# Chemical Regeneration of Thermally Conditioned Basalt Fibres  
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Footnote 1: email: {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 10, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 51, 52, 53, 54, 55, 56, 57, 58, 59, 61, 62, 63, 64, 65, 66, 67, 68, 69, 71, 72, 73, 74, 75, 76, 77, 78, 7 been focusing on alternatives to the most common synthetic fibres. In this perspective, the use of fibres of mineral origin, such as basalt, is a plausible solution for the reduction of the environmental impact of composite materials [3,4]. Due to its lower environmental impact, its higher chemical stability and similar mechanical properties, basalt fibre has proven to be a viable alternative to glass fibres as a reinforcement in composite materials [5]. Unfortunately, similar to the case of composite materials reinforced with glass fibres, composite materials with basalt fibres show a drastic decrease in mechanical properties after the typical thermal recycling process [6, 7, 8, 9]. In a previous work, basalt fibres heat treated at 600 \({}^{\circ}\)C for 1 h in air showed a decrease of \(\sim\)75% of the original tensile strength [10]. Several mechanisms, classified as superficial and structural phenomena, have been proposed as responsible for the loss of fibre strength [11, 12, 13], even though this is still an open issue in the literature. [PERSON] et al. [14] have recently shown by single edge notch tension and nanoindentation micro-pillar splitting methods that microstructural changes occur in the fibres after thermal exposure. [PERSON] and co-workers [15,16] demonstrated that the tensile strength of heat-treated glass fibres can be significantly recovered by a post treatment in an acid or an alkaline solution. [PERSON] et al. [15], in a pioneering work on strength regeneration of glass fibres, investigated a treatment based on 1 v% HF aqueous solution and reported almost a triple increase in the strength of degraded fibres after a 3 min treatment. This approach was also effective at the composite scale, for a chopped strand mat in an epoxy matrix. Later, [PERSON] et al. [16] proposed a more environmentally friendly chemical treatment based on a 3 M NaOH solution for 10 min at 90 \({}^{\circ}\)C compared to hydrofluoric acid. The results of single fibre tensile testing displayed a maximum recovery of 75%. [PERSON] et al. [17] carried out a detailed study in order to assess the effectiveness of other alkaline treatments (KOH and LiOH) in restoring the tensile strength of thermally degraded glass fibres. Results suggested that LiOH was unable to regenerate the strength while KOH proved to be a milder glass fibre etchant compared to NaOH, producing improvements in fibre strength only at higher molarity and longer treatment times. These treatments allow to obtain a closed-loop recycling process [15], resulting in a considerable reduction of the CO\({}_{2}\) produced [18]. These studies on chemical treatments of glass fibres [17,19] demonstrated that the strength regeneration is governed by a dissolution reaction that occurs during the immersion of the fibres in the corrosive solution. In this regard, a controlled dissolution in aqueous HF-solution can be performed to remove the superficial layers of silicate glass [20]. This chemical reaction can not only remove the already damaged surface through a glass dissolution but also modify the pre-existing defects on the glass fibre surface.