When stone wool fibres (SWFs, iron-rich aluminosilicate glasses) are heat treated near the glass transition temperature, tg, in air, ferrous iron is oxidised to ferric iron. The oxidation process induces the diffusion of network modifying cations (primarily Mg2+) from the interior towards the surface, and consequently, an MgO enriched surface layer forms. In earlier studies, it has been found that the high temperature staiblity (HTS) of SWFs, i.e. the ability of SWFs to maintain their geometry at high temperature, is improved as a result of oxidation. However, the chemical origin of the improved HTS is not fully clear, since several chemical processes occur during the heat treatment around Tb. To address this question, the surface layer was deliberately removed by treating the SWFs in an acidic solution in order to clarify the impact of this layer on the HTS of the SWFs. During this treatment, Mg2+ ions from the SWFs leach into the solution. The dissolution occurs in two diffusion-controlled regimes: an outer layer with relatively fast diffusio and the next layer with relatively slow diffusion. Based on the dissolution experiments and HTS tests, it is inferred tha the MgO-rich layer is responsible for the improved HTS and that the layer benefits the chemical durability of the SWFs.
