Effect of Boric Acid Content in Aluminosilicate Matrix On

materials

Article Eﬀect of Boric Acid Content in Aluminosilicate Matrix on Mechanical Properties of Carbon Prepreg Composites

Eliška Haincová * and Pavlína Hájková

Unipetrol Centre for Research and Education, Revoluˇcní 84, 40001 Ústí nad Labem, Czech Republic; [email protected] * Correspondence: [email protected]; Tel.: +420-475-309-233

Received: 30 October 2020; Accepted: 25 November 2020; Published: 27 November 2020

Abstract: This work presents carbon fabric reinforced aluminosilicate matrix composites with content of boric acid, where boron replaces aluminum ions in the matrix and can increase the mechanical properties of composites. Five different amounts of boric acid were added to the alkaline activator for preparing six types (including alkaline activator without boric acid) of composites by the prepreg method. The influence of boric acid content in the matrix on the tensile strength, Young’s modulus and interlaminar strength of composites was studied. Attention was also paid to the influence of boron content on the behavior of the matrix and on the internal structure of composites, which was monitored using a scanning electron microscope. The advantage of the aluminosilicate matrix is its resistance to high temperatures; therefore, tests were also performed on samples affected by temperatures of 400–800 ◦C. The interlaminar strength obtained by short-beam test were measured on samples exposed to 500 ◦C either hot (i.e. measured at 500 ◦C) or cooled down to room temperature. The results showed that the addition of boron to the aluminosilicate matrix of the prepared composites did not have any significant effect on their mechanical properties. The presence of boron affected the brittleness and swelling of the matrix and the differences in mechanical properties were evident in samples exposed to temperatures above 500 ◦C. All six prepared composites showed tensile strength higher than 320 MPa at laboratory temperature. The boron-free composite had the highest strength 385 MPa. All samples showed a tensile strength higher than 230 MPa at elevated temperatures up to 500 ◦C.

Keywords: composite; carbon ﬁber; prepreg; temperature resistant; tensile strength; short-beam

1. Introduction Alkali activated aluminosilicates, called geopolymers under the certain conditions, have gained great popularity in research in recent years. The advantage of these inorganic materials is easy and fast preparation, during which it is not necessary to use a high temperature for the final hardening [1]. This inorganic binder based on alkali activated aluminosilicates is generally formed by mixing powdered aluminosilicates with a liquid alkaline activator. Material containing Si and Al, such as metakaolin, is usually dissolved in a liquid alkali silicate and alkali metal hydroxide solution [1,2]. The use of industrial aluminosilicate waste materials, such as coal ash and blast furnace slag, is an advantage, which leads to a reduction in the price and environmental footprint of the material [3]. The type and ratio of aluminosilicate and activator affect the resulting properties of the material [4–6]. Just the excellent mechanical properties, such as strength, resistance to chemicals and high temperature, is another reason for the interest in these materials [1,2,7]. Many fillers and additives can also be added to the binder, which specifically affect the properties of the resulting material [8]. Widely used fillers are sand [8,9] and fibers [10,11]. Some studies also

Materials 2020, 13, 5409; doi:10.3390/ma13235409 www.mdpi.com/journal/materials Materials 2020, 13, 5409 2 of 11 use additives, such as compounds based on boron or phosphate [12–14]. Boron ions are usually represented by borax or boric acid and materials containing these in the binder are often referred to as boroaluminosilicate materials [14]. Boron is a glass-forming cation and can be four-coordinated. So, it is assumed that these characteristics can lead to improved interactions between the cation and the aluminosilicate network. The studies work with the idea that the boron ions replace the aluminum ones in the initial structure and thus the mechanism of boroaluminosilicates formation is similar to formation of pure aluminosilicates. Some authors report an increase in the strength of composites after the addition of boron to the matrix [13–16]. The use of alkali activated aluminosilicates is focused primarily on construction industry [17,18] or the immobilization of hazardous waste [19,20]. Recently, it has also been studied as a matrix of composite laminate [7,11]. Commonly used organic material matrices have a very high strength, the disadvantage is that the organic matrix (e.g., epoxy resin) cannot be used at temperatures above 300 ◦C [11]. Inorganic materials based on alkali activated aluminosilicates are resistant to the temperatures up to 1000 ◦C with no emission of toxic fumes when heated [21], including the composites with carbon fibers [22]. Under the laboratory conditions, composite laminate with inorganic matrix is often produced by applying the matrix simultaneously with the layering of fibers, roving, or fabrics. Another option is production of composite laminate with the assistance of prepregs. Thus, first the matrix is applied to the fabric, then pre-impregnated fabrics (prepregs) are stored and used to prepare composites after a certain time. Alkali activated aluminosilicates mature under normal air conditions for about 24 hours. Therefore, the composition of these materials must be adjusted to prevent hardening. Modified material used in this study can be stored at 18 C. At this temperature, the matrix does not harden during the − ◦ first month. In this article, the influence of boron content in a matrix based on alkali-activated aluminosilicates on the mechanical properties of composites is studied. Carbon fabrics and six matrices with different boron contents were used to prepare composite plates, which were then tested for tensile strength, interlaminar strength and Young’s modulus. The tests were performed on samples cured at room conditions (ca 25 ◦C) and on samples heated to 400 ◦C, 500 ◦C, 600 ◦C and 800 ◦C for 1 h. Interlaminar strength (short-beam test) was also measured during the heating of samples.

2. Materials and Methods

2.1. Materials For preparation of alkali activated aluminosilicate matrix (A-matrix) were used commercial metakaolinite-rich material produced by the calcination of kaolinitic claystone [23–25] in a rotary kiln at ca 750 ◦C(Ceskˇ é lupkové závody, a.s., Nové Strašecí, Czech Republic), silica fume (Ceskˇ é lupkové závody, a.s., Nové Strašecí, Czech Republic), commercial potassium water glass with molar ratio SiO2:M2O equal to 1.7 (Vodní sklo, a.s., Prague, Czech Republic), potassium hydroxide flakes (Lach-Ner, s.r.o., Neratovice, Czech Republic), boric acid (Penta, s.r.o., Prague, Czech Republic), and distilled water. Composites were prepared from A-matrix and carbon plain weave fabric with the area weights of 200 g/m2 (Carbon fabric eSpread 200 CHT, Porcher Industries, La Voulte-sur-Rhône, France). The carbon fabric was chosen as the reinforcement due to the high alkalinity of the matrix. Although carbon fibers degrade at high temperatures in an oxidizing atmosphere, the use of materials such as glass or basalt in such an alkaline matrix is unsuitable. The conventional acid-base titration method and an inductively coupled plasma optical emission spectrometer OPTIMA 8000 (PerkinElmer, Waltham, MA, USA) were used for the chemical analysis of water glass while powdered materials were analyzed by X-ray fluorescence (XRF, Bruker S8 Tiger, Billerica, MA, USA). The chemical compositions of the raw materials are given in Table1. The results of X-ray diffraction analysis (XRD, Bruker D8 Advanced, Billerica, MA, USA) are shown in Figure1. The thermal analysis of the carbon fabric (TG/DTA – thermogravimetry/differential thermal analysis) Materials 2020, 13, 5409 3 of 12 Materials 2020,, 13,, 54095409 3 of 1112 thermalthermal analysis)analysis) waswas performedperformed usingusing aa DiscoveryDiscovery SeSeriesries thermalthermal analysisanalysis systemsystem (TA(TA Instruments,Instruments, Newwas performed Castle, DE, using USA). a The Discovery sample Series was heated thermal at analysis a heating system rate of (TA 10 Instruments,°C/min in flowing New Castle,air (Figure DE, 2). USA).2). The sample was heated at a heating rate of 10 ◦C/min in flowing air (Figure2).

Table 1. Chemical composition of raw materials. Table 1.1. Chemical composition of raw materials. Material Composition (%) MaterialMaterial Composition Composition (%) (%) MaterialMaterial Material H2O SiO2 Al2O3 Na2O K2O CaO P2O5 Fe2O3 ZrO2 H2O SiOH22O AlSiO2O32 AlNa2O2OK3 Na22O K2 CaOO CaO P 2O5P2O5 Fe2FeO32O3 ZrOZrO2 2 Potassium water glass 63.9 18.9 0.23 16.7 PotassiumPotassium water glasswater glass 63.9 18.963.9 18.9 0.23 0.23 16.7 16.7 Metakaolinite-richMetakaolinite-rich material 1.69 52.8 41.7 0.84 0.16 0.08 0.92 Metakaolinite-rich material1.69 52.81.69 52.8 41.7 41.7 0.84 0.84 0.16 0.16 0.080.08 0.920.92 materialSilica fume 0.99 96.4 0.42 0.04 0.09 0.42 1.27 SilicaSilica fume fume 0.99 96.40.99 96.4 0.42 0.42 0.04 0.04 0.09 0.09 0.420.42 1.271.27

Figure 1. X-ray powder diffraction analysis of metakaolinite-rich and silica material. Figure 1. X-ray powder diffraction diﬀraction analysis of metakaolinite-richmetakaolinite-rich and silica material.

Figure 2. TheThe thermal thermal analysis analysis curve curve describing describing the the degradat degradationion of of carbon carbon fibers fibers in in the the air air atmosphere. atmosphere. Figure 2. The thermal analysis curve describing the degradation of carbon fibers in the air atmosphere. 2.2. Matrix 2.2. Matrix Starting alkaline activator A0 was prepared with composition of molar ratio SiO2/K2O = 1.15 Starting alkaline activator A0 was prepared with composition of molar ratio SiO2/K2O = 1.15 by by mixingStarting the alkaline commercial activator potassium A0 was water prepared glass, with potassium composition hydroxide of molar solution ratio(KOH:H SiO2/K2O2O = =1.151:1 by in mixing the commercial potassium water glass, potassium hydroxide solution (KOH:H2O = 1:1 in weightmixing ratio),the commercial and distilled potassium water. After water mixing, glass, five potassium different batcheshydroxide of solidsolution boric (KOH:H acid were2O added= 1:1 toin weight ratio), and distilled water. After mixing, five different batches of solid boric acid were added

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the activator A0 so that the resulting amounts of boric acid in the newly formed activators were 0.8%, 4%, 8%, 12% and 16%. Six activators, including reference activator A0, were labeled according to the amount of boric acid: A0,A0.8,A4,A8,A12 and A16. Activators were mixed in a blender (Kenwood KVL8400S Chef XL Titanium, Havant, UK) for 24 hours and stored in the fridge at 5 ◦C for one day. Then, the metakaolinite-rich material and silica fume were added to the alkaline activators to form six matrices named after the activators (M0,M0.8,M4,M8,M12 and M16). This mixtures of matrix with composition of SiO2/Al2O3 = 33.9, K2O/Al2O3 = 3.98, H2O/K2O = 12.1 (molar ratio) was blended ca 30 min, stored in a freezer at 18 C for 24 h. This matrix composition was designed and veriﬁed in − ◦ previous studies [12,26].

2.3. Laminate Composites

Matrices M0 – 16 were used to prepare six-layer composite plates with analogous names C0,C0.8, C ,C ,C and C . Matrices were applied to 50 30 cm carbon fabric pieces using a paint roller, 4 8 12 16 × then the impregnated fabrics (prepregs) were individually placed between two pieces of plastic foil and stored in a freezer at 18 C. After one week, the prepregs were taken out of the freezer, − ◦ stripped of plastic foil and stacked one by one (always in the same direction) to get the composite plate. Every prepared composite plate was placed between two pieces of peel-ply fabric, wrapped in a plastic foil, compressed at 440 kPa for one hour and then cured in the oven at 65 ◦C for 3 h. After this time, the plates were unwrapped from the plastic foil and peel-ply fabric and ﬁnally cured for 28 days at laboratory conditions [26].

2.4. Pyrometric Cone Refractoriness Refractoriness of the prepared matrices was obtained by a method for determining the refractory conformity of refractory raw materials with pyrometric reference cones according to the European standard EN 993-12. A heat microscope (Clasic CZ, type 0116 VAK, Revnice,ˇ Czech Republic), pyrometric reference cones and test cones (30 8.5 mm, prepared according to EN 993-13) were × used to the determination of matrix refractoriness. During heating (5 ◦C/min), the tested samples were observed and compared with the pyrometric reference cones. The results were veriﬁed by three measurements.

2.5. Tensile Strength and Young’s Modulus Prepared six-layer composite plates were cut into 250 25 mm samples by water jets. The ends of × the samples were coated with epoxy resin and covered with sandpaper to protect the sample surface from sharp grips. Tensile strength and Young’s modulus were measured on samples stored at room temperature (ca 25 ◦C) and on samples that were treated with temperatures of 400 ◦C, 500 ◦C, 600 ◦C and 800 ◦C for one hour using the universal testing machine LabTest 6.200 (LaborTech, s.r.o., Opava, Czech Republic) complying with ASTM D3039. The treatment temperature was added to the name of the composites (C0-25, C0-400, C0-500, C0-600, C0-800, etc.).

2.6. Interlaminar Strength (Short-Beam Test) Short-beam strength test was measured on samples prepared from 12 layers of carbon fabrics in the same way as the six-layer tensile strength samples. A large number of layers was used to comply with conditions of the standard ASTM D2344. Prepared samples 4 mm 8 mm 24 mm were tested × × by the universal testing machine LabTest 6.200 (LaborTech, s.r.o., Opava, Czech Republic) with a three-point bending loading conﬁguration at a rate of crosshead speed of 1 mm/min.

2.7. Scanning Electron Microscope The saturation of the carbon fabric with the matrix and changes in the morphology of the composites aﬀected by diﬀerent amounts of boric acid in the aluminosilicate matrix and by the action Materials 2020, 13, 5409 5 of 11

of diMaterialsﬀerent 2020 high, 13,temperatures 5409 on the composite were monitored by scanning electron microscope5 of 12 (SEM, JEOL JSM-IT500HR, Tokyo, Japan). Samples were coated with about 5 nm of gold, to make them conductive(SEM, JEOL and toJSM-IT500HR, prevent charging Tokyo, ofJapan). the samples Samples in were the coated microscope. with about 5 nm of gold, to make them conductive and to prevent charging of the samples in the microscope. 3. Results and Discussion 3. Results and Discussion 3.1. Pyrometric Cone Refractoriness 3.1. Pyrometric Cone Refractoriness The results of refractoriness for the six prepared matrices are shown in Figure3a. The pyrometric cone refractorinessThe results ofof matricesrefractoriness was for determined the six prepared by simultaneously matrices are shown melting in Figure of two 3a. reference The pyrometric cones with cone refractoriness of matrices was determined by simultaneously melting of two reference cones with diﬀerent melting point and tested matrix cone placed in the middle (Figure3b). The results show that different melting point and tested matrix cone placed in the middle (Figure 3b). The results show that all all prepared matrices melt at a temperature higher than 800 C, so the matrices are suitable for high prepared matrices melt at a temperature higher than 800 °C,◦ so the matrices are suitable for high temperature applications. Especially matrix without boric acid (M ) melted at a temperature of up to temperature applications. Especially matrix without boric acid (M0) melted0 at a temperature of up to 940 940 ◦°C.C. The high high temperature temperature caused caused a visible a visible foaming foaming of the tested of the cones. tested The cones. addition The of addition boric acid ofled boric to a acid led toreduction a reduction in the in refractoriness, the refractoriness, but also to but a reduction also to a in reduction the undesired in the swelling undesired of the swellingmatrix. of the matrix.

FigureFigure 3. (a 3.) Pyrometric(a) Pyrometric refractoriness refractoriness of of sixsix matrices with with different diﬀerent content content of boric of boric acid acidand ( andb) tested (b) tested matrixmatrix cone cone between between two two reference reference cones. cones.

It isIt known is known that that materials materials containing containing elemental elemental Si, Si, such such asas silicasilica fumefume used used for for preparation preparation of of the matrix,the maymatrix, be usedmay asbe aused gas-forming as a gas-forming agent at elevatedagent at elevated temperature, temperature, thereby thereby causing causing the geopolymer the to swellgeopolymer [27–29]. to Incorporation swell [27–29]. ofIncorporation boron into of the boron aluminosilicate into the aluminosilicate network [ 13network,15] can [13,15] suppress can this suppress this effect [13]. However, the effect of matrix expansion can have a positive effect in effect [13]. However, the effect of matrix expansion can have a positive effect in applications where it is applications where it is necessary to provide an increase in thermal insulation capabilities. After necessary to provide an increase in thermal insulation capabilities. After heating the aluminosilicate heating the aluminosilicate matrix without boron and with lower boron content, the porosity matrixincreases without [27–29]. boron and with lower boron content, the porosity increases [27–29].

3.2. Expansion3.2. Expansion of Aluminosilicate of Aluminosilicate Laminate Laminate CompositeComposite An expansionAn expansion of theof the matrix matrix volume volume waswas also observed observed for for laminate laminate composite composite samples samples after afterheat heat treatment.treatment. Asin As the in casethe case of refractoriness of refractoriness measurements, measurements, this this effect effect decreased decreased withwith increasing boric boric acid contentacid in content the matrix. in the Inmatrix. the caseIn the of case laminate of laminate composites composites with with borosilicate borosilicate matrix, matrix, the the effect effect of of sample expansionsample is expansion suppressed is suppressed at boric acid at contentboric acid higher content than higher 4 wt.% than in 4 thewt. alkali% in the activator alkali activator (ca 2 wt.% (ca 2 in wt.matrix). % in matrix). Composite sample C4 showed the highest change in sample thickness up to 66% at Composite sample C4 showed the highest change in sample thickness up to 66% at temperature 500 ◦C. temperature 500 °C. Compared to that, composite sample C16 had a volume change only 5% at 500 °C. Compared to that, composite sample C16 had a volume change only 5% at 500 ◦C. Foaming of geopolymer with high SiO2 content (>50 wt. %) is described by E. Prud’homme at al. Foaming of geopolymer with high SiO content (>50 wt.%) is described by E. Prud’homme at al. [27]. [27]. The authors describe the possibilities2 of the formation of foamed aluminosilicates, which they The authors describe the possibilities of the formation of foamed aluminosilicates, which they are the are the production of a gas (H2), an increase of the viscosity and a consolidation of material. Hydrogen productionis produced of a gas (Equation (H2), an (1)) increase by water of thereduction viscosity and and by athe consolidation oxidation of ofsilicon material. in a basic Hydrogen environment is produced (Equationat temperatures (1)) by water above reduction 70 °C. and by the oxidation of silicon in a basic environment at temperatures above 70 ◦C. 2 0 → 2 4. 4H O + Si0 2H + Si(OH) (1) 4H2O + Si 2H2 + Si(OH)4. (1) Thus, the addition of boric acid reduces the→ foaming of the matrix. These changes may be due to reductionThus, the in addition the percentage of boric of silicon acid reduces in the matrix the foaming, the incorporation of the matrix. of boron These into changesthe aluminosilicate may be due to reductionstructure, in the or percentagethe acidification of silicon of the in thebasic matrix, activator the incorporationwith acid. Figure of boron4 graphically into the aluminosilicateshows the structure, or the acidiﬁcation of the basic activator with acid. Figure4 graphically shows the percentage MaterialsMaterials 20202020, ,1313, ,5409 5409 66 of of 12 11 Materials 2020, 13, 5409 6 of 12 percentage increase in sample volumes after heat exposure. The thickness of the samples was percentage increase in sample volumes after heat exposure. The thickness of the samples was measuredincrease in by sample a caliper volumes after cooling after heatthe samples exposure. down The to thickness the room of temperature. the samples was measured by a measuredcaliper after by cooling a caliper the after samples cooling down the tosamples the room down temperature. to the room temperature.

Figure 4. Percentage increase in sample thickness due to matrix swelling. FigureFigure 4. 4. PercentagePercentage increase increase in in sample sample thickness thickness due due to matrix swelling. The tensile strength, Young’s modulus and interlaminar strength results (in MPa) were related TheThe tensile strength, Young’sYoung’s modulusmodulus andand interlaminar interlaminar strength strength results results (in (in MPa) MPa) were were related related to to the original dimensions of the samples, (before the dimensional changes caused by high tothe the original original dimensions dimensions of the of samples, the samples, (before the(before dimensional the dimensional changes caused changes by high caused temperatures). by high temperatures). This process was chosen due to the practical use of composites, especially in terms of temperatures).This process was This chosen process due was to chosen the practical due to the use pr ofactical composites, use of composites, especially inespecially terms of in design terms of design of construction before thermal exposure. designconstruction of construction before thermal before exposure. thermal exposure.

3.3.3.3. Tensile Tensile Strength Strength and and Young’s Young’s Modulus 3.3. Tensile Strength and Young’s Modulus ItIt can can be be seen seen in Figure5 5 that that the the boron boron content content in in the the matrix matrix did did not not have have a significant a significant e ff ecteffect on on the It can be seen in Figure 5 that the boron content in the matrix did not have a significant effect on thetensile tensile strength strength of the of laminatesthe laminates up to up a temperature to a temperature of 500 ofC. 500 This °C. correlates This correlates with the with other the literature, other the tensile strength of the laminates up to a temperature ◦of 500 °C. This correlates with the other literature,where the where effect the ofboron effect of on boron the mechanical on the mechanic propertiesal properties of composites of composit is stronglyes is strongly dependent dependent on the literature, where the effect of boron on the mechanical properties of composites is strongly dependent oncomposition the composition of the matrix of the binder matrix [14 binder,15]. The [14,15]. author’s The results author's show results the e ffshowect of the boron effect depending of boron on on the composition of the matrix binder [14,15]. The author's results show the effect of boron dependingthe content on of the fly, content slug or sodiumof fly, slug silicate. or sodium silicate. depending on the content of fly, slug or sodium silicate.

FigureFigure 5. InﬂuenceInfluence of of temperature temperature and and boron boron content content in the in matrix the onmatrix the tensile on the strength tensile of strength composites. of Figure 5. Influence of temperature and boron content in the matrix on the tensile strength of composites. composites.Samples without high temperature treatment had tensile strength from 385 to 324 MPa. These strengths correspond to the tensile strength of conventional duralumin. After one-hour Samples without high temperature treatment had tensile strength from 385 to 324 MPa. These expositionSamples of thewithout samples high to temperature temperatures treatment of 400 C had and 500tensileC, strength the samples from still 385 had to very324 MPa. high tensileThese strengths correspond to the tensile strength of conventional◦ duralumi◦ n. After one-hour exposition of strengthsstrength 329–270 correspond MPa, to 259–239 the tensile MPa, strength respectively. of conventional At these temperatures, duralumin. After laminate one-hour composites exposition with anof the samples to temperatures of 400 °C and 500 °C, the samples still had very high tensile strength theorganic samples matrix to aretemperatures not useable. of In400 addition, °C and heating500 °C, the or ﬁre samples of epoxy still resin had (thevery most high common tensile strength organic 329–270 MPa, 259–239 MPa, respectively. At these temperatures, laminate composites with an organic 329–270matrix) leadsMPa, to259–239 the release MPa, ofrespectively. toxic and irritating At these gasestemperatures, and vapors. laminate composites with an organic matrix are not useable. In addition, heating or fire of epoxy resin (the most common organic matrix) matrix are not useable. In addition, heating or fire of epoxy resin (the most common organic matrix) leads to the release of toxic and irritating gases and vapors. leads to the release of toxic and irritating gases and vapors.

Materials 2020, 13, 5409 7 of 11 Materials 2020, 13, 5409 7 of 12

AtAt higherhigher temperatures,temperatures, there there was was a sharp a sharp decrease decrea inse tensile in tensile strength strength of composites of composites with matrices with matriceswith and with without and boron without content. boron This content. was probably This was due probably to the thermal due to degradation the thermal of degradation carbon fibers of in carbonthe oxidizing fibers in atmosphere, the oxidizing which atmosphere, takes place which at this ta temperature.kes place at this The temperature. best results afterThe exposurebest results to after600 ◦ Cexposure/1 h were to achieved 600 °C/1 inh casewere of achieved composite in Ccase4, which of composite also corresponds C4, which to also the corresponds composition to of the the compositionmatrix with theof the largest matrix increase with the in volume,largest increase as discussed in volume, earlier. as After discussed one hour earlier. at the After temperature one hour at of the600 temperature◦C, this sample of reached600 °C, this a tensile sample strength reached of144 a te MPansile and strength after exposureof 144 MP toa temperatureand after exposure 800 ◦C/1 to h, temperatureit was 34 MPa. 800 °C/1 h, it was 34 MPa. FigureFigure 66 shows shows the the e effectffect of of temperature temperature and and matrix matrix composition composition on theon the stiff stiffnessness of a laminateof a laminate with withan inorganic an inorganic matrix. matrix. It is clear It is that clear the that diff erentthe different boron content boron incontent the matrix in the did matrix not have did anot significant have a significanteffect on the effect Young’s on the modulus Young’s upmodulus to a temperature up to a temperature of 500 ◦C, of which500 °C, was which mainly was mainly influenced influenced by the bysti fftheness stiffness of the carbonof the carbon fibers. Upfibers. to theseUp to temperatures, these temperatures, the modulus the modulus reached reached values aroundvalues around 30 GPa. 30A significantGPa. A significant decrease decrease of the sti offf nessthe stiffness of the samples of the samples occurred occurred at the temperature at the temperature 600 ◦C 600 or higher. °C or higher.At 600 ◦AtC, a600 loss °C, of a the loss sti offfness the canstiffness be seen can depending be seen depending on the boron on contentthe boron in thecontent matrix. in the The matrix. higher Thethe amounthigher the of boron,amount the of lower boron, the the sti fflowerness ofthe the stiffn samples.ess of Thisthe samples. was probably This was due probably to the effect due of to boron the effecton the of reduction boron on of the the reduction melting temperature of the melting of the temperature binder (Figure of the3a) and,binder thus, (Figure its higher 3a) and, degradation thus, its higherin samples degradation with a higher in samples boron with content. a higher boron content.

FigureFigure 6. 6. InfluenceInfluence of of temperature temperature and and boron boron content content in inthe the matrix matrix on onthe theYoung’s Young’s modulus modulus of composites.of composites. 3.4. Interlaminar Strength (Short-Beam Test) 3.4. Interlaminar Strength (Short-Beam Test) During the measurement of the tensile strengths of the laminates, the mechanical properties of During the measurement of the tensile strengths of the laminates, the mechanical properties of used fabric were manifested, primarily because the loading force acted in the direction of the fibers. used fabric were manifested, primarily because the loading force acted in the direction of the fibers. In the short-beam test, when the adhesion of individual layers (interlaminar strength) is determined, In the short-beam test, when the adhesion of individual layers (interlaminar strength) is determined, the force acts perpendicular to the direction of the fibers and therefore the properties of the matrix, the force acts perpendicular to the direction of the fibers and therefore the properties of the matrix, and its adhesion to the reinforcement are more pronounced here. and its adhesion to the reinforcement are more pronounced here. From the results shown in Figure7, it is evident that the boron content has a negative e ffect on the From the results shown in Figure 7, it is evident that the boron content has a negative effect on interlaminar strength of the aluminosilicate matrix, which is based on clay shell, silica and carbon fibers. the interlaminar strength of the aluminosilicate matrix, which is based on clay shell, silica and carbon The interlaminar strength first decreased with the addition of boron. The lowest value of strength was fibers. The interlaminar strength first decreased with the addition of boron. The lowest value of shown by the C8 composite, the composition of which seems to be the least suitable. With the further strength was shown by the C8 composite, the composition of which seems to be the least suitable. addition of boron, the interlaminar strength increased again, but only up to a temperature of 500 C. With the further addition of boron, the interlaminar strength increased again, but only up to◦ a High temperature resistance is a significant advantage of the composites investigated in this work. temperature of 500 °C. High temperature resistance is a significant advantage of the composites The reduction in strength of samples with a higher boron content affected by a temperature higher investigated in this work. The reduction in strength of samples with a higher boron content affected than 500 C is therefore very undesirable, and the effect of the addition of boron to the matrix appears by a temperature◦ higher than 500 °C is therefore very undesirable, and the effect of the addition of to be negative. boron to the matrix appears to be negative.

MaterialsMaterials 20202020, ,1313, ,5409 5409 88 of of 12 11 Materials 2020, 13, 5409 8 of 12

Figure 7. Influence of boron content in the matrix on the interlaminar strength of composites. FigureFigure 7. 7. InfluenceInﬂuence of of boron boron content content in in the the matrix matrix on on the the interlamin interlaminarar strength strength of of composites. composites. Figure 7 shows the interlaminar strength of the samples after exposing the samples to the FigureFigure 77 showsshows thethe interlaminarinterlaminar strengthstrength ofof thethe samplessamples afterafter exposingexposing thethe samplessamples toto thethe appropriate temperature for 1 h. In contrast, interlaminar strength was also measured directly at the appropriateappropriate temperature temperature for for 1 1 h. h. In In contrast, contrast, interlaminar interlaminar strength strength was was also also measured measured directly directly at at the the temperaturetemperature 500 500 °C.C. Except Except for for the the sample sample with with the the highest highest boron boron content content in the matrix matrix (C 16),), a a higher higher temperature 500 °C.◦ Except for the sample with the highest boron content in the matrix (C16), a higher interlaminar strength was always found when measured at elevated temperature (Figure 8). interlaminarinterlaminar strength strength was was always always found found when when measured measured at at elevated elevated temperature temperature (Figure 88).).

Figure 8. Comparison of interlaminar strength results of samples measured during and after Figure 8. Comparison of interlaminar strength results of samples measured during and after heat Figureheat treatment. 8. Comparison of interlaminar strength results of samples measured during and after heat treatment. treatment. This was probably due to the fact that the sample was cooled after high temperature and the This was probably due to the fact that the sample was cooled after high temperature and the matrixThis became was probably brittle due due to to volume the fact changes. that the Assample the temperature was cooled rises,after thehigh plasticity temperature of the and matrix the matrix became brittle due to volume changes. As the temperature rises, the plasticity of the matrix matrixmaterial became also appears brittle due to increaseto volume and changes. thus its As sti ffthnesse temperature decreases. rises, However, the plasticity it was not of possiblethe matrix to material also appears to increase and thus its stiffness decreases. However, it was not possible to materialperform also tensile appears tests atto higherincrease temperatures, and thus its sostif unfortunately,fness decreases. we However, do not have it was this not data possible available, to perform tensile tests at higher temperatures, so unfortunately, we do not have this data available, performand we cantensile only tests deduce at higher it from temperatures, the results obtained so unfo inrtunately, the graph we in Figuredo not8 have. The this increased data available, plasticity and we can only deduce it from the results obtained in the graph in Figure 8. The increased plasticity andand, we thus, can the only reduced deduce brittleness it from the of results the matrix obtained aids thein the mutual graph adhesion in Figure of 8. the The individual increased layersplasticity and and, thus, the reduced brittleness of the matrix aids the mutual adhesion of the individual layers and and,therefore thus, thethe measurementsreduced brittleness performed of the atmatrix elevated aids temperature the mutual adhesion showed a of higher the individual interlaminar layers strength and therefore the measurements performed at elevated temperature showed a higher interlaminar thereforethan after the being measurements cooled down. performed at elevated temperature showed a higher interlaminar strength than after being cooled down. strength than after being cooled down. 3.5. Scanning Electron Microscope 3.5. Scanning Electron Microscope 3.5. ScanningSEM photos Electron showed Microscope that the fabrics were very well saturated with matrix in all prepared composite SEM photos showed that the fabrics were very well saturated with matrix in all prepared plates.SEM Figure photos9 shows showed SEM that photos the offabrics a composite were very with well a matrix saturated without with boric matrix acid Cin0 all(Figure prepared9a,b), composite plates. Figure 9 shows SEM photos of a composite with a matrix without boric acid C0 compositecomposite plates. C8 (Figure Figure9c,d) 9 shows and composite SEM photos with of a matrixa composite with thewith highest a matrix amount without of boric boric acid acid C C160 (Figure 9a,b), composite C8 (Figure 9c,d) and composite with a matrix with the highest amount of (Figure(Figure 99a,b),e,f). Therecomposite is a clearlyC8 (Figure visible 9c,d) di ffanderence composite between with the a matrices. matrix with Matrix the ofhighest C0-25 amount composite of boric acid C16 (Figure 9e,f). There is a clearly visible difference between the matrices. Matrix of C0-25 boric(Figure acid9a) C is16 smooth(Figure and9e,f). fused There which is a clearly confirms visible the toughnessdifference ofbetween the sample, the matrices. while, in Matrix the case of ofC0 the-25 composite (Figure 9a) is smooth and fused which confirms the toughness of the sample, while, in the compositeC16-25 composite (Figure (Figure9a) is smooth9e), a fragmentedand fused which matrix confirms can be seen,the toughness which has of become the sample, brittle while, due toin the case of the C16-25 composite (Figure 9e), a fragmented matrix can be seen, which has become brittle case of the C16-25 composite (Figure 9e), a fragmented matrix can be seen, which has become brittle

Materials 2020, 13, 5409 9 of 11

presence ofMaterials boron. 2020, As 13, 5409 a result of the embrittlement of the matrix, the matrix was more9 often of 12 chipped out of the composite. This phenomenon was also observed while cutting the composite to obtain a due to the presence of boron. As a result of the embrittlement of the matrix, the matrix was more smaller sampleoften chipped that could out of be the placed composite. in theThis SEM.phenomen Furthermore,on was also observed we can wh seeile in cutting Figure the9 composite that the matrix of the C0 compositeto obtain a became smaller sample porous that after coulda be temperature placed in the SEM. of 500 Furthermore,◦C (Figure we 9canb—the see in Figure pores 9 arethat visible in the white borderthe matrix area), of the whileC0 composite the increased became porous boron after content a temperature of the of C 50016 °Csample (Figure signiﬁcantly 9b—the pores are reduced the porosity (Figurevisible 9inf). the white border area), while the increased boron content of the C16 sample significantly reduced the porosity (Figure 9f).

Figure 9. Photos of composite samples (a) C0-25, (b) C0-500, (c) C8-25, (d) C8-500, (e) C16-25, (f) C16-500 Figure 9. Photos of composite samples (a)C0-25, (b)C0-500, (c)C8-25, (d)C8-500, (e)C16-25, (f)C16-500 obtained using scanning electron microscope. obtained using scanning electron microscope. Differences in the interface between the fibers and the matrix are also visible in the presented DifferencesSEM photos. in the Thus, interface the addition between of boron the to the fibers matrix and appears the matrix to affect arethe adhesion also visible of the infiber the to presented SEM photos.the matrix, Thus, which the addition may have caused of boron differences to the in matrix interlaminar appears strength. to a Thisffect supports the adhesion SEM photo of of the fiber to the matrix,the which C8 composite may have(Figure caused 9c,d), where diff aerences significant in decrease interlaminar in the adhesion strength. of the This matrix supports to the fibers SEM photo is seen. It was sample C8 that showed the lowest interlaminar strength. The results show that the of the C8 compositecomposition of (Figure the matrix9c,d), M8 is where the least a suitable significant for the decrease preparation in of the composites, adhesion andof that the with matrix an to the fibers is seen.increasing It was amount sample of boron, C8 that the strength showed can the increase lowest again. interlaminar strength. The results show that the composition of the matrix M8 is the least suitable for the preparation of composites, and that with an increasing amount of boron, the strength can increase again.

4. Conclusions This paper presents a comparison of carbon composites prepared from six inorganic matrices differing in boric acid content. The composites were made of carbon plain wave fabrics and aluminosilicate matrices in prepreg form. The obtained results can be drawn in the following conclusions: Materials 2020, 13, 5409 10 of 11

A boric acid content of up to 16 wt.% in the alkaline activator (approximately 8 wt.% in the • matrix) has not been shown to be significantly suitable for improving the mechanical properties of aluminosilicate laminates prepared by the prepreg method. The boric acid content affected the behavior of the matrix. The higher boron content reduced the • swelling of the matrix when heated to higher temperatures, but caused its embrittlement. Up to a temperature of 500 C, all samples were relatively stable and always showed a tensile • ◦ strength higher than 230 MPa. Without temperature treatment, the highest tensile strength showed samples without boron • content C (385 14 MPa) and the lowest tensile strength showed samples C (324 6 MPa). 0 ± 8 ± Laminate C with the biggest volume change had the highest tensile strength (144 13 MPa) after • 4 ± 600 ◦C. Up to 500 C, Young’s modulus was most affected by the properties of the reinforcement (carbon • ◦ fiber) and by the boron content at temperature above 600 ◦C. Boron content had a negative effect on interlaminar strength. •

Author Contributions: Conceptualization, E.H. and P.H.; Methodology, E.H. and P.H.; Formal analysis, E.H.; Investigation, E.H. and P.H.; resources, E.H. and P.H.; data curation, E.H.; writing—original draft preparation, E.H.; writing—review and editing, E.H. and P.H. All authors have read and agreed to the published version of the manuscript. Funding: The publication is a result of the project Development of the UniCRE Center (LO1606) which has been financially supported by the Ministry of Education, Youth and Sports of the Czech Republic (MEYS) under the National Sustainability Programme I. The result was achieved using the infrastructure of the project Efficient Use of Energy Resources Using Catalytic Processes (LM2015039) which has been financially supported by MEYS within the targeted support of large infrastructures. Conflicts of Interest: The authors declare no conflict of interest.

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