# Chemical Regeneration of Thermally Conditioned Basalt Fibres
## 4 Conclusions  
In the present work, it has been established that both chemical treatments in alkaline (NaOH) and acid solution (HF) are able to recover the mechanical strength lost during the thermal recycling process. In particular, the 10 min chemical treatment with a 3 molar solution of NaOH was able to almost  
Figure 8: SEM micrograph of the cross-section of an artificially notched basalt fibre after tensile test (white arrows indicate the location of the artificial notch).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Temperature** & **Chemical Treatment** & **a/D** & **SIF–Y** & **Residual Strength** & **Facture Toughness** & **Reference** \\ (C) & & & & **(MPa/m)** & **(MPa/m)** & **Reference** \\ \hline R.T. & - & 0.15 (0.06) & 1.28 (0.16) & 420.85 (195.93) & 1.17 (0.45) & [14] \\
600 & - & 0.13 (0.05) & 1.30 (0.17) & 569.88 (168.08) & 1.62 (0.46) & [14] \\
600 & NaOH 15 M 5 min & 0.10 (0.04) & 1.26 (0.03) & 893.86 (166.26) & 2.14 (0.57) & This study \\
600 & NaOH 3 M 10 min & 0.13 (0.04) & 1.24 (0.02) & 984.89 (295.63) & 2.53 (0.59) & This study \\
600 & HF 1 \(\%\)\(\approx\) 2.5 min & 0.16 (0.03) & 1.24 (0.02) & 991.16 (315.94) & 2.88 (0.53) & This study \\ \hline \hline \end{tabular}
\end{table}
Table 3: Fracture characteristic values obtained after SENT tests of untreated, thermally conditioned and regenerated basalt fibres. The values include fibre notch depth/diameter ratio (_a/D_), geometric factor (_Y_), residual strength (_c/d_), fracture toughness (\(K_{lc}\)), expressed as average value of the investigated property (standard deviation).  
completely regenerate the strength of heat-treated basalt fibres at 400 \({}^{\circ}\)C for 25 min, featuring a strength recovery of 94%, while this ability decreased with increasing exposure temperature, reaching a value of 45% for a treatment performed at 600 \({}^{\circ}\)C for 25 min. Alkaline solution proved to be considerably more effective than the acid solution, thus eliminating problems in terms of safety and toxicity related to hydrofluoric acid, especially after treatments at 400 and 500 \({}^{\circ}\)C. From morphological investigations, it was noted that the chemical treatments promoted an etching reaction on the fibre surface, reducing the exposed flaws after the sizing degradation during the heat treatment. Data obtained from the tensile tests on single notched fibres showed a significant increase in the fracture toughness for the regenerated fibres, in comparison with as-received and heat-treated basalt fibres. Values of 2.53 and 2.88 MPa\(\cdot\)m\({}^{1/2}\) were obtained for fibres exposed to 600 \({}^{\circ}\)C and subjected to treatments in NaOH 3 M for 10 min and HF 1v% for 2.5 min, respectively, compared to 1.17 MPa\(\cdot\)m\({}^{1/2}\) for neat basalt fibres. This increase confirms that the observed strength regeneration is due to superficial and bulk structural re-organizations, which deserve specific investigations.  
Conceptualization, F.S., J.T., and C.G.; methodology, F.S., J.T., C.G. and C.S.L.; validation, F.S., J.T., C.G. and C.S.L.; investigation, M.L., L.D.F., and A.F.; resources, F.S., J.T., C.G. and C.S.L.; data curation, J.T., M.L., L.D.F., and A.F.; writing--original draft preparation, F.S., M.L., and L.D.F.; writing--review and editing, J.T., A.F., C.G. and C.S.L.; visualization, M.L. and L.D.F; supervision, F.S., J.T., C.G. and C.S.L. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors declare no conflict of interest.