# Chemical Regeneration of Thermally Conditioned Basalt Fibres
## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c} \hline
**Heat Treatment Temperature (\({}^{\circ}\)C)** & **Chemical Treatment with NaOH** & **Tensile Strength (MPa)** & **Weibull Modulus—\(\alpha_{\sigma}\)** & **Scale Parameter—\(\sigma_{0}\) (MPa)** \\ \hline R.T. & - & 2514.85 (276.67) & 10.85 & 2634.19 \\ \hline \multirow{4}{*}{400} & - & 1304.77 (304.96) & 4.82 & 1425.17 \\  & 1.5 M-5 min & 17539.89 (394.36) & 5.25 & 1905.02 \\  & 1.5 M-10 min & 1653.53 (189.30) & 10.24 & 1736.76 \\  & 3 M-5 min & 1694.83 (356.98) & 5.53 & 1837.44 \\  & 3 M-10 min & 2366.63 (327.84) & 8.16 & 2511.32 \\ \hline \multirow{4}{*}{500} & - & 1056.67 (305.80) & 4.16 & 1162.30 \\  & 1.5 M-5 min & 1291.53 (299.30) & 5.31 & 1402.48 \\ \cline{1-1}  & 1.5 M-10 min & 1327.48 (214.57) & 7.20 & 1417.86 \\ \cline{1-1}  & 3 M-5 min & 1183.72 (192.49) & 7.33 & 1263.30 \\ \cline{1-1}  & 3 M-10 min & 1414.40 (199.43) & 8.63 & 1496.41 \\ \hline \multirow{4}{*}{600} & - & 663.96 (166.22) & 4.63 & 726.29 \\  & 1.5 M-5 min & 882.16 (242.65) & 4.34 & 939.16 \\ \cline{1-1}  & 1.5 M-10 min & 838.42 (182.42) & 5.61 & 908.06 \\ \cline{1-1}  & 3 M-5 min & 739.90 (163.08) & 5.48 & 803.56 \\ \cline{1-1}  & 3 M-10 min & 1122.05 (315.50) & 4.10 & 1236.64 \\ \hline \end{tabular}
\end{table} 
Table 1: Average tensile strength for regenerated basalt fibres after NaOH treatment and Weibull parameters of the strength distribution (standard deviation in parentheses). \begin{table}
\begin{tabular}{c c c c c} \hline
**Heat Treatment Temperature (\({}^{\circ}\)C)** & **Chemical Treatment with HF** & **Tensile Strength (MPa)** & **Weibull Modulus—\(\alpha_{\sigma}\)** & **Scale Parameter—\(\sigma_{0}\) (MPa)** \\ \hline R.T. & - & 2514.85 (276.67) & 10.85 & 2634.19 \\ \hline \multirow{4}{*}{400} & - & 1304.77 (304.96) & 4.82 & 1425.17 \\  & 30 s & 1789.97 (289.39) & 7.31 & 1909.11 \\  & 60 s & 1630.58 (271.17) & 7.01 & 1743.09 \\  & 150 s & 1571.87 (281.29) & 6.66 & 1685.90 \\ \hline \multirow{4}{*}{500} & - & 1056.67 (305.80) & 4.16 & 1162.30 \\  & 30 s & 1279.32 (283.81) & 5.43 & 1387.58 \\ \cline{1-1}  & 60 s & 1351.33 (278.03) & 5.97 & 1457.70 \\ \cline{1-1}  & 150 s & 1260.74 (276.38) & 5.55 & 1365.60 \\ \hline \multirow{4}{*}{600} & - & 663.96 (166.22) & 4.63 & 726.29 \\ \cline{1-1}  & 30 s & 892.49 (177.62) & 5.96 & 962.34 \\ \cline{1-1}  & 60 s & 1073.20 (209.79) & 5.77 & 1159.87 \\ \cline{1-1}  & 150 s & 1098.44 (189.66) & 6.81 & 1175.79 \\ \hline \end{tabular}
\end{table} 
Table 2: Average tensile strength for regenerated basalt fibres after HF treatment and Weibull parameters of the strength distribution (standard deviation in parentheses). Figure 2 shows the regenerative ability of HF treatments as a function of treatment duration. Tensile tests confirmed a general improvement of the fibre strength properties after HF treatment, but the plot underlines a strength reduction with increasing chemical treatment time, limiting the recovery to a maximum of \(\sim\)70%. Thus, the hydrofluoric acid solution allows a partial strength recovery for the basalt fibres, offering smaller improvements compared to the treatments in NaOH solution. This result points towards a more environmentally friendly regeneration process, excluding the issues related to the use and disposal of harmful HF. Microscopic analysis of the fracture surfaces of tested fibres has already demonstrated that surface flaws are the responsible for the failure of fibres during tensile tests [10; 14]. Additionally, in this case, the fracture mechanism was not changed by the thermal recycling process and the nature of the flaws was not altered by the high temperature. Considering the fracture surface of the regenerated fibres, the SEM analysis of the cross-section area confirmed the presence of three distinct zones, i.e., mirror, mist and hackle, typical of the fracture mechanism of brittle materials (Figure 3). This behaviour confirms that the failure of regenerated fibres hinges on the presence of superficial flaws even after the chemical treatment.