Variation on the morphology and tensile strength of basalt fiber processed in alkali solutions

This paper presented the effect of concentration, temperature, and immersing time of alkali solutions (NaOH, KOH, Ca(OH)2) on the morphology and mechanical property of basalt fiber (BF). Various techniques, including scanning electron microscopy, tensile test of single filament, infrared and Raman spectroscopy, were employed to characterize the fiber sample before and after treatment. The results showed that the surface of BF underwent changes from smooth surface to the rough one with deposited particles, and then the presence of shell with plate-like structures as the alkali condition became harsh. The tensile strength of BF followed a downward trend with the aggravation of the degradation. Unexpectedly, the strength of BF increased by more than 30% after KOH treatment due to the ion exchange between the Na+ in fiber and K+ in alkaline solution. In general, the degradation of BF in the alkali solution involved the breaking of Si-O-Si bond under OH− attack, and this mechanism was partly confirmed by the variation on the chemical composition of BF as characterized by Raman technique. In NaOH and KOH solutions, the in-situ formed outer shell and spherical particles on the BF surface were insoluble metal hydroxides (Fe and Mg) and silicates (Ca), and CaCO3 cluster was noticed on the fiber surface when BF was treated in Ca(OH)2 solution.