ALKALI RESISTANCE ENHANCEMENT OF BASALT FIBERS BY HYDRATED ZIRCONIA FILMS FORMED BY THE SOL-GEL PROCESS

Basalt fibers were dip-coated in zirconiuM-n-propoxide, unstabilized o r stabilized by chelation with ethyl acetoacetate. The thermal transfo rmations of the hydrated zirconia coatings formed were investigated by dynamic x-ray diffraction and differential thermal analysis. The chan ges in the surface chemical compositions of coated and uncoated fibers , following alkali immersion extending to 90 days, were characterized by EDXA and IR spectral analysis. Fiber strengths were also measured a fter immersion in 0.1 M NaOH for different durations. It was found tha t the transition of the amorphous zirconia coating to the tetragonal c rystalline phase is shifted to higher temperatures by chelation of the zirconium alkoxide. Alkali corrosion of the uncoated basalt fibers re sults in dissolution of the oxides of Si, Al, and Ca, and the formatio n of unsoluble hydroxides of Fe, Mg, and Ti from the chemical constitu ents of basalt. These reactions are suppressed by the protective zirco nia coating on basalt fibers formed by the unstabilized zirconium alko xide. However, the coating formed from zirconium propoxide stabilized by ethyl acetoacetate does not form an effective barrier against alkal i attack since it is easily detached from the fiber surface during alk ali immersion. The tensile strength of uncoated basalt fibers is drast ically reduced by alkali attack. But the strength of zirconia-coated b asalt fibers is maintained even after 90 days of alkali immersion. The vastly improved alkaline durability of the coated fibers shows the po tential of zirconia-coated basalt fibers for cement reinforcement.