Basalt Fibre

9.3 Spin Process and Fiber Properties

For the preparation of fibers from basalt stones, a content of silica of 46% or more is necessary. Only under this precondition it is possible to melt the stone completely without residues, to reach an adequate viscosity for fiber formation, and to gain after freezing a homogenous amorphous phase without crystalline areas [23]. In general, the preparation of basalt fibers can be outlined in following steps: preparation of raw materials, melting of the stones, homogenization of the melt, the spinning of the fibers, and finally the application of the size [14]. Compared to the preparation of melts for glass fiber production, the melting of stones for the basalt fiber production is more challenging. The reason for this is the low thermal conductivity and low transparency for infrared (IR) radiation of basalt fibers. The IR radiation is also named as heat radiation and a material which has a good transparency to heat radiation is warmed homogeneously and liquefy more easily. For this reason, the melting of transparent glass is easier compared to the IR-intransparent basalt. To reach a melt of basalt stones a preheating at 1450°C is described [14]. Another challenge during the preparation of the basalt melt is posed by possible inhomogenities of the natural basalt stones [31]. A sufficient temperature for the spinning of basalt fibers is reported to be in the range of 1350–1420°C [23].

After realizing a homogeneous melt as starting material for the spinning process, the next step is the spinning including the filament formation accompanied by the cooling and solidification of the melt. During this step a problematic crystallization can occur, which can be avoided by thermo-isolation and controlled cooling procedures [26,32]. A fast cooling process leads to a high amorphous basalt fiber, while a slow cooling process increases the crystallization rate of the basalt fiber [32]. If the cooling process is done step by step and not continuously, different types of crystalline phases such as plagioclase, magnetite, and pyroxene can occur [22]. Altogether it should be clear that the exact controlling of temperature of molten basalt and temperature of the cooling is absolutely necessary to obtain basalt fibers of excellent and reproducible properties.

Besides attaining the above-mentioned properties by the size, other properties are also often aimed such as an improved corrosion stability, antistatic properties, and an improved abrasion stability [26]. A special development is the combination of the size with new materials such as carbon nanotubes (CNTs). A silane treatment of basalt fibers can as well be used to apply CNTs onto the fibers. In this application the silane is used to fix and arrange the CNTs on the basalt fiber surface. In this way modified basalt fibers are used for the preparation of fiber-reinforced materials, which are described as CNT/epoxy/basalt composites and exhibit significant improved fracture toughness [34,35]. Other innovative sizing agents are reported by Wei et al. [36,37]. They described a surface modification of basalt fibers with the so-called hybrid sizings containing nanosilica and epoxy functions.

Such systems can be realized by sol-gel process using tetraethoxysilane (TEOS) and epoxy modified silane compounds, for example, GLYMO shown in Fig. 9.5. The silica particles exhibit diameters of only few nanometers and the epoxy function introduces an enhanced adhesion to the polymer matrix in the final fiber-reinforced material. The main idea is here to realize a compound at the interface of basalt fiber surface to the polymer matrix, which contains an inorganic silica component and an organic epoxy function. The final goal is to improve the adhesion of polymer matrix to the basalt fibers [36,37]. Another aspect of using the size during basalt fiber production is avoiding micro-cracks on the fiber surface by the sizing. By the application of the size the growth of these micro-cracks can be avoided and the tenancy of the fibers can be stabilized [38]. The mechanical stability guaranteed by the size is reported to be absolutely necessary for the production steps like the production of hybrid yarns, weaving, knitting, and the finishing processes. The mechanical forces acting on the fibers during these processes are quite high, so a size giving the fiber a sufficient elasticity and flexibility is necessary [38].

One point to be kept in mind is that if the sizing compounds are made from organic material, then they have a higher thermal sensitivity as the inorganic basalt fibers. It was observed that rovings made from basalt fibers have already lost significant strength after a heat treatment at 300°C [28,39]. For these materials, it was determined that by heat treatment the amount of carbon on the basalt fiber surface can be deleted [28]. Before the heat treatment a significant amount of carbon (15%) was detected on the basalt fiber surface, probably related to an organic sizing agent. By heating process in air, this size is probably burned away and the positive influence of the size on the strength of the rovings is also eliminated [28].

One conclusion from this investigation is that it is necessary to develop sizing agents of high thermal stability especially for use in inorganic fiber with high thermal stability. Only with a thermally stable size, it is possible to take the full benefit of the thermal stability of the inorganic basalt fiber.

These investigations lead to the following results. First, the supplied basalt roving already exhibited a mechanical stability at 400°C. Second, by the application of the polysiloxane size the mechanical stability of the basalt roving is significantly improved, probably because the sizing agent glues the basalt fibers together strongly. Third, both additional sizing agents (polysilazane and polysiloxane) lead to improved mechanical properties after heat treatment at 500°C compared to the original basalt roving. However, a thermal treatment at 600°C eliminates mostly the mechanical stability for all the samples [40].

It is concluded that the sizing agents that form a metalorganic polymer film onto the basalt fiber surface act as a protective barrier layer against heat. By this crystallization processes introduced by heat are suppressed and the fiber strength is retained [40]. Further, this polymer film could also act as a barrier layer against oxygen from air. The oxidation of FeO present in the basalt fiber is avoided and the following crystallization is suppressed. The heating up to higher temperatures of 600°C probably also destruct the metalorganic polymer film, so its protective properties for the basalt fibers are diminished.

Altogether, it can be concluded that the sizing agent is an elemental component of basalt fibers, which influences the properties of the basalt fibers significantly. The type of sizing agent used has to be selected according to the demand and the type of application of basalt fibers.

20.4 Influence of temperature on mechanical behavior of basalt fibers

Exposure to higher temperatures will generally promote crystallization and therefore the loss of mechanical characteristics. Militký et al. (2007, 2009) investigated the strength distribution of basalt multifilament roving samples tempered in an oven at temperatures T_T = 20, 50, 100, 200, 300, 400, and 500°C in time intervals t_T = 15 and 60 min. The 50 samples of strength Pi were collected. These values were recalculated to stress at break values σ i (GPa).

The shear modulus is comparatively high. The prolongation of tempering leads to a large drop of G.

BFs show long term thermal stability characteristics. They are resistant to the influence of high temperature for a short time period up to 750°C, whereas for longer exposure the working temperature is in the range from 260 to 700°C, with excursions being possible up to 1000°C. After exposure under 400°C, BF loses only 20%–25% of its initial strength without loss of insulation properties, whilst the strength of E-glass under similar conditions drops more than 40%–45% (Kamenny Vek, 2015; Gilewicz et al., 2013; Milman et al., 1996). The results found for basalt multifilaments were much worse (Militký et al., 2002), see Table 20.8.

Up to 200°C, the variation in strength was neither significant nor clear in tendency. Over 200°C, however, a decrease in strength became distinctive as the heating temperature increased. The reduction was more significant in the carbon and the glass fibers but the BFs retained about 90% of their normal temperature strength up to 600°C. The BFs maintained their shape and seemed to have not lost the mechanical integrity till 1200°C. Li (2011) confirmed this conclusion.

This high thermal stability, i.e., heat resistance, is based on the material characteristics of natural basalt rocks, which nucleate at high temperature. Desirable physical and mechanical properties of fibrous materials are attributed to the homogeneous and fine distribution of crystalline phases. This desired microstructure can be obtained by adding a nucleating agent to some competing fibers such as TiO₂, ZrO₂, or P₂O₅. The basalt rocks, however, do not require this but contain a natural nucleating agent (nucleates at high temperature) such as Fe₃O₄ during the melting process, hence giving advantages over the other fibers, where nucleating agents are necessary to obtain a similar microstructure (Yilmaz et al., 1996).

Van de Velde et al. (2002) investigated the mechanical properties of basalt and glass after 2 h of thermal treatment. They found that basalt can retain its properties over a greater temperature range than glass. At temperatures over 400–500°C, it becomes weaker than glass, but it does retain integrity and still provides protection against heat.

Knotkova et al. (2007) found that the heating of basalt glass up to a temperature higher than 600°C is accompanied by the competitive processes of oxidation of Fe²⁺ with the evolution of spinel phases ((Mg, Fe)₃O₄) and the crystallization of a glass matrix (including the formation of a mixture of crystalline silicates and alumosilicates).

24.3.1.4 Basalt fibers

Basalt fibers are fabricated by melting crushed basalt rocks at 1400°C and drawing the molten material. Basalt fibers have better mechanical and physical properties than glass fibers, their main advantages being: fire resistance, good resistance to chemically active environments, vibration and acoustic insulation capacity (fib, 2007). The improved production facilities and quality control capacity enable fabrication of basalt fibers with high quality and low variability of properties (Matthys, 2014). Basalt fibers are more expensive than E-glass fibers, but much cheaper than carbon fibers. Some typical properties of basalt fibers are given in Table 24.2, while the architectural forms of these fibers are illustrated in Figs. 24.8–24.11.

8.6.6 Basalt fibres

Basalt fibres are obtained from a naturally occurring complex silica/alumina/other oxide basalt rock similar to glass in composition and used as an asbestos replacement. They are available in filament and nonwoven forms with claimed superiority to glass fibres in terms of temperature performance as indicated in Table 8.5. Initially they contained high levels of impurity and were brown in colour, although current versions are bronze. This improved purity in fibres such as Basaltex® (Basaltex NV, Belgium) enables their production as continuous filaments with nominal diameters in the range 9–24 μm. Rovings and chopped fibres are also available. Derived woven fabrics and nonwoven mats are used as fire barriers, thermal insulation, and composite reinforcements.

Structure and properties

Basalt is a common extrusive volcanic rock, which is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic, containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey. The manufacture of basalt fibre requires the melting of the quarried basalt rock to about 1400 °C, this is then extruded through small nozzles to produce continuous filaments of basalt fibre. There are three main manufacturing techniques, which are centrifugal-blowing, centrifugal-multiroll and die-blowing. The fibres typically have a filament diameter of 9–13 μm, which is far enough above the respiratory limit of 5 μm to make basalt fibre a suitable replacement for asbestos. They also have a high elastic modulus, resulting in an excellent specific tenacity that is three times that of steel. The continuous basalt fibres derived from basalt rock have proven technical characteristics and performance specifications.

9.4.3 Inorganic natural fibre grid: basalt fiber reinforced polymer grid

Basalt fibre grids embedded in cement-based mortar have been used as external confinement of concrete cylinders (Di Ludovico et al., 2010). The application required the patching of a layer of cement-based mortar with a thickness higher than 4 mm. Then the bidirectional basalt fibre grid was preimpregnated with epoxy resin or latex. Finally, a second layer of cement-based mortar was applied with a thickness higher than 4 mm. The experimental results showed a substantial gain in strength and ductility of columns and a gradual failure of the specimens. The results also revealed the need for higher mechanical interaction between the cementitious matrix and the grid.

In a later investigation, Al-Salloum et al. (2012) applied two different cementitious mortars reinforced with basalt textiles to increase the shear strength of reinforced concrete beams. The first mortar was a common cementitious one and the second polymer modified cementitious mortar. External shear strengthening of concrete members is extremely demanding, as it is a bond critical application, while the developed inclined tensile strains are very low prior to shear cracking formation. Therefore successful external strengthening requires a high bond between the mortar and concrete substrate, as well as a high interaction between the mortar matrix and basalt textile grid. The experimental results of the study were very promising, as the shear capacity of the beams was substantially raised. Retrofits with polymer-modified cementitious mortar matrix presented a higher shear strength increase.

Basalt fibre grids are widely applied in masonry structures. In Balsamo et al. (2014), basalt textile grids were embedded in lime-based mortar of very low strength and bonded externally on both sides of masonry panels. Masonry panels consisting of uncoursed masonry were representative of existing buildings in L'Aquila, Italy. Also, a yellow tuff masonry panel was included. The shear performance of the panels was remarkably enhanced. The failure mode of strengthened masonry panels was far more ductile. External seismic strengthening of masonry buildings with cementitious basalt grids may provide a reliable, efficient and sustainable alternative. Further analytical research is required to accurately quantify the enhancement of this technique with respect to the retrofit system, the reinforcement layout and the masonry type. Efficient analytical tools will allow for the retrofit of a large number of existing masonry structures with damages (environmental deterioration, inadequate construction techniques and materials, design for gravity loads only) or high seismic vulnerability. A basalt grid with an inorganic matrix provides high strength-to-weight ratio, low influence on global structural mass, corrosion and fatigue resistance, easy handling and installation and negligible architectural impact (Balsamo et al., 2014).

Basalt textile reinforcements are also included (together with carbon, glass and steel ones) in the study by Ascione et al. (2015). The authors proposed a method for the qualification of externally bonded FRCM systems, based on combined direct tension and shear bond tests. The maximum stress and mode of failure of the FRCM system are estimated with shear bond tests. Then the qualification strain is estimated more reliably with direct tension tests, given the failure stress (from shear bond tests). The study presents the characteristic three-stage behavior of FRCMs under tension, as well as their typical force-slip behavior for different modes of failures. The study considers cement or lime mortars as matrices and modern bricks or historic bricks or tuff units as substrates.

14.4.4 Effect of hybridization

Hybridization is defined as the mixing of two or more natural fibers or natural/synthetic fibers in intra-ply or inter-ply combination in a common matrix to tailor its mechanical properties, and to suit the need for structural applications [48,49]. The purpose of hybridization is to obtain a new type of material which can retain the advantages of its constituents [16].

3.2.2.4 Basalt Fibers

Similarly to glass fibers, basalt fibers are made by drawing a fiber from a molten material. However, in this case, the melt is 100% basalt with no further constituent ingredients or additives. Therefore, they are considered considerably more ecofriendly than carbon fibers. They have a long history of engineering application use since the 1920s. Basalt fibers are typically 9–13 μm and have a higher strength and stiffness than glass fibers and are not as expensive as carbon. They are mainly used in civil applications needing superior properties to glass but despite being not as widely used as either glass or carbon appear to offer considerable further multifunctional potential uses in conjunction with other materials.

10.4.2 Fire insulators

Because of their unique structure, based on quartz or basalt fibers in combination with non-decomposable binder for example, fibrous materials are capable of withstanding high heating temperatures for a long time (Zverev et al., 2008). Therefore, fibrous heat-resistant materials, especially their composites (Bai & Keller, 2007), are widely used in practice in various high-temperature technologies such as fire insulators in buildings.

Bai et al. (2007) developed models for assembling the material properties of fibrous composites, thermo-chemical and thermo-mechanical models to predict the thermal and mechanical responses of the composites under elevated and high temperatures. The post-fire mechanical properties of fibrous materials have also been evaluated widely (e.g. Pering et al., 1980; Springer, 1984). Haddad et al. (2008) evaluated the bond behavior between fiber-reinforced concrete and reinforcing steel rebars under elevated temperatures. Results showed that the use of fibers minimized the damage in the steel–concrete bond under elevated temperatures and hence the reduction in bond strength.

In high performance concrete, the addition of polypropylene fibers is widely used as an effective method to prevent explosive spalling and its efficiency was investigated by Liu et al. (2008).

Some experimental studies and results on the fire behavior of glass fiber-reinforced polymer load-carrying slabs for building applications were presented in a series of papers (Keller et al., 2005; 2006a; 2006b).

2.5.3 Effect of the fiber type

The same trend is observed irrespective of the fiber type (E-glass, basalt, and carbon fibers were investigated). The main effect of changing the fiber type is a change in the modulus, resulting from the intrinsic fiber properties, as highlighted with carbon fibers. Higher strength is also obtained with carbon fibers, but not in the same proportion as the intrinsic fibers properties. E-glass and basalt fibers lead to similar behaviors and mechanical properties of composites, with slightly better properties obtained with basalt fibers (refer to Balea et al., 2014 for detailed results).