# Chemical Regeneration of Thermally Conditioned Basalt Fibres
## 3 Results and Discussion
The Weibull distribution of strength for the treatments in alkaline solutions (Figure 4) and for the treatments in acid solutions (Figure 5) underlined a linear trend, thus validating the occurrence of a single population of defects as already shown by untreated fibres [10; 14]. The homogenization of the basalt fibre surfaces could be responsible for the regeneration  
Figure 1: Percentage of strength retention for basalt fibres after NaOH treatment as a function of pre-treatment temperature. Figure 2: Percentage of strength retention for basalt fibres after HF treatment as a function of treatment time. of the strength lost during the heat treatment. In fact, while the thermal recycling process increased the surface defects size and caused the exposure of the fibre flaws because of the sizing degradation, the subsequent chemical treatment generated a dissolution reaction able to smooth the fibres surface, thus removing the superficial defects. Studies related to the glass fibres strength loss as a result of corrosion phenomena by immersion in an alkaline environment can be extended to basalt fibres. In particular, the corrosion in NaOH solution is controlled by the dissolution of the silica network (SiO\({}_{2}\)), which is the main component of glass and basalt fibres [30]. The hydroxyl ion OH\({}^{-}\) in aqueous solution is able to break the siloxane bonds network, generating the migration of the silicates in solution according to the following reaction [31]:  
\[[-\text{Si}-\text{O}-\text{Si}-]+\text{OH}^{-}\rightarrow[-\text{Si}-\text{ OH}]+[-\text{SiO}]^{-}, \tag{3}\]  
Figure 4: Weibull plots for the tensile strength of basalt fibres after NaOH treatment as a function of thermal treatment temperature: (a) 400 \({}^{\circ}\)C, (b) 500 \({}^{\circ}\)C, (c) 600 \({}^{\circ}\)C.  
Figure 3: SEM micrograph of fracture surface of a basalt fibre heat treated at 500 \({}^{\circ}\)C and in a 3 M NaOH solution for 10 min. This etching reaction showed negative effects on the as-received fibres [32], while in the case of thermally conditioned fibres, it has positive results by removing surface defects and allowing the recovery of the mechanical strength lost during the heat treatment. In fact, the detrimental effect of alkali treatments on glass fibres is always presented in an attempt to maintain the performance of strong (virgin) fibres for long contact times, while in the present case the effects of shorter alkali treatments are discussed with a view to changing the strength of very weak fibres, as those thermally conditioned, with a limited dissolution of the glass network. With regard to the chemical regeneration with HF solution, recent studies on glass fibres highlighted the ability of the hydrofluoric acid solution to remove the damaged surface layer of the fibre previously heat-treated [15]. During this etching reaction, the controlling mechanism is the breakage of the siloxane bonds in the silica network due to the surface adsorption of undissociated HF molecules and \(\text{HF}_{2}\)- ions and the catalytic action of \(\text{H}^{+}\) ions [20]. The reaction (4) expresses the heterogeneous dissolution of the silica network that occurs as a result of the reaction with HF:  
\[\text{SiO}_{2}+6\text{HF}\rightarrow\text{H}_{2}\text{SiF}_{6}+2\text{H}_{2} \text{O}, \tag{4}\]  
The dissolution reaction is controlled by the adsorption and the chemisorption kinetics of the reactive species. The etching process not only removes material from the fibre surface, but it is also able to modify the morphology of the surface itself. As already shown for glass fibres [20], the aqueous HF solution transforms the surface with closed flaws into a surface with cusp-like structures. In the present paper, it is supposed that the corrosion produced by HF solution on heat-treated basalt fibres strengthens fibres due to the removal of the damage surface layers.