# Chemical Regeneration of Thermally Conditioned Basalt Fibres
## 3 Results and Discussion
Comparing SEM micrographs of the lateral surfaces of the as-received, heat-treated and regenerated basalt fibres (Figure 6), the ability of the two chemical treatments to remove the surface defects and homogenize the fibre surface can be confirmed. In Figure 6a, it is possible to notice how the polymer sizing coating ensures the uniformity of the untreated basalt fibre, offering adequate protection to the fibre. The heat treatment damaged the silane sizing, thus exposing the fibre superficial defects and decreasing the mechanical strength (Figure 6b). As can be clearly seen in Figure 6c,d, the chemical treatments smoothened the fibre surface due to the etching reaction promoted by the corrosion occurring in solution. Figure 5: Weibull plots for the tensile strength of basalt fibres after HF treatment as a function of thermal treatment temperature: (**a**) 400 \({}^{\circ}\)C, (**b**) 500 \({}^{\circ}\)C, (**c**) 600 \({}^{\circ}\)C.  
The reduction of surface flaw seems to be the controlling mechanism of the regeneration of the basalt fibre tensile strength, but it cannot be ruled out that structural modifications of the bulk fibre might control the mechanical behaviour. To this purpose, fracture properties of regenerated basalt fibres were analysed, by introducing artificial defects on the regenerated basalt fibres using the focused ion beam milling technique [33] (Figure 7). After tensile tests on single notched fibres, the fracture toughness values were evaluated assuming a linear elastic fracture mechanics (LEFM) [34]. Later, the introduction of a straight-fronted edge crack in a cylinder subjected to uniaxial tension along its axis was confirmed by SEM analysis of the regenerated fibre fracture surface (Figure 8). The fracture properties of chemically-treated fibres are expressed in terms of mode I fracture toughness (\(K_{lc}\)) [35], according to the Equation (5):  
\[K_{lc}=Y[a/D]\cdot\sigma_{f}\sqrt{\pi a}, \tag{5}\]  
where \"\(a\)\" represents the notch depth, \"\(Y\)\" is the stress intensity factor (SIF) or geometric factor, \"\(D\)\" is the fibre diameter and \"\(\sigma_{f}\)\" is the fibre tensile strength. The SIF is calculated for each fibre using the [PERSON]'s equation (Equation (6)), as already done for carbon and basalt fibres [14; 21]. This equation  
Figure 6: SEM micrographs of (**a**) as-received basalt fibres, (**b**) heat-treated at 600 \({}^{\circ}\)C, (**c**) heat treated at 600 \({}^{\circ}\)C + chemically treated in 3 M NaOH solution for 10 min, (**d**) heat treated at 600 \({}^{\circ}\)C + chemically treated in HF solution for 2.5 min. Figure 1:0 SEM micrographs of artificially notched basalt fibre. was obtained by finite element method in a previous work for surface cracks in round bars subjected to tension loading [36]:  
Figure 1:1  
where \(\xi=~{}a/D\) depends on the ratio \"notch depth/ fibre diameter\" (_a/D_). Table 3 shows the fracture properties of the basalt fibres regenerated by NaOH chemical treatments at different concentration and by chemical treatments in HF acid solution. The tensile tests on the single notched fibres were carried out on chemically treated fibres previously conditioned at 600 \({}^{\circ}\)C. This temperature was chosen to highlight any structural changes in the bulk fibre during the recycling treatment [37]. For the chemical solution able to guarantee the highest regenerative ability, i.e., the most aggressive solution (NaOH 3 M-10 min), it was possible to obtain a fracture toughness (\(K_{lc}\)) value of \(2.53\pm 0.59\) MPa\(\sqrt{\text{m}}\). On the contrary, in the case of HF treatment, the \(K_{lc}\) obtained is even higher, equal to \(2.88\pm 0.53\) MPa\(\sqrt{\text{m}}\), although the treatment in acid solution on un-notched fibres offered a lower regeneration effect.