Fire Safety and Impact and Frost Resistance of Basalt Fiber-Reinforced Polysiloxane Matrix Composite Processed under Partial Pyrolysis Conditions

Abstract

The study focuses on developing a fiber-reinforced composite that would exhibit good mechanical properties and climate resistance, and fire safety parameters would surpass commonly used fiber-reinforced polymers. The subject of development is a polysiloxane thermoset matrix reinforced with basalt fibers, which is processed by partial pyrolysis at 650 °C after curing. The heat release rate test showed virtually zero heat released, and the toxicity test showed only a very low amount of carbon monoxide. The flammability test showed no ignition, no radiation, and no glow. Composites for mechanical tests were prepared in three variants differing in the distribution of reinforcement. Due to the intended use of the composite for thin-walled panels or shells of buildings, the mechanical properties were compared in identical tests with fiber cement plates. The flexural strength of the composites was 3 to 10 times, and the impact energy was 10 to 100 times higher than the values measured on fiber cement, depending on the type and orientation of the composite. The flexural strength measured after 240 freeze–thaw conditioning cycles is higher than fiber cement by 1.3 to 2 times. The climate resistance of the composite should be the subject of further development.

1. Introduction

Light mechanically stressed transport and building structures can be produced using shell or plate parts from composites with a polymer matrix. However, when assessing fire resistance, these materials will present varying degrees of risk depending on the type of matrix chosen and the possible use of fillers added to ensure self-extinguishing. With these materials, the formation of smoke and gaseous combustion products must be expected, from the case with a specific excess of oxygen to the case close to pyrolysis conditions, which can occur primarily in the internal parts of the structure [1,2,3,4,5,6]. When using self-supporting structures made of fiber-reinforced polymers (FRP), it is also necessary to consider a particular reduction in the structure’s load-bearing capacity even at temperatures several 10s of degrees Celsius above standard room temperature [7,8,9,10,11,12,13,14,15].

1.1. Basalt Fibers and Their Use in Terms of Temperature and Fire Resistance

Basalt fibers for FRP are a suitable choice in terms of both mechanical properties and fire resistance [16]. Their non-negligible advantage is their low price. Basalt fibers are also considered ecological due to the lowest energy consumption of their production among industrially produced fibers used for reinforcing composites [17,18]. Basalt fibers alone enable the preparation of composites meeting fire safety A1, as they are entirely non-combustible and do not produce gaseous emissions or release drops of burning or molten material at the temperatures reached during a fire [19].

In some older popularization studies, basalt fibers were referred as a refractory materials with reference to the temperature parameters of natural basalt rock or basalt technical materials obtained by melting and subsequent long-term crystallization (temperature transformations of crystalline phases of basalt used for petrurgic purposes are shown, for example, in [20,21]). When spinning from a very pure melt, at a drawing speed of 2000 to 5000 m/min [22], the fibers cool down quickly, leading to their completely amorphous structure, and compared to crystalline basalt, their mechanical properties and thermal characteristics are very different.

These fibers do not surpass E-glass fibers by the softening temperature, as shown by the experiments presented in [23]. The deformation level ɛ = 0.001 on the creep curve under constant load was reached by basalt fibers at 630 °C, E-glass fibers at 700 °C, and R-glass fibers at 840 °C. The crystallization of basalt fibers in real-time can technically occur at 650 °C; in some types, a broad diffuse peak of spinel appears on XRD after one hour of annealing. After annealing at 750 °C, basalt fibers show sharp diffractions of clinopyroxene and spinel, while the diffractogram of E-glass fibers remains practically unchanged compared to the unannealed state [23].

In production methods, chemical composition, mechanical properties, prerequisites for fire safety, and some physical properties, basalt fibers are similar to glass fibers, whereas E-glass fibers are the cheapest and most commonly used fibers for reinforcing composites. The tensile strength reported by various authors is in the range of 2175–4840 MPa, and the modulus of elasticity is in the range of 75–115 GPa, as reported in the review [18] based on the data of 23 publications. The advantage of basalt fibers compared to E-glass fibers is their higher resistance to chemical attack, which improves their prerequisites for reinforcing concretes and mortars. Reviews [16,24,25] summarize the results of works focused on the effect of the amount of added fibers and their length and report the positive effect of the addition of basalt fibers on compressive and flexural strength at an optimal dosage of 1% and a fiber length of 10 to 20 mm in chopped form. This addition also leads to an increase in temperature durability and freeze–thaw cycle resistance. However, the review [18] also reports studies with less favorable results of direct concrete reinforcement with basalt fibers. The findings mentioned above indicate that the positive influence of basalt fibers in direct contact with reinforcing concrete can most likely be found in their bridging effect in the concrete matrix [25].

Several recent works have been devoted to basalt reinforcements of concretes and mortars where fibers are placed in a polymer matrix to prevent chemical attacks on the fibers. The parameters and use of basalt fiber-reinforced polymer (BFRP) rebars for concrete are determined by the standard [26] from 2023. One of the critical aspects of this application is the fire behavior of this concrete/polymer/basalt-fiber hybrid composite. A fire resistance test and measurement of compressive strength at elevated temperatures were performed on a concrete column, where basalt fiber-reinforced vinyl ester rebar [27] was used as reinforcement. The column withstood the fire resistance test for 240 min under a load on level 1/3 of its calculated load-bearing capacity. The compressive strength decreased at 100 °C to approx. 20% and at 200 °C to 7% of the original strength value.

The fire performance of BFRP with a matrix of vinyl ester resin was investigated in detail in [28,29], and similar results were obtained in comparative tests with glass fiber reinforcement. When applying a heat flux of 25 and 50 kW.m⁻², the basalt composite exhibits an approximately 100 °C higher surface temperature, higher creep speed (axial extension), and a shorter time to the fracture. In this work, the effect of temperature on the tensile strength of fibers without matrix was also investigated, where both types of fibers reached very similar values. Basalt reinforcement has slightly better parameters at the lower temperatures of these tests.

In conclusion, it is worth noting that when comparing the temperature properties of silicate fibers, E-CR fibers should also be considered, as they show increased temperature and chemical resistance and are currently at a similar price level as E-glass and basalt fibers.

1.2. Polymer Matrices for BFRP and Their Fire Safety

Due to the relatively high fire safety of basalt fibers, the fire properties of BFRP depend primarily on the chemical structure of the polymer matrix. The highest temperature resistance is shown by polymeric ceramizable matrices, whose temperature properties correspond to refractory materials. However, these matrices are not suitable for reinforcement with basalt fibers, as their ceramization requires temperatures above 800 °C during several hours of exposure, which leads to the crystallization of the basalt fibers and overall brittleness of the composite [30,31,32]. Another option is to use highly non-flammable polymer materials with a ladder structure, where two chains of the polymer skeleton running parallel are connected in each monomer unit by a covalent bond or with a more complicated double-strand cross-linking. Currently, these polymers are rather the subject of development for electronic use [33]. Polymers with cyclic aromatic components such as polybenzoxazoles (PBOs), polybenzimidazoles, polybenzthiazoles (PBTs), or polyimides also exhibit high fire resistance, whose gaseous products arising during combustion prevent the spread of fire [34]. Due to the high price, difficult processability, and problematic assessment of the toxicity of the fumes produced during a fire, neither ladder structure polymers nor polymers with cyclic aromatic components are completely ideal solutions for fire safety.

Currently, the most widespread solution to the fire safety of polymers is using flame-retardant additives and fillers. These additives either reduce the thermal potential of the combustion of free radicals H and OH in the gas phase (bromine, chlorine, or phosphorus compounds) or slow down or prevent combustion in the solid phase (metal oxides and hydroxides and salts). Phosphorus and nitrogen compounds work effectively in both the liquid and gas phases of the fire-affected polymer. For example, a summary of the actual state of the art using flame retardants for fiber composite matrices was published in [35,36]. Even in the case of polymers protected by flame-retardant additives, the problem of the toxicity of the fumes arising from burning or pyrolysis at a sufficiently high temperature in the later stages of the fire when the heat source outside these materials partly persists.

A very variable group of polymers is represented by inorganic polymers, i.e., whose backbones do not contain carbon. The reduced flammability of inorganic polymers lies in the eventual non-flammability of the backbone. For example, polysiloxanes with a backbone (-Si-O-Si-) represent a polymer with limited flammability, which depends mainly on the oxidation potential of specific side groups. Polysiloxanes are commonly industrially produced in many polymer structure variants with good processability and a price acceptable for many industrial areas [37]. The results of the experiments below, which are the subject of this study, show that the relatively good fire resistance of the polysiloxane matrix can be further improved by subsequent high-temperature processing.

1.3. Aim of This Study

This work aims to design and test BFRP, whose fire properties are further improved by partial pyrolysis of the composite conducted at up to 650 °C. The technology of these partially pyrolyzed composites (PPC) is relatively simple, and the fibers and polymer precursor used are among the cheapest in the field of composite materials. Such materials could be used as building envelopes or lightweight fire-resistant walls for vehicles. The matrix precursor of the composite is polymethylsiloxane thermoset. As previously published, the best mechanical properties are achieved by controlled heating of the cured composite in a protective atmosphere up to 650 °C, when changes in the chemical structure of the polysiloxane begin to occur, primarily by the release of some less stable side groups of the cross-linked thermoset. The preparation and properties of these materials have already been tested both by the authors of this study [38,39,40] and in the laboratories of other authors [41,42,43,44]. Carbon fibers and several types of glass fibers were also tested for reinforcement of partially pyrolyzed composites, but by far, the best results were obtained with basalt fiber reinforcement, as shown in [45].

The temperature resistance of these composites was investigated in [46] during long-term exposure in the range of 300 to 600 °C. Annealing for 1000 h resulted in a significant reduction in flexural strength even at lower temperatures: at 300 °C, the strength is reduced to approximately half the value, at 400 °C, it drops to 18%, at 500 °C to 12%, and at 600 °C, to 7.5% of the original value. It is, therefore, clear that this composite material is not very suitable for mechanically stressed applications permanently exposed to elevated temperatures. However, compared to other types of FRP, it has good prerequisites for achieving good results in various fire safety tests. Moreover, thermogravimetric experiments performed on polymethylsiloxane resin after pyrolysis at 650 °C, carried out up to 1000 °C in both air and under pyrolysis conditions, show very low weight losses in both cases, from which the low risk of release of toxic of vapors at low and high temperatures can be concluded [47]. Therefore, a heat release rate test, standardized toxicity test, and flammability test were performed as part of this study. A targeted assessment of the toxicity of the resulting combustion products was not carried out, as it is an experimentally demanding specialized problem [48,49] requiring a separate study.

A characteristic feature of partially pyrolyzed composites is the acceptable bending strength of the unidirectionally reinforced composite, reaching 860 MPa [38] and a satisfactory impact energy to failure of 5 kJ/m² [40] for a number of applications. Unfortunately, a bending strength of only 130 MPa [50] can be achieved in the case of reinforcement with woven reinforcement with a plain weave. A similar excessive flexural strength reduction also occurs in lamination, which has not yet been systematically investigated.

To clarify the adverse effect of low flexural strength of laminated partially pyrolyzed composites, three compositional combinations of unidirectional and fabric reinforcement were investigated in this study: a composite with only woven reinforcement, a composite with unidirectional reinforcement with a composition of 0°/90°/0°, and a composite combining woven and textile reinforcement.

One of the intended uses of the partially pyrolyzed composite is as a plate or shell element of the outer shell of buildings or their roof coverings, where it is necessary to take into account the effects of demanding climatic conditions. Due to the higher porosity of this composite compared to conventional FRPs, there is a need to perform cyclic freeze–thaw tests of frost resistance. In order to assess the effect of the freeze–thaw process, two types of fiber cement coverings, which have been developed and expertly discussed worldwide for a long time, were tested in parallel [51,52,53,54].

In order to estimate the potential of the partially pyrolyzed composite to withstand the impact of hail and falling ice frosts or to enable problem-free installation or technological adjustments, impact tests were performed and fiber cement boards were also tested in parallel as a comparative material.

2. Materials and Methods

2.1. Materials

The composites in the investigation were prepared using standard composite technology for thermoset matrices, and then high-temperature treatment was applied as the final technological step. Kamenny Vek KV13 (Basfiber®, Dubna, Russia) basalt fibers were used as reinforcement as roving for unidirectional reinforced layers. According to the roving manufacturer, the roving fiber has a strength of 2070 MPa and a modulus of elasticity of 71 GPa; the linear density of roving is 340 tex and the determined density of the roving fiber is 2.60 g/cm³. The plain weave was custom made (VÚLV, spol.sr.o., Šumperk, Czech Republic). According to the fabric manufacturer’s data, the fabric fiber has a strength of 1870 MPa and a modulus of elasticity of 85 GPa, the determined density is 2.67 g/m³, the areal density of the fabric is 186 g/m², the linear density of the wrap and weft is 110 tex, and the thread count (warp/weft) is 110/90. The polymethylsiloxane resin Lukosil M130 (Lučební závody a. s., Kolín, Czech rep.) was used as a matrix precursor. Figure 1 shows the chemical structure of this resin and its changes during the processing performed in this work.

Prepregs for the production of composites were prepared by saturating the fibers in diluted resin. The fabric layers of the reinforcement were saturated by hand application. Unidirectional prepregs for unidirectional layers were prepared by wet winding. After drying the solvent, the resulting prepregs were assembled into a mold according to the type of composition, pressed at 0.3 MPa, and cured at a uniformly increasing temperature up to 250 °C. Cured composite bodies were inserted into the high-temperature mold and mechanically pre-tensioned. High-temperature treatment was carried out in a pyrolysis furnace under a protective nitrogen atmosphere at a heating rate of 15 °C per hour up to 650 °C followed by a ten-hour dwell at this temperature. Subsequently, the processed material was cooled at a rate of 50 °C per hour. The details about the used technology are described in [26,29]. No catalysts affecting curing, adhesion-improving additives, or additional impregnations were used for the production of the composites.

Three compositional variants of the composite in the investigation were prepared:

• (1). A composite composed of plain weave fabric (2D layers) with a thickness of 4.2 mm, hereinafter referred to as T composite;

• (2). A composite composed of two outer 1D (unidirectionally reinforced) layers with a thickness of 0.6 mm and one inner layer reinforced with plain weave fabric with a thickness of 3 mm, hereinafter referred to as UT composite;

• (3). A composite composed of two outer 1D layers with a thickness of 0.5 mm and one inner 1D layer with a thickness of 2.7 mm oriented perpendicular to the surface layers, hereinafter referred to as U composite. The compositional concept of the second and third production variants of the composite assumes that the unidirectional surface layers will protect the somewhat porous and permeable textile core against penetrating water or moisture. The in-plane dimensions of the resulting composite boards were approx. 120 mm × 120 mm and the nominal thickness of composite T and UT was 4.2 mm and composite U was 3.6 mm. The cross-sectional and top views of prepared composite boards are illustrated in macro photographs in Figure 2. The manufacturing process for preparing samples for fire safety tests is shown in Figure 3A. The manufacturing procedure for preparing samples for static mechanical property tests, impact tests, and flexural strength tests on freeze–thaw conditioned samples is shown in Figure 3B.

To compare the utility properties of composites in the investigation, the basic physical and mechanical properties, impact resistance, and frost resistance were tested using an identical methodology on fiber cement boards Cembrit (Czech Rep.)—coverings for altitudes of central Europe to 600 m.a.s.l. (C1) and for altitudes of 600 to 900 m.a.s.l. (C2). The field of application of fiber cement approximately corresponds to the intended applications of composites in investigation, mainly concerning their climate resistance and fire properties.

2.2. Experimental Methods

Flexural strength tests were measured on an Inspekt 100 universal testing machine (Hegewald and Peschke, Nossen, Germany) using three-point bending with a support span of 40 mm. The modulus of elasticity was measured using a four-point bending fixture with an external support span of 40 mm and an internal load span of 20 mm according to EN 843-2. Flexural strength was used to monitor degradation during freeze–thaw cyclic experiments.

The response of the materials to the impact loading was simulated by the Charpy test of unnotched specimens. An instrumented impact tester B5113.303 (Zwick/Roell, Ulm, Germany) equipped with the instrumented hammer of nominal capacity of 15 J and a support span of 40 mm was used. The initial impact energy was lowered by setting the release angle to 30°, which corresponds to an energy of 1 J and impact speed of 1 m/s. Loading traces were recorded and further analyzed to determine the development of consumed impact energy; the total consumed energy was calculated from the difference between the release and rise angles. The nominal dimensions of the testing bars were 7 × 4 × 50 mm. At least 12 measurements were conducted per material.

The toxicity test of combustion products was carried out according to the technical standard ČSN EN 45545-2. The specimens with dimensions of 75 × 75 × 2 mm were exposed to the radiant heat of a conical radiator with an intensity of 50 kW/m² with a pilot flame. The test took place in a smoke chamber (equipment according to ČSN EN ISO 5659-2). In the 4th and 8th minutes of the test, the gaseous byproducts of the thermal degradation of the material were taken via a heated path to the FTIR cell of the IGS Antaris spectrometer (Nicolet, Thermo Fisher Scientific Inc., Waltham, USA), where the concentrations of the gaseous components CO, CO₂, HCl, HBr, HF, HCN, SO₂, and NO_X were measured. The optical length of the FTIR cell was 2 m.

The heat release rate (HRR) was determined by a cone calorimeter according to ISO 5660-1 (Reaction-to-fire tests), which evaluates the effect of radiant heat flux on the surface of the test object. A test sample with dimensions of 100 × 100 × 2 mm was exposed to a radiant heat flux of 50 kW/m². This test is terminated depending on the behavior of the tested material according to the state that occurs first: (a) after 32 min in the case of continuous burning of the sample, (b) after 30 min if the sample does not start burning, (c) when the sample burns out, i.e., after reaching oxygen concentration values when it returns to the initial value, and (d) if the mass of the sample does not become zero.

The ignitability of materials was tested according to the ČSN 64 0149 standard. A sample of material weighing 2–3 g was heated in a chamber furnace by an air stream of a certain constant temperature and constant flow rate. During the test, it was observed whether the material burst into flames, ignited, or glowed within 15 min. The formation of a flame or flameless burning (glow) was detected visually and simultaneously by monitoring the temperature of the sample during the test.

For observation of the microstructure of composites by scanning electron microscopy (SEM), a Quanta 450 microscope (FEI, Hillsboro, USA) was used.

The freeze–thaw cyclic process was carried out in a laboratory Q-cell 80 ZN (Pol-Lab, Wilkowice, Poland) programmable freezer equipped with self-constructed automatics for regular heating and moistening of climatically cycled samples. The scheme of the testing system is shown in Figure 4a and an example of placement of specimens is shown in Figure 4b. During the tests, the requirements of the EN 539-2 standard were followed. The cooling time between +1 and −3 °C lasted 30 to 45 min and during the next 30 min; the samples were cooled down to −18 °C. Heating and moistening of the samples was ensured by the flow of water at 15 °C. Intensive wetting of the porous structure of the tested material is ensured by wrapping the samples in a linen woven fabric as is shown in Figure 4b. The scheme in Figure 5 shows the conditioning process at the cyclic freeze–thaw test.

3. Results

3.1. Characterization and Basic Properties of Investigated Composites

Three production variants of the developed composite, T, UT, and U, were subjected to mechanical tests together with two reference fiber cements, C1 and C2, used for comparison. The basic properties measured on developed composites and reference materials are listed in Table 1.

In terms of geometric and physical parameters (thickness of board materials, density, and porosity), developmental composites and comparative fiber cement materials have similar values. As can be expected, the composites show higher values of flexural strength and also higher values of elastic moduli measured during static bending. The UT composite with textile reinforcement shows balanced strength in both directions. However, it is necessary to assume a lower strength of composite in the direction ±45° to the warp of the woven layers.

The main intention for the development of composite variants UT and U was to achieve a surface layer with reduced porosity that would better resist the destructive effects of the climate. Objectively, the material nature of the composites processed at 650 °C does not provide for the lamination of differently oriented layers with such high strength as could be expected from the parameters measured on unidirectional composites of this type. It should also be mentioned that fabric-reinforced composites in the investigation have flexural strength lower than what could be expected from the parameters of unidirectionally reinforced composites [29]. The cause of these effects is most likely the lower toughness of the matrix manifested by the formation of cracks between the individual reinforcing layers already during the production of these materials. These effects are also visible in the strength of the UT composite, where one would expect a higher strength value when loaded in the direction of the fibers of the surface layer, and also a higher strength compared to T composite boards. Also, the difference in strength in two perpendicular directions should be rather larger.

The lower interlaminar cohesion is evident when comparing the flexural strength of the composite board measured in mutually perpendicular directions. When stressed in bending in the direction perpendicular to the surface fibers, more than three times the strength was achieved. The failure of the laminate structure is shown in the photographs in Figure 6 taken after the end of the flexural strength experiment, whereas in Figure 6A, there is almost imperceptible destruction of the specimen stressed perpendicular to the surface fibers. On the contrary, the sample in Figure 6B, stressed in the direction of the fibers, shows shear damage of the inner layer and also a significant failure of the adhesion of both outer layers to the inner layer. Figure 6C shows the condition after the experiment for twice the span (i.e., 80 mm) with the same result. Despite the reduction in the shear stress component in this configuration of the experiment, there was no increase in the measured value of the bending strength.

3.2. Impact Load Resistance of Investigated Composites

The impact resistance of partially pyrolyzed composite plates of types T, UT, and U was tested employing the Charpy impact test. Geometrically identical specimens extracted from fiber-cement plates C1 and C2 were used as reference materials. The results, in the form of consumed impact energy normalized by the specimen cross-section (Work of Fracture) on the thickness of the specimen, are shown in the graph in Figure 7. From the presented data, it is evident that composite materials consume several times higher portion of energy during the fracture process than the fiber-cement plates. A comparison of selected typical load curves for cement board C1 and all pyrolyzed composites is shown in Figure 8. The effectiveness of long fibers in terms of strength and overall resistance is obvious. The resulting impact strength and energy consumption are also dependent on processing parameters and can be further optimized as shown in previous research [20]. Photographs of the samples after the impact test are shown in Figure 9.

3.3. Fire Resistance of the Investigated Composites

The goal of the fire tests was to clarify the effect of high-temperature treatment on the fire resistance of the composites in comparison with a composite with a matrix in a cross-linked thermoset state, i.e., an identical composite that was not subjected to pyrolysis treatment at 650 °C. The polymer component usually worsens the fire resistance of the composite compared to metal, ceramic, or silicate components mainly due to the release of gaseous combustion products and in most cases, also due to the release of heat during combustion, which also applies to polysiloxane thermosets.

The toxicity test was performed on a 75 × 75 × 2 mm T-type composite plate that was cured and then pyrolyzed at 650 °C. As a reference material, a composite plate made of the same composite was used, which was only cured (cross-linked) at 250 °C. Each of the materials was subjected to a heat flux of 50 kW/m² and CO, CO₂, HCl, HBr, HF, HCN, SO₂, and NO_X were measured by FTIR analysis. Of the monitored substances, only CO and CO₂ were detected. A comparison of the results of the amount of CO₂ and mass loss of the cured composite and the composite after heat treatment is shown in Table 2. It was observed that a non-negligible part of the released fumes from the cured composite consists of gases with heavier molecules, which is not assumed by this standardized test. A list of gaseous pyrolysis products of very similar polysiloxane structures can be found in [30].

Reaction-to-fire tests, which determine the HRR, were performed on the cured and pyrolyzed composites. This quantity indicates the size of the fire hazard, expresses the degree of flammability of the material, and at the same time, serves as a tool for modeling fire development and simulating fire scenarios. The T composite cured at 250 °C and the composite pyrolyzed at 650 °C were subjected to a heat flux of 50 kW/m² in a cone calorimeter. In addition to HRR, the release of CO and CO₂ was also monitored during the experiment. For the cured composite, the test was stopped after 12 min because the detected amount of oxygen returned to the initial value, i.e., the sample burned out, according to condition (c) described in the Experimental section chapter. The pyrolyzed composite was exposed to heat flow for 30 min without signs of burning, see definition of condition (a).

Figure 10 shows the course of monitored parameters in the first seven minutes of the experiment. The temperature in the displayed interval varied between 756 and 761 °C. The cured composite shows burning with a rapidly rising heat flux contribution in the 40th second of the experiment and reaches its peak in the next 20 s. This thermal effect wears off after about another 130 s. The CO₂ detection record corresponds quite precisely to the course of the thermal contribution. In contrast to the cured composite, the partially pyrolyzed composite practically shows no thermal contribution from combustion throughout the entire range of the experiment, which corresponds to the detected amount of released CO₂. CO is produced in the case of both material variants in a smaller amount approaching the resolution capacity of the method used. The highest value is reached after 2.2 min from the start of the experiment. This maximum is about 20% lower for pyrolyzed material than for cured material. The value of the CO concentration then decreases slightly for both material variants. A visual comparison of the state of the partially pyrolyzed composite with only the cured composite after the reaction-to-fire test performed at a constant heat flow of 50 kW/m² is shown in Figure 11.

Both production variants, partially pyrolyzed composite and only cured composite, were subjected to an ignitability test. During the entire test, carried out up to 650 °C in flowing air, flare, ignition, and glow were not observed for any of the two materials.

3.4. Frost Resistance of Investigated Composites

Due to the porous structure of composites processed at 650 °C and the assumption of the propagation of microcracks that were created in the matrix at processing temperatures, it is necessary to assume a deterioration of the mechanical properties during repeated freezing of the water-saturated material. For testing the frost resistance of the composites, the freeze–thaw mode was chosen according to the frost resistance standard EN 539-2 for burnt roof coverings, which, unlike the standardized tests for fiber cement, is designed for testing during permanent wetting. The standardized freeze resistance test for fiber cement assumes the crystallization of dissolved salts and therefore its cycle includes a drying phase. However, the components of high-temperature processed composites are insoluble in water and therefore do not have a leaching effect. The time dependence of temperature recorded during the cyclic freeze–thaw process shown in Figure 12 represents the temperatures: in the test chamber, on the surface of the sample under the textile cover, and in a hole with a depth of 4 mm drilled on the face of the sample.

The material degradation after the cyclic freeze–thaw process was determined by the remaining flexural strength at three-point bending. The results of these tests are shown in the graph in Figure 13. The materials were mechanically tested in three groups: on unprocessed samples and after 120 and 240 freeze–thaw cycles. The results show that partially pyrolyzed composite materials retain higher strength compared to fiber cement materials, but all of them also show a certain decrease in strength with an increasing number of freeze–thaw cycles. The causes of this decrease can most likely be found in the lower toughness of the pyrolyzed polysiloxane matrix and in its porous structure.

The relatively good results of the composite T can most probably be explained by the easy manufacturing process and also by the fact that the tested composite boards represent a homogeneous body in terms of elasticity without sudden changes in the elastic parameters. The measured strength after 240 cycles reaches approx. twice the value measured on commercial fiber cement boards. The variance in the results does not change much with the degree of freeze–thaw, which partially improves the prediction of strength at an order of magnitude higher for the number of freeze cycles than for the number performed in this experiment.

After freeze–thaw cycling, the UT-type composite showed similar flexural strength results to the T-type composite. However, it shows a higher dispersion of results in all the observed states. When considering the problems in the production of larger UT-type composite panels resulting from the differences in pyrolysis shrinkage between the fabric and unidirectional layers, it is necessary to evaluate this composite as it is less promising for further development.

As shown in Table 1, the U-type composite shows a significant difference in flexural strength in the longitudinal and transverse directions. As in the case of the UT composite, the orientation of the samples with fibers in the outer layer in the longitudinal direction of the sample was chosen for testing. U-composite samples showed the lowest strength values in all monitored states among the developed composites. Also, the variances of the values are rather higher. The biggest drawback of this composite is its generally low strength in the direction of the fibers of the outer layers as was stated above. A favorable feature of the U variant is good surface quality without significant cracks between the fibers and the matrix.

4. Discussion

4.1. Mechanical Properties of PPC

The results of the bending strength measurements on the three compositional types of PPC met expectations only in the case of the T composite. The measured strength values correspond to the results measured on similarly reinforced composites shown in [41,50]. As already mentioned above, the measured values are lower than would be expected based on the results obtained on unidirectionally reinforced composites (parameters of unidirectional PPCs are given, e.g., in [38,41]) and their conversion to the strength of the woven textile structure. The flexural failure mechanism of the composite occurs on the compression side by buckling of the compressed textile layers near the surface. The thickness of the group of flexible layers differs for individual T composite samples.

The flexural strength of the UT composite in the direction of the fibers in the surface layer is 10% lower than the strength of the T composite, which does not correspond to the higher degree of reinforcement of the UT composite with a 1D surface layer. Failure occurs on the pressure side of the bend and the resilient layer is always in the full thickness of the one-way surface layer (i.e., the entire shell). In the perpendicular direction of the UT composite, the strength is lower by 20%, but a lower value can be expected in the direction perpendicular to the surface fibers. The flexural strength results could be improved here by strengthening or at least ensuring adhesion between individual layers, e.g., by increasing the amount of resin (i.e., by reducing the volume fraction of fibers). It is also necessary to mention the technological problems in the production of this composite, when in the preparatory experiments for the production of larger plates, delamination of the surface layer occurred during pyrolysis. The reason is probably the different pyrolysis shrinkage between the surface and inner layer in the direction of the fibers of the surface layer, where it is a collision of two parallel less relaxingly yielding directions. The perpendicular direction in the surface layer, i.e., the direction perpendicular to the fibers, is on the other hand more relaxingly yielding due to the ability of the cross-linked matrix to partially plastically deform at higher temperatures of pyrolysis heating and cooling. The development of a certain healing of the structure during pyrolysis is documented microscopically and also by the development of open porosity in [50].

The U-type composite exhibits significantly greater strength in the direction perpendicular to the fibers of the surface layer. It is partly a consequence of the design of the composite thickness ratios in such a way as to both respect the relatively fragile nature of the matrix and at the same time, allow for some damage to the outer layer during long-term climatic exposure (for example, linear damage by pulling out a group of fibers). In such an extreme case, it is necessary that only the inner layer can take over the load-bearing function. The proposal therefore did not consider the classical lamination theory, but formulated the task as a case of equal bending strength in both directions of the board, when only the layer with longitudinal fibers is considered for the calculation, i.e., surface layers with a mutually invariable distance have the same bending strength in the direction of the fibers as the central plate has in the direction of its fibers. As can be seen from the strength measurement results, the effect of the strong center plate is obvious. However, this is due not only to the ratio of the thicknesses of the composite layers but also to the buckling failure mechanism—buckling of the pressed layer, which is evident from Figure 6c (Figure 6b shows a rather later and more extensive destruction where shear also acts).

Elimination of the buckling mechanism or at least its limitation would very likely lead to an increase in bending strength, e.g., in the case of too close mutual contact of the fibers, a certain increase in the proportion of the matrix leads to an improvement in the interlaminar properties. In similar cases of FRP, increasing the adhesion between the fibers and the matrix using additives could be considered. In the case of this material, however, it would probably not be possible to find suitable chemical agents, as the composite technology here assumes several hours of exposure to temperatures of 650 °C. Moreover, the low adhesion of the fibers to the matrix will not be the only fundamental problem of this type of composite. Low adhesion would then be manifested even in the case of 1D PPC, where, however, significantly greater flexural strength is achieved. Regarding the problem of sufficient adhesion between the basalt fibers and the matrix, it should be noted that PPCs are generally composites with properties at the transition between FRP and CMC. In most cases of CMC, excessive fiber–matrix adhesion must be reduced to avoid brittle fracture propagating indefinitely across the fibers and matrix with zero bridging effect, which is the fundamental problem of CMC composites. In the case of PPC, the matrix has a rather polymeric character, but the shrinkage is at a non-negligible level. The formula for volume shrinkage due to pyrolysis s_V is

(1)$s_V=1-{\displaystyle \frac{V_{pp}}{V_0}}=1-{\displaystyle \frac{m_{pp}{\rho }_0}{{\rho }_{pp}{m}_0}}$

where V₀ and V_pp are volumes before and after partial pyrolysis, ρ₀ and ρ_pp are the densities before and after partial pyrolysis, and m₀ and m_pp are the mass before and mass after partial pyrolysis. When considering the values according to [55,56], the volume shrinkage value is 18% compared to the original state of the cross-linked thermoset. Such shrinkage itself will cause significant fiber clamping causing a frictional force between the fiber and the matrix. Realistically, however, this shrinkage-induced stress will be lower, as also relaxation takes place during pyrolysis.

Figure 14 shows the delamination fracture surface of the U composite during buckling at different magnifications. This area was taken from the sample after its additional layering because the sample remained cohesive after the test. Figure 14a shows the fracture surface that occurred at the dividing plane of two differently oriented layers, where longitudinal fibers are visible in the lower layer and only impressions of transverse fibers in the matrix are visible in the upper layer. The fiber decals are smooth and continuous, with brittle fracture surfaces between them that are left over from the matrix that surrounds the fibers, as seen in Figure 14b,c. Figure 14d shows fibers with a smooth surface without residual matrix adhering to the surface of the fibers. These facts show that the region of the fracture surfaces suffers both from low adhesion of the fibers to the matrix and the fact that the matrix itself lacks sufficient toughness.

An effective way to reduce buckling is to increase the thickness of the outer layers, a change at the expense of the inner layer, but this increase could have a progressive effect on flexural strength, as generally, layer thickness contributes to its buckling resistance by the third power. A number of the above-mentioned problems could also be solved by using 3D-woven reinforcement, where a preferential plane for the propagation of the cracks would not arise.

4.2. Impact Behavior of PPC

Similar to the measurement of bending strength, the highest breaking work value was achieved in the impact tests for the U-type composite when the sample was oriented perpendicular to the fibers of the surface layer, when its value is 105 J/m². On the other hand, this material in the longitudinal orientation of the surface fibers shows the lowest value of 11.6 J/m² among PPC. In this case, however, the difference between these two values is very significant and is therefore completely beyond the calculation assumptions. Also, compared to the T composite, the difference in values is large. This result can most likely be attributed to the fact that the fibers of the inner layer are protected by the outer layer, which is not heavily loaded, but protects the outer layer from the direct effects of impact. Conversely, the longitudinally oriented direction of the unidirectional U composite suffers from longitudinal delamination, which weakens the loaded cross-section and allows easy shear deformation and further crack propagation.

The samples from all types of fiber cement boards were completely broken into two separate parts after the test, which corresponds well with the obtained fracture energies and load shape curves where the force dropped to zero after reaching the maximum value. Unlike fiber cement, all developed composites were delaminated at the end of the experiment but were still able to transfer loads fully mirroring the shape of the load curves. In the case of the U-type material, fragmentation of the sample along the fibers was observed, but the complete separation was not observed as in the cemented boards. The fracture behavior is in good agreement with previous work investigating woven and unidirectional composites [31,32]. The impact resistance of partially pyrolyzed composites can contribute to longer service life and expand the field of application compared to fiber cement.

4.3. PPC Fire Safety Tests

The results of the experiment’s toxicity test, ignitability test, and reaction to fire test show that PPC fulfills the requirements of a fire-safe material mainly from the point of view of passive safety, i.e., they do not become the cause of ignition, release a very low amount of gases and zero amount of liquids in the heat, and also have zero thermal contribution to ambient combustion if exposed to external heat flow. However, from the point of view of the toxicity test, it should be noted that this is a standardized test designed for a wide range of materials, which only monitors the commonly occurring toxic gases that are released when they are heated. High-temperature oxidation and pyrolysis of polysiloxanes is a specific process that requires a separate targeted analysis. According to [57], the pyrolysis of polymethylsiloxanes produces a small amount but a diverse spectrum of simpler and more complex compounds, where the level of their toxicity cannot be found in the literature for many of them. A comprehensive treatise on the toxicity of polysiloxanes is, e.g., in [58,59].

Ongoing changes in the chemical structure of the polysiloxane precursor can be detected based on the escaping gaseous substances. The possibilities of this method are relatively limited for recognizing the structural changes in the precursor and their quantity, but some findings are of significant importance for the study of pyrolysis. According to [57], elemental analysis found that hydrogen is released relatively evenly in terms of temperature dependence. The molar ratio of hydrogen to silicon in polymethylsiloxane in cross-linked polymer state is 3.5. Its uniform decrease occurs at a temperature of around 600 °C, and at a temperature of 900 °C, hydrogen is still present in the structure at an H/Si ratio of 0.5. Carbon loss occurs in a much narrower temperature interval between 600 and 700 °C and the C/Si ratio changes from 1 to 0.7. In relation to PPC, it follows from the above that the H/Si ratio in the case of pyrolysis at 650 °C is around 2 to 2.5, i.e., a state close to the polymer, from which one could infer a high potential for the release of gaseous products. However, both hydrogen and carbon are well fixed in the chemical structure after partial pyrolysis, and their leakage is low both under pyrolysis conditions and under oxidation. The use of PPC for load-bearing elements of structures that would resist fire is only limited. The main cause of this limitation is the creep of basalt fibers, which occurs at 650 °C. To increase the creep resistance of PPC, it is possible to use the pyrolysis production process with a five-hour delay at a maximum temperature of 800 °C, when the creep resistance of the composite reinforced with basalt fabric increased to 740 °C. However, in the case of this pyrolysis process, a reduction in flexural strength at room temperature of about 30% must be expected. It is also possible for this purpose to consider a methylphenylsiloxane matrix precursor when using which the composite does not suffer from a reduction in mechanical strength at higher temperatures of partial pyrolysis, but the creep resistance results do not reach the values as at the use of the polymethylsiloxane precursor.

4.4. Frost Resistance of PPC Measured by the Freeze–Thaw Cyclic Experiment

The results of bending strength measurements after 120 and 240 freeze–thaw conditioning cycles are not completely satisfactory. However, it should be noted that they were achieved on composite samples without any treatment to increase frost resistance. As part of the solution to this problem, it is possible to consider, for example, the creation of a coating (to maintain high fire safety, e.g., based on Tetraethoxysilicate), or to perform a hydrophobic treatment of the surface and open porosity and thus prevent the destruction of the composite by penetrating and subsequently freezing water. However, these modifications can have a limited lifetime, so it is generally better to reduce the open porosity of the matrix, especially in the form of microcracks.

From the point of view of comparing the individual types of investigated composites, the results obtained for T and UT composites can be considered comparable. However, due to the problems in the production of larger plates of UT composite, this material seems to be less promising for further development. Due to the limited space of the freezing chamber, only samples of the weaker direction of the U-type composite (in the direction of the fibers of the outer layer), which shows the lowest bending strength, were included in this three-month experiment. However, it can be assumed that the stronger direction would show better results and the improvement in the weaker direction could be achieved by changing the technology and changing the ratio of the thicknesses of the outer and inner layers. Therefore, the U composite can be considered suitable for further development.

5. Conclusions

Polymethylsiloxane thermoset was reinforced with long basalt fibers, which was processed by partial pyrolysis after cross-linking, and was subsequently examined in terms of mechanical properties, fire safety, and frost resistance. The problem of PPC is lamination and also partially reinforcement with fabric when the mechanical strength results are significantly lower than what would be expected from the strength of a unidirectionally reinforced partially pyrolyzed composite. For this reason, the composite was investigated in three variants with different reinforcement geometries: T composite reinforced with plain weave fabric, UT composite with unidirectionally reinforced surface layers and fabric-reinforced core, and U composite reinforced with three 1D layers [0°/90°/0°]. From the point of view of possible applications of PPC as boards and skins of external building envelopes, their mechanical properties and frost resistance were compared with the parameters of fiber cement boards obtained using identical tests. The results of the conducted experiments can be summarized as follows:

• (1). All variants of the composite showed satisfactory bending strength: the strength value of the T composite was 92.3 MPa and the UT composite was 84.5 MPa. For U composite bending strength is 65.4 MPa, resp. is 207 MPain the longitudinal resp. transverse orientation, which is approximately 2.5 to 10 times more than what was measured on fiber cement boards. The density of PPC ranges from 2.07 to 2.25 g/cm³ and the open porosity ranges from 1.9% for U composite to 6.5% for T composite;

• (2). The tendency of the composite to brittle failure was assessed by measuring the impact energy required to fracture the specimen. Impact loading revealed significant differences between the investigated materials by type and orientation. The measured values of impact energy range from approx. 10 to 100 J/m², which exceeds the values measured on fiber cement boards by 10 to 100 times;

• (3). Fire safety properties of T-type composite were investigated by the ignitability test, toxicity test, and heat release rate test. In the case of fire safety properties, this partially pyrolyzed composite was compared with an identical composite that was not partially pyrolyzed and whose matrix was, therefore, a cross-linked polymethylsiloxane thermoset. The ignitability test did not show either ignition, heat, or glow for both evaluated composites. The toxicity test showed only the formation of a small amount of CO₂ and a very small amount of CO at the detection limit of the device, when the partially pyrolyzed composite showed lower values in both cases. The heat release rate test showed a higher fire resistance of the partially pyrolyzed composite, as it did not detect any released heat, while the composite with a matrix in a cross-linked polymer state shows a standard course of released heat from the polymer material;

• (4). Frost resistance was investigated by a cyclic freeze–thaw test. The flexural strength measured after 120 freeze–thaw cycles is 1.8 to 3 and after 240 cycles 1.3 to 2 times higher than that of fiber cement, depending on the type of composite and its orientation. The composite material would meet the bending strength prescribed by the standard for freeze–thaw conditioned fiber cement, but the problem is rather the decreasing tendency of the strength value.

Partially pyrolyzed composites could be applied as light fire-resistant walls or shells of buildings or means of transport. However, the ability to contribute to the load-bearing capacity of the structure should be further investigated. Likewise, the frost resistance of these composites should be investigated and improved.

Author Contributions

Conceptualization, M.Č.; methodology, M.Č., Z.C., P.K., M.R., L.Š., J.S. (Jana Schweigstillová) and J.S. (Jaroslava Svítilová); formal analysis, M.Č., Z.C., P.K., M.R., and A.S.; investigation, M.Č., Z.C., J.K., M.R., L.Š., and J.S. (Jaroslava Svítilová); project administration, M.Č.; funding acquisition, M.Č.; Validation, A.S. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data will be made available to the corresponding author upon request.

Acknowledgments

This work was supported by the Technology Agency of the Czech Republic under the project TP01010055—subproject: Partially pyrolyzed composites as a light roofing—verification of climatic resistance and optimization of suitable textile reinforcement. and The Czech Academy of Sciences, Czechia, under the project RVO:67985891.

Conflicts of Interest

The authors declare that they have no known conflicts of interest or personal relationships that could have appeared to influence the work reported in this paper.

Footnotes

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Figures and Tables

Figure 1. Simplified structure of (a) the liquid oligomeric polymethylsiloxane resin, (b) of the same resin after cure to an infinite network, and (c) of the partly pyrolyzed polymer, which contains structural units CH3SiO1.5 of the cured network as well as ones of completely pyrolyzed “SiOC” (units SiO1.5C0.x); the chemical changes during the processing in this work are represented by the sequence (a–c).

Figure 2. Cross-section and an outer surface of T, UT, and U composites after temperature treatment at 650 °C. T1, UT1, and U1: polished cross-section; T2, UT2, and U2: natural outer surface—overview; T3, UT3, and U3: natural outer surface—detail of indicated materials.

Figure 3. Material preparation procedure and follow-up experiments of this study: (A) preparation of cured and PPC samples for fire safety tests and (B) preparation of PPC samples for static tests of mechanical properties, impact tests, and flexural strength tests on samples conditioned by freeze–thaw process.

Figure 4. The scheme of the device for the cyclic freeze–thaw process (a) and placement of samples in the freezing device (b)—the arrow indicates the reference sample with thermocouples.

Figure 5. Freeze–thaw process for 120 and 240 conditioning cycles on PPC samples and comparative fiber cement samples for flexural strength tests.

Figure 6. Three-point bending test of the U-type composite; a comparison of the two basic orientations of the reinforcement in the sample: (A) fibers of the outer layers are perpendicular to the length of the sample; (B) fibers of the outer layers are parallel to the length of the sample; and (C) same material orientation as case (B) but the span of the three-point bending experiment is doubled.

Figure 7. Results of impact tests of three compositional variants of developed composites in comparison with values measured on fiber cement materials with statistical evaluation. Types of composite materials are indicated.

Figure 8. Example of typical loading curves (Force–Deflection) for selected materials from impact tests with the highlighted area under the curve corresponding to the fracture energy. Types of composite materials are indicated.

Figure 9. Example of typical fracture patterns for selected specimens after the impact test. Types of composite materials are indicated.

Figure 10. Reaction-to-fire test results: the amount of CO (wt.‰) and CO2 (wt.%) and the heat release rate (HRR) at a heat flux of 50 kW/m2 measured on the composite after processing by partial pyrolysis at 650 °C compared to the results measured on the cured composite.

Figure 11. Samples after reaction-to-fire test—visual comparison of the state of the cured T composite with the T composite after partial pyrolysis at 650 °C. Both composites are after a standardized reaction-to-fire test performed at a constant heat flow of 50 kW/m2. In the lower part, an untreated cut with a diamond saw on post-fire samples.

Figure 12. Recording of two cycles of the freeze–thaw conditioning—the temperature in the test chamber, temperature on the surface of the reference sample, and temperature inside the reference sample.

Figure 13. Frost resistance measured by a cyclic freeze–thaw experiment: flexural strength of U, UT, and T composites processed by partial pyrolysis at 650 °C after 0, 120, and 240 conditioning cycles of freeze–thaw conditioning—comparison with values measured on fiber-cement material.

Figure 14. Delamination fracture surface caused by buckling during the bending test of the U composite (SEM, ET detector); (a) dividing fracture plane of two differently oriented layers, (b) detail of the fiber placement in the lower layer and the remain of the matrix with the grooves of the fibers of the upper layer, (c) detail of the smooth grooves and the brittle fracture of the matrix between them, (d) detail of the smooth surface of the fibers and their grip in the residue of the matrix.

Table 1

Basic mechanical properties of investigated PPC; a comparison with fiber cement boards.

Type	CompoSite Structure	Thickness	Density	Porosity	Flexural Strength *ǁ	Flexural Strength * ⊥	Modulus of Elasticity *ǁ	Modulus of Elasticity *⊥
		(mm)	(g.cm−3)	(%)	(MPa)	(MPa)	(GPa)	(GPa)
T	2D (plain weave)	4.23	2.07 ± 0.05	6.5 ± 1.3	92.3 ± 15	-	32.4 ± 2.3	-
UT	1D/2D/1D	4.25	2.09 ± 0.05	5.5 ± 1.6	84.9 ± 19	74.8 ± 11	42.8 ± 3.0	18.2 ± 2.8
U	1D—[0°/90°/0°]	3.57	2.25 ± 0.03	1.9 ± 0.7	65.4 ± 17	208 ± 35	43.1 ± 2.5	31.9 ± 2.2
C1	Fiber cement	4.28	2.05 ± 0.04	11.2 ± 0.1	27.3 ± 3	-	28.1 ± 2.4	-
C2		5.07	2.01 ± 0.03	10.9 ± 0.9	21.7 ± 2.0	-	20.4 ± 2.7	-

* ǁ…fibers of the outer layers parallel to the length of the sample; ⊥…fibers of the outer layers perpendicular to the length of the sample.

Table 2

Comparison of the amount of CO, CO₂, and mass loss of the sample, measured in the fourth and eighth minutes of the experiment.

	Time (min)	CO2 (mg/m3)	CO (mg/m3)	CO/CO2 (%)	Δm (%)
T compositecured at 250 °C	4	11,701	80	0.68	2.6
	8	20,472	156	0.76
T composite processed at 650 °C	4	10,322	259	2.51	0.41
	8	17,514	106	0.61

References

1. Mouritz, A.P.; Feih, S.; Kandare, E.; Mathys, Z.; Gibson, A.G.; Des Jardin, P.E.; Case, S.W.; Lattimer, B.Y. Review of fire structural modelling of polymer composites. Compos. Part A Appl. Sci. Manuf.; 2009; 40, pp. 1800-1814. [DOI: https://dx.doi.org/10.1016/j.compositesa.2009.09.001]

2. Mouritz, A.P.; Mathys, Z.; Gibson, A.G. Heat release of polymer composites in fire. Compos. Part A Appl. Sci. Manuf.; 2006; 37, pp. 1040-1054. [DOI: https://dx.doi.org/10.1016/j.compositesa.2005.01.030]

3. Gibson, A.G.; Otheguy Torres, M.E.; Browne, T.N.A.; Feih, S.; Mouritz, A.P. High temperature and fire behaviour of continuous glass fibre/polypropylene laminates. Compos. Part A Appl. Sci. Manuf.; 2010; 41, pp. 1219-1231. [DOI: https://dx.doi.org/10.1016/j.compositesa.2010.05.004]

4. Dao, Q.D.; Luche, J.; Richard, F.; Rogaume, T.; Bourhy-Weber, C.; Ruban, S. Determination of characteristic parameters for the thermal decomposition of epoxy resin/carbon fibre composites in cone calorimeter. Int. J. Hydrog. Energy; 2013; 38, pp. 8167-8178. [DOI: https://dx.doi.org/10.1016/j.ijhydene.2012.05.116]

5. Arslan, C.; Dogan, M. The effects of silane coupling agents on the mechanical properties of basalt fibre reinforced poly(butylene terephthalate) composites. Compos. Part B Eng.; 2018; 146, pp. 145-154. [DOI: https://dx.doi.org/10.1016/j.compositesb.2018.04.023]

6. Kim, M.; Choe, J.; Lee, D.G. Development of the fire retardant glass fabric/carbonized phenolic composite. Compos. Struct.; 2016; 148, pp. 191-197. [DOI: https://dx.doi.org/10.1016/j.compstruct.2016.04.003]

7. Correia, J.R.; Bai, Y.; Keller, T. A review of the fire behaviour of pultruded GFRP structural profiles for civil engineering applications. Compos. Struct.; 2015; 127, pp. 267-287. [DOI: https://dx.doi.org/10.1016/j.compstruct.2015.03.006]

8. Harries, K.A.; Guo, Q.; Cardoso, D. Creep and creep buckling of pultruded glass-reinforced polymer members. Compos. Struct.; 2017; 181, pp. 315-324. [DOI: https://dx.doi.org/10.1016/j.compstruct.2017.08.098]

9. Kessler, E.; Gadow, R.; Straub, J. Basalt, glass and carbon fibres and their fibre reinforced polymer composites under thermal and mechanical load. AIMS Mater. Sci.; 2009; 3, pp. 1561-1576. [DOI: https://dx.doi.org/10.3934/matersci.2016.4.1561]

10. Pereira, D.; Gago, A.; Proença, J.; Morgado, T. Fire performance of sandwich wall assemblies. Compos. Part B Eng.; 2016; 93, pp. 123-131. [DOI: https://dx.doi.org/10.1016/j.compositesb.2016.03.001]

11. Jang, K.S. Mechanics and rheology of basalt fibre-reinforced polycarbonate composites. Polymer; 2018; 147, pp. 133-141. [DOI: https://dx.doi.org/10.1016/j.polymer.2018.06.004]

12. Morgado, T.; Correia, J.R.; Silvestre, N.; Branco, F.A. Experimental study on the fire resistance of GFRP pultruded tubular beams. Compos. Part B Eng.; 2018; 139, pp. 106-116. [DOI: https://dx.doi.org/10.1016/j.compositesb.2017.11.036]

13. Vejmelkova, E.; Konakova, D.; Scheinherrova, L.; Dolezelova, M.; Keppert, M.; Cerny, R. High temperature durability of fibre reinforced high alumina cement composites. Constr. Build. Mater.; 2018; 162, pp. 881-891. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2018.01.076]

14. Hajiloo, H.; Green, M.F.; Noël, M.; Bénichou, N.; Sultan, M. Fire tests on full-scale FRP reinforced concrete slabs. Compos. Struct.; 2017; 179, pp. 705-719. [DOI: https://dx.doi.org/10.1016/j.compstruct.2017.07.060]

15. Hajiloo, H.; Green, M.F. Post-fire residual properties of GFRP reinforced concrete slabs: A holistic investigation. Compos. Struct.; 2018; 201, pp. 398-413. [DOI: https://dx.doi.org/10.1016/j.compstruct.2018.06.047]

16. Li, Y.; Zhang, J.; He, Y.; Huang, G.; Li, J.; Niu, Z.; Gao, B. A review on durability of basalt fibre reinforced concrete. Compos. Sci. Technol.; 2022; 225, 109519. [DOI: https://dx.doi.org/10.1016/j.compscitech.2022.109519]

17. Huang, T.; Zhang, Y.X. Multiscale modeling of multiple-cracking fracture behavior of engineered cementitious composite (ECC). Advances in Engineered Cementitious Composites; Woodhead Publishing: Cambridge, UK, 2022; pp. 337-388. [DOI: https://dx.doi.org/10.1016/B978-0-323-85149-7.00003-3]

18. Al-Rousan, E.T.; Khalid, H.R.; Rahman, M.K. Fresh, mechanical, and durability properties of basalt fibre-reinforced concrete (BFRC): A review. Dev. Built Environ.; 2023; 14, 100155. [DOI: https://dx.doi.org/10.1016/j.dibe.2023.100155]

19. EN 13501-1:2018 Fire Classification of Construction Products and Building Elements-Part 1: Classification Using Data from Reaction to Fire Tests; European Committee for Standardization (CEN): Brussels, Belgium, 2018.

20. Babievskaya, I.Z.; Drobot, N.F.; Fomichev, S.V.; Krenev, V.A. Physicochemical modeling of basalt melt generation for petrurgy. Inorg. Mater.; 2008; 44, pp. 1334-1340. [DOI: https://dx.doi.org/10.1134/S0020168508120133]

21. De Lima, L.F.; Zorzi, J.E.; Cruz, R.C.D. Basaltic glass-ceramic: A short review. Bol. Soc. Esp. Ceram. Vidr.; 2022; 61, pp. 2-12. [DOI: https://dx.doi.org/10.1016/j.bsecv.2020.07.005]

22. Jagadeesh, P.; Rangappa, S.M.; Siengchin, S. Basalt fibres: An environmentally acceptable and sustainable green material for polymer composites. Constr. Build. Mater.; 2024; 436, 136834. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2024.136834]

23. Černý, M.; Glogar, P.; Goliáš, V.; Hruška, J.; Jakeš, P.; Sucharda, Z.; Vávrová, I. Comparison of mechanical properties and structural changes of continuous basalt and glass fibres at elevated temperatures. Ceram-Silik.; 2007; 51, pp. 82-88. Available online: https://www2.irsm.cas.cz/materialy/cs_content/2007/Cerny_CS_2007_0000.pdf (accessed on 19 July 2024).

24. Zheng, Y.; Zhang, Y.; Zhuo, J.; Zhang, Y.; Wan, C. A review of the mechanical properties and durability of basalt fibre-reinforced concrete. Constr. Build. Mater.; 2022; 359, 129360. [DOI: https://dx.doi.org/10.1016/j.conbuildmat.2022.129360]

25. Elshazli, M.T.; Ramirez, K.; Ibrahim, A.; Badran, M. Mechanical, Durability and Corrosion Properties of Basalt Fibre Concrete. Fibres; 2022; 10, 10. [DOI: https://dx.doi.org/10.3390/fib10020010]

26. ASTM D8505-23 Standard Specification for Basalt and Glass Fibre Reinforced Polymer (FRP) Bars for Concrete Reinforcement; ASTM International: West Conshohocken, PA, USA, 2023.

27. Wydra, M.; Turkowski, P.; Dolny, P.; Sadowski, G.; Grochowska, N.; Michalski, P.P.; Wieczorek-Czarnocka, M.; Pakieła, Z.; Fangrat, J. Basalt fibre reinforced polymer bars as main reinforcement of axially compressed concrete column—Experimental and numerical considerations of fire resistance. Fire Saf. J.; 2023; 140, 103898. [DOI: https://dx.doi.org/10.1016/j.firesaf.2023.103898]

28. Bhat, T.; Chevali, V.; Liu, X.; Feih, S.; Mouritz, A.P. Fire structural resistance of basalt fibre composite. Compos. Part A Appl. Sci. Manuf.; 2015; 71, pp. 107-115. [DOI: https://dx.doi.org/10.1016/j.compositesa.2015.01.006]

29. Bhat, T.; Kandare, E.; Gibson, A.G.; Di Modica, P.; Mouritz, A.P. Compressive softening and failure of basalt fibre composites in fire: Modelling and experimentation. Compos. Struct.; 2017; 165, pp. 15-24. [DOI: https://dx.doi.org/10.1016/j.compstruct.2017.01.003]

30. Li, Y.M.; Hu, S.-L.; Wang, D.-Y. Polymer-based ceramifiable composites for flame retardant applications: A review. Compos. Commun.; 2020; 21, 100405. [DOI: https://dx.doi.org/10.1016/j.coco.2020.100405]

31. Smulski, K.; Imiela, M.; Gozdek, T.; Bieliński, D.M. New elastomeric ceramizable composites based on butadiene-acrylonitrile rubber filled with talc. Polym. Test.; 2024; 134, 108438. [DOI: https://dx.doi.org/10.1016/j.polymertesting.2024.108438]

32. Duan, L.; Luo, L.; Wang, Y. Oxidation and ablation behavior of a ceramizable resin matrix composite based on carbon fibre/phenolic resin. Mater. Today Commun.; 2022; 33, 104901. [DOI: https://dx.doi.org/10.1016/j.mtcomm.2022.104901]

33. Yang, J.S.-J.; Fang, L. Conjugated ladder polymers: Advances from syntheses to applications. Chem; 2024; 10, pp. 1668-1724. [DOI: https://dx.doi.org/10.1016/j.chempr.2024.04.002]

34. Pielichowski, K.; Njuguna, J.; Majka, T.M. 9—Thermal degradation of high temperature-resistant polymers. Therm. Degrad. Polym. Mater.; 2023; pp. 187-211. [DOI: https://dx.doi.org/10.1016/B978-0-12-823023-7.00003-4]

35. Shi, X.-H.; Li, X.-L.; Li, Y.-M.; Li, Z.; Wang, D.-Y. Flame-retardant strategy and mechanism of fibre reinforced polymeric composite: A review. Compos. Part B Eng.; 2022; 233, 109663. [DOI: https://dx.doi.org/10.1016/j.compositesb.2022.109663]

36. Hasnat, M.R.; Hassan, M.K.; Saha, S. Flame Retardant Polymer Composite and Recent Inclusion of Magnesium Hydroxide Filler Material: A Bibliometric Analysis towards Further Study Scope. Fire; 2023; 6, 180. [DOI: https://dx.doi.org/10.3390/fire6050180]

37. Zhang, Y.; Li, J.; Liu, H.; Ji, Y. Recent Advances in Rochow-Müller Process Research: Driving to Molecular Catalysis and to A More Sustainable Silicone Industry. Chem. Cat. Chem.; 2019; 11, pp. 2757-2779. [DOI: https://dx.doi.org/10.1002/cctc.201900385]

38. Černý, M.; Glogar, P.; Sucharda, Z.; Chlup, Z.; Kotek, J. Partially pyrolyzed composites with basalt fibres—Mechanical properties at laboratory and elevated temperatures. Compos. Part A Appl Sci Manuf.; 2009; 40, pp. 1650-1659. [DOI: https://dx.doi.org/10.1016/j.compositesa.2009.08.002]

39. Chlup, Z.; Černý, M.; Strachota, A.; Sucharda, Z.; Halasová, M.; Dlouhý, I. Influence of pyrolysis temperature on fracture response in SiOC based composites reinforced by basalt woven fabric. J. Eur. Ceram. Soc.; 2014; 34, pp. 3389-3398. [DOI: https://dx.doi.org/10.1016/j.jeurceramsoc.2014.03.003]

40. Halasova, M.; Cerny, M.; Strachota, A.; Chlup, Z.; Dlouhy, I. Fracture response of SiOC based composites on dynamic loading. J. Compos. Mater.; 2016; 50, pp. 1547-1554. [DOI: https://dx.doi.org/10.1177/0021998315594682]

41. Weichand, P.; Gadow, R. Basalt fibre reinforced SiOC-matrix composites: Manufacturing technologies and characterisation. J. Eur. Ceram. Soc.; 2015; 35, pp. 4025-4030. [DOI: https://dx.doi.org/10.1016/j.jeurceramsoc.2015.06.002]

42. Gadow, R.; Weichand, P. Novel Intermediate Temperature Ceramic Composites, Materials and Processing for Siloxane based Basalt Fibre Composites. Key Eng. Mater.; 2014; 611, pp. 382-390.

43. Mingazzini, C.; Scafè, M.; Caretti, D.; Nanni, D.; Burresi, E.; Brentari, A. Poly-Siloxane Impregnation and Pyrolysis of Basalt Fibres for the Cost-Effective Production of CFCCs. Adv. Sci. Technol.; 2014; 89, pp. 139-144. [DOI: https://dx.doi.org/10.4028/www.scientific.net/AST.89.139]

44. Cox, S.B.; Lui, D.; Wang, X.; Gou, J. Processing and Characterization of Basalt Fibre Reinforced Ceramic Composites for High Temperature Applications Using Polymer Precursors. Proceedings of the CAMX 2014—Composites and Advanced Materials Expo: Combined Strength. Unsurpassed Innovation; Daytona Beach, FL, USA, 26–31 January 2014; Available online: https://ntrs.nasa.gov/api/citations/20150000241/downloads/20150000241.pdf (accessed on 19 July 2024).

45. +Cerny, M.; Chlup, Z.; Strachota, A.; Halasová, M.; Schweigstillová, J.; Kácha, P.; Svítilová, J. Potential Of Glass, Basalt Or Carbon Fibres For Reinforcement Of Partially Pyrolysed Composites With Improved Temperature And Fire Resistance. Ceram. Silik.; 2020; 64, pp. 115-124. [DOI: https://dx.doi.org/10.13168/cs.2019.0056]

46. Chlup, Z.; Černý, M.; Strachota, A.; Hadraba, H.; Halasová, M. Effect of the exposition temperature on the behaviour of partially pyrolysed hybrid basalt fibre composites. Compos. Part B Eng.; 2018; 147, pp. 122-127. [DOI: https://dx.doi.org/10.1016/j.compositesb.2018.04.021]

47. Chlup, Z.; Černý, M.; Strachota, A.; Rýglová, Š.; Schweigstillová, J.; Svítilová, J.; Trško, L.; Hadzima, B. Effect of Pyrolysis Temperature on the Behaviour of Environmentally Friendly Hybrid Basalt Fibre Reinforced Composites. Appl. Compos. Mater.; 2022; 29, pp. 829-843. [DOI: https://dx.doi.org/10.1007/s10443-021-09990-z]

48. Hull, T.R.; Stec, A.A.; Lebek, K.; Price, D. Factors affecting the combustion toxicity of polymeric materials. Polym. Degrad. Stab.; 2007; 92, pp. 2239-2246. [DOI: https://dx.doi.org/10.1016/j.polymdegradstab.2007.03.032]

49. Rogula-Kozłowska, W.; Krasuski, A.; Rybak, J.; Wrobel, M. The ecotoxicity and mutagenicity of fire water runoff from small-scale furnishing materials fire tests. Sci. Total Environ.; 2024; 906, 167394. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2023.167394] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/37793445]

50. Černý, M.; Halasová, M.; Schwaigstillová, J.; Chlup, Z.; Sucharda, Z.; Glogar, P.; Svítilová, J.; Strachota, A.; Rýglová, Š. Mechanical properties of partially pyrolysed composites with plain weave basalt fibre reinforcement. Ceram. Int.; 2014; 40, pp. 7507-7521. [DOI: https://dx.doi.org/10.1016/j.ceramint.2013.12.102]

51. Ranachowski, Z.; Schabowicz, K. The Fabrication, Testing and Application of Fibre Cement Boards; Cambridge Scholars Publishing: Newcastle upon Tyne, UK, 2018; ISBN 1-5275-0576-6

52. Veliseicik, T.; Zurauskiene, R.; Valentukeviciene, M. Determining the impact of high temperature fire conditions on fibre cement boards using thermogravimetric analysis. Symmetry; 2020; 12, 1717. [DOI: https://dx.doi.org/10.3390/sym12101717]

53. Hendawitharana, S.; Ariyanayagam, A.; Mahendran, M.; Steau, E. Bushfire resistance of external light steel wall systems lined with fibre cement boards. Fire Saf. J.; 2023; 139, 103806. [DOI: https://dx.doi.org/10.1016/j.firesaf.2023.103806]

54. Doll, R.; Peto, J. Effects on Health of Exposure to Asbestos; Health & Safety Commission: London, UK, 1985.

55. Černý, M.; Chlup, Z.; Strachota, A.; Brus, J. In-situ measurement of mechanical properties and dimensional changes of preceramic thermosets during their pyrolysis conversion to ceramics using thermomechanical analysis. Ceram. Int.; 2021; 47, pp. 23285-23294. [DOI: https://dx.doi.org/10.1016/j.ceramint.2021.05.041]

56. Černý, M.; Chlup, Z.; Strachota, A.; Halasová, M.; Havelcová, M. Changes in structure and in mechanical properties during the pyrolysis conversion of crosslinked polymethylsiloxane and polymethylphenylsiloxane resins to silicon oxycarbide glass. Ceram. Int.; 2015; 41, pp. 6237-6247. [DOI: https://dx.doi.org/10.1016/j.ceramint.2015.01.034]

57. Havelcova, M.; Strachota, A.; Cerny, M.; Sucharda, Z.; Slouf, M. Effect of the dimethylsilyloxy co-monomer “D” on the chemistry of polysiloxane pyrolysis to SiOC. J. Anal. Appl. Pyrol.; 2016; 117, pp. 30-45. [DOI: https://dx.doi.org/10.1016/j.jaap.2015.12.018]

58. Lassen, C.; Hansen, C.L.; Mikkelsen, S.H.; Maag, J. Siloxanes-Consumption, Toxicity and Alternatives. Danish Environmental Protection Agency, Environmental Protection Agency, Environmental Project no. 1031; Danish Ministry of the Environment: Copenhagen, Denmark, 2005; Available online: https://www2.mst.dk/udgiv/publications/2005/87-7614-756-8/pdf/87-7614-757-6.pdf (accessed on 19 July 2024).

59. De Arespacochaga, N.; Valderrama, C.; Raich-Montiu, J.; Crest, M.; Mehta, S.; Cortina, J.L. Understanding the effects of the origin, occurrence, monitoring, control, fate and removal of siloxanes on the energetic valorization of sewage biogas—A review. Renew. Sustain. Energy Rev.; 2015; 52, pp. 366-381. [DOI: https://dx.doi.org/10.1016/j.rser.2015.07.106]