Fiber-reinforced polymer composites are widely used as lightweight structural materials in aerospace, naval, navigation, electronics, and automotive industries. In most cases, composites are made from organic matrix and carbon, basalt, or glass fibers. These materials exhibit excellent properties such as high tensile and flexural strength, low density, and corrosion resistance [1
]. The use of nanofiber structures as composite fillers is a recent trend in the field of composites [4
]. The disadvantage of composites prepared from organic matrix is that they cannot be used at temperatures above 200 °C. This is one of the reasons why an inorganic matrix has been studied in recent years [3
One of the perspective substitutes for an organic matrix for the preparation of composites is inorganic material based on the alkali-activated aluminosilicates (A-matrix), material that can be called geopolymer under certain conditions. A-matrix is formed by mixing powdered aluminosilicates with a liquid alkaline activator. A liquid alkali silicate and alkali metal hydroxide solution are usually used to dissolve material containing Si and Al such as metakaolin [6
]. A-matrix is often reinforced by solid materials to improve its properties [9
]. The properties of prepared A-matrix depend on the type and amount of aluminosilicate, activator, additive, water, Si/Al molar ratio, Na/Al molar ratio, Na+
, or K+
content and the curing conditions [9
]. A-matrix can be cured at the low temperatures (even at a room temperature). This material has good mechanical properties and it is resistant to chemicals and temperatures up to 1000 °C.
Various kinds of composites with inorganic matrix based on alkali-activated aluminosilicates, including particulate [13
], continuous fiber [15
] and short fiber [6
], were investigated in many studies. Glass [18
], carbon [15
] and basalt [17
] fabrics or unidirectional continuous fibers were usually layered in 6–16 layers [15
]. Mechanical properties were studied on samples cured at laboratory temperature and even after high-temperature heat treatment [6
]. Commonly, compressive strength [14
], flexural strength [6
], Young’s modulus [6
], tensile properties [22
], and impact resistance [18
] were investigated on composites with A-matrix. The properties of A-matrix composites depended on the type of fiber reinforcement, number of layers of fiber reinforcement, composition of A-matrix, A-matrix/fiber reinforcement weight ratio, and cure conditions of composites. Krystek et al. [23
] prepared composites with A-matrix and carbon fabric. The prepared samples reached a tensile strength up to 265 MPa.
Epoxy resins are usually used for carbon fiber sizing. Unfortunately, the carbon fibre sizing has made it difficult to apply the A-matrix to the surface of the fiber reinforcement; therefore, many authors removed it in various ways. Yan et al. [24
] washed the carbon fibers by an ultrasonic vibrator in acetone and then dried them in oven at 60 °C for 5 h. Lin et al. [25
] separated the short carbon fibers by an ultrasonic vibrator in ethanol, then the fibers were filtered out by a wire sieve to get sheet-like short carbon fiber preforms with a thickness in the range of 0.15–0.2 mm. Yuan et al. [26
] treated the fibers at 370 °C in air atmosphere for 2 h.
Composites can be also made from pre-impregnated fabric reinforcements called prepregs. In that case, the fabrics are commonly laminated by resin and stored for several months in a tempered equipment before layering. This method is typical for composites with organic matrix [18
]. The advantages of composites prepared from prepregs are easy manipulation with fiber reinforcement, better fiber reinforcement saturation with epoxy matrix, and the possibility of dividing the preparation of composites for fiber reinforcement impregnation and layering of pre-impregnated reinforcement. The preparation of geopolymer prepregs is not yet widely published.
This work is focused on a comparison of mechanical properties of geopolymer composite materials prepared in two ways. Conventional lamination of the carbon fabric, which is subsequently layered into a composite, is compared to the production of prepreg composites. The preparation of prepregs is part of this work. The properties of geopolymer composites made from untreated carbon fabrics and treated carbon fabric at 300 °C were also compared. The tensile strength, modulus of elasticity and the composite structure were studied on geopolymer composite samples after thermal exposure at a temperature up to 800 °C.
2. Materials and Methods
Carbon plain weave fabric with the area weights of 200 g/m2
(Carbon fabric eSpread 200 CHT, Porcher Industries, La Voulte-sur-Rhône, France) was used as the reinforcement for composites. Commercial metakaolinite-rich material produced by the calcination of kaolinitic claystone in rotary kiln at c. 750 °C (České lupkové závody, a.s., Nové Strašecí, Czech Republic) [28
], silica fume (České lupkové závody, a.s., Nové Strašecí, Czech Republic), commercial potassium water glass with molar ratio SiO2
O 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 were used for preparation of A-matrix.
The chemical compositions of powdered raw materials determined by X-ray fluorescence (XRF, Bruker S8 Tiger, Billerica, MA, USA) can be seen in Table 1
. The structural properties shown in Figure 1
and Figure 2
were specified by a BRUKER D8 Advanced X-ray diffraction system (XRD) equipped with a BRUKER SSD 160 detector and operating with Cu-Kα radiation. Size distribution of powdered raw materials was determined by a Mastersizer 2000 laser diffraction particle size analyser (MALVERN Instruments, Malvern, United Kingdom). The morphology of carbon fabric and prepared composites was observed by a scanning electron microscope (SEM, JEOL JSM-IT500HR, Tokyo, Japan). The conventional acid-base titration method and an inductively coupled plasma optical emission spectrometer OPTIMA 8000 (PerkinElmer, Waltham, MA, USA) were used for chemical analysis of water glass (Table 1
Alkaline activator was prepared with composition of molar ratio SiO2/K2O = 1.15 and K2O/B2O3 = 5.15 by mixing the commercial potassium water glass, potassium hydroxide solution (KOH:H2O = 1:1 in weight ratio), solid boric acid, and distilled water. Activator was mixed in a blender (Kenwood KVL8400S Chef XL Titanium, Havant, United Kingdom) for 24 h and stored in the fridge at 5 °C for two days. Then, the metakaolinite-rich material and silica fume were added to the alkaline activator. This mixture of A- matrix with composition of SiO2/Al2O3 = 33.9, K2O/Al2O3 = 3.98, H2O/K2O = 12.1 (molar ratio) was blended c. 30 min, stored in a freezer at −18 °C for 24 h and then used to prepare four six-layer composite plates.
Four six-layer composite plates were prepared from 24 pieces of 50 cm × 30 cm carbon fabric. Composite series were different in the combination of fabric pre-treatment and preparation method. For clarity, the scheme of composites preparation with the sample marking system is shown in Figure 3
. Twelve source pieces of carbon fabric for preparation of the first and second composite plates were kept in the air (CA) until the composite preparation. The remaining twelve pieces of fabric were placed into an oven at 300 °C for one hour to remove the epoxy layer from the surface of the carbon fibers (CO). Then, all fabrics were impregnated in a conventional manner with the A-matrix using a paint roller. Six impregnated CA fabric pieces were used for composite plate by classic method (CAC). The CAC plate was prepared by stacking impregnated fabrics one by one. The other six pieces of fabric were used for preparation of composite plate by prepreg method (CAP). In this case, the impregnated carbon fabrics were individually placed between two pieces of plastic foil (two pieces for every fabric) to prepare the prepregs. Prepregs were stored in a freezer at −18 °C. After seven days, the prepregs were taken out of the freezer, stripped of plastic foil, and used for preparation of composites by stacking one by one to get the CAP composite plate as in the case of CAC plate. Twelve impregnated carbon CO fabrics were used for preparation of the COC plate by a classic method and the COP plate by the prepreg method in the same way as CAC and CAP composite plates. 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. The fabric mass fraction of the plates is presented in Table 2
Four prepared composite plates were cut into 250 × 25 mm samples (Figure 4
) by water jets. The obtained samples were kept at a laboratory temperature (LT) or treated with temperatures of 400, 500, and 600 °C for one hour. The treated temperatures were added to the sample names (CAC-LT, COP-400, etc.). All samples were tested for tensile strength and modulus of elasticity using the universal testing machine LabTest 6.200 (maximum load of the sensor 200 kN) at a loading speed of 2 mm/min. (LaborTech, s.r.o., Opava, Czech Republic) complying with ASTM 3039 (Figure 5
). The ends of the samples were reinforced with epoxy resin coating and covered with sandpaper to protect the composite surface from sharp grips. Prepared composite samples were studied by a scanning electron microscope.