide compositions) are characterized by a large dispersion of the obtained values. Based on these, it is difficult to determine whether we are dealing with dehydroxylation of portlandite (decrease in the amount of calcium hydroxide in the cement matrix), this process takes place at a temperature of about 500 °C. The effect of the temperature in the range of 600–700 °C, resulting in, among other things, the decomposition of calcium carbonate with a simultaneous increase in the volume of calcium compounds, which causes concrete cracking and a significant decrease in its strength, is not shown.

4. Conclusions

The analysis of the obtained results of the experiments allowed the formulation of the following conclusions:

• The tests of fresh cement mortar showed that the introduction of PPF fibers had a significant effect on consistency. However, much smaller differences occur in the density of the mixture. The results in this area are varied and depend on the type, length and shape of the fibres.

• In all cases tested as part of the strength tests, the samples after 56 days of maturation obtained higher strength. These differences were particularly large for type I fiber and amounted to about 80% for bending strength and 13% for compression in relation to tests carried out after 28 days of maturation.

• It has been shown that the shape and type of PPF fibers have a significant impact on strength at normal temperatures of 20 °C. Type I fibers have the highest strength (higher than reference samples) and type III fibers have the lowest strength. These differences are quite significant.

• Similar to I, during the tests at elevated temperatures, the highest residual strength was shown by type I fibers. Importantly, the addition of fibers of all types resulted in a decrease in the strength decrease both during compression and bending. In general, it can be stated that the addition of PPF fibers has a positive effect on the residual strength of mortars.

• Spalling of samples with PPF fibers was not observed during annealing. This confirms the previous results of studies widely described in the literature.

• The obtained results of the EDS analyses (oxide compositions) are characterized by a large dispersion of the obtained values. It is difficult to clearly indicate the effect of high temperatures on the formation of mortar cracking.

Author Contributions

Conceptualization, G.R., A.P. and P.O.; methodology, G.R., A.P. and P.O.; software, A.P.; validation, A.P., J.S., K.W. and G.R.; formal analysis, A.P. and P.H.; investigation, A.P. and M.Ż.; resources, P.O.; data curation, G.R., A.P. and P.O.; writing—original draft preparation, G.R., A.P., J.S. and P.O.; writing—review and editing, G.R., A.P. and P.O.; visualization, A.P. and G.R.; supervision, G.R.; project administration, A.P. and M.Ż.; funding acquisition, G.R. and P.O. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data presented in the study are publicly available at the Institute of Civil Engineering, Warsaw University of Life Sciences, 166 Nowoursynowska Street, 02-787 Warsaw, Poland.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Characteristics of fibers [42]: (a) Polypropylene fibers; (b) Polymer microfibers; (c) Polymer macrofibers.

Figure 1. Characteristics of fibers [42]: (a) Polypropylene fibers; (b) Polymer microfibers; (c) Polymer macrofibers.

Figure 2. Flexural strength tests: (a) Sample under test; (b) Schematic of the sample.

Figure 3. Annealing furnace annealed samples.

Figure 4. Consistency testing for mortar with polymeric microfiber CEM II 3: (a) mortar in a cone; (b) mortar flow.

Figure 5. Consistency and density of cement mortars.

Figure 6. Flexural strength after 28 and 56 days of maturing at 20 °C.

Figure 7. Compressive strength after 28 and 56 days of maturing at 20 °C.

Figure 8. Flexural strength for individual temperatures.

Figure 9. Compressive strength for individual temperatures.

Figure 10. SEM photographs of fractures of concrete samples of the CEM II 1 series (magnification 2000×): (a) base sample (20 °C); (b) sample heated at 300 °C; (c) sample heated at 500 °C; (d) sample heated at 700 °C.

Figure 11. Base sample (20 °C): (a) SEM photograph with points A and B marked; (b) the results of the EDS microanalysis of the sample point A; (c) the results of the EDS microanalysis of point B of the sample.

Figure 11. Base sample (20 °C): (a) SEM photograph with points A and B marked; (b) the results of the EDS microanalysis of the sample point A; (c) the results of the EDS microanalysis of point B of the sample.

Figure 12. Base sample (300 °C): (a) SEM photograph with points A and B marked; (b) results of the EDS microanalysis of the sample point A; (c) results of EDS microanalysis of sample point B.

Figure 12. Base sample (300 °C): (a) SEM photograph with points A and B marked; (b) results of the EDS microanalysis of the sample point A; (c) results of EDS microanalysis of sample point B.

Figure 13. Base sample (500 °C): (a) SEM photograph with points A and B marked; (b) the results of the EDS microanalysis of the sample point A; (c) the results of the EDS microanalysis of the sample point B.

Figure 13. Base sample (500 °C): (a) SEM photograph with points A and B marked; (b) the results of the EDS microanalysis of the sample point A; (c) the results of the EDS microanalysis of the sample point B.

Figure 14. Baseline sample (700 °C): (a) SEM photograph with points A and B marked; (b) the results of the EDS microanalysis of the sample point A; (c) the results of the EDS microanalysis of the sample point B.

Figure 14. Baseline sample (700 °C): (a) SEM photograph with points A and B marked; (b) the results of the EDS microanalysis of the sample point A; (c) the results of the EDS microanalysis of the sample point B.

Table 1

Mechanical, chemical, and physical parameters of cement [39].

Parameters	Standard RequirementsPN-EN 197-1	Average Values Achieved
Compressive strength, MPa
after 2 days	≥20	28.6
after 28 days	≥42.5 ≤ 62.5	58.2
Loss on ignition, %	≤5.0	2.88
The residue is insoluble, %	≤5.0	0.83
Sulphate content SO3, %	≤4.0	2.65
Chloride content Cl−, %	≤0.10	0.04
Start of setting time, min	≤10	197
Consistency, mm	≤10	0.1
Specific surface, cm2/g	no requirements	3802
Alkali content Na2Oeq	no requirements	0.67

Table 2

Size distribution of standard sand grains [40].

Square mesh side dimension, mm	2.00	1.60	1.00	0.50	0.16	0.08
Total sieve residue, %	0	7 ± 5	33 ± 5	67 ± 5	87 ± 5	99 ± 1

Table 3

Concrete mix proportions by weight.

Specification	Mass of Concrete Ingredients, g
	Aggregate	Water	Cement	Fibers
CEM I, CEM II	1350	225	450	-
CEM II 1, CEM II 2, CEM II 3	1350	225	405	41

Table 4

List of selected descriptive statistics.

Variable	Descriptive Statistics (Mortars with Fiber)
	Valid N	Mean	Median	Mode	Minimum	Maximum	Variance	StandardDeviation	StandardError	Skewness	StandardError Skewness	Kurtosis	StandardError Kurtosis
Flexural20 °C	25	7.8	7.0	6.57	5.4	12.7	4.290	2.071	0.414	1.055	0.464	0.188	0.902
Flexural300 °C	25	7.3	7.1	6.81	6.1	9.4	0.626	0.791	0.158	0.725	0.464	0.284	0.902
Flexural500 °C	25	5.2	5.4	5.40	2.3	7.3	1.935	1.391	0.278	−0.694	0.464	−0.026	0.902
Flexural700 °C	25	0.5	0.5	0.001	0.0	1.2	0.148	0.385	0.077	0.168	0.464	−0.830	0.902
Compressive 20 °C	25	45.6	45.0	50.94	39.1	53.4	17.565	4.191	0.838	0.295	0.464	−1.096	0.902
Compressive 300 °C	25	43.3	42.5	0.001	34.1	53.1	28.976	5.383	1.077	0.414	0.464	−0.685	0.902
Compressive 500 °C	25	29.6	26.9	0.001	20.9	40.3	33.531	5.791	1.158	0.582	0.464	−0.872	0.902
Compressive 700 °C	25	13.0	13.1	14.06	7.5	18.4	10.206	3.195	0.639	−0.023	0.464	−0.914	0.902

Table 5

Oxide compositions of four samples (measurement points: fiber site and binder).

Temperature	Measurement Site	CaO	SiO2	Al2O5	Fe2O3
20 °C	fiber site	41.54	26.68	5.85	1.52
	binder	51.36	17.62	5.54	2.00
300 °C	fiber site	59.71	4.25	1.10	0.41
	binder	63.59	4.41	1.95	0.55
500 °C	fiber site	65.54	10.75	2.66	0.76
	binder	63.75	7.60	9.33	1.64
700 °C	fiber site	47.77	17.53	4.02	0.65
	binder	62.39	11.90	2.98	1.13

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