Alkali resistance enhancement of basalt fibers by hydrated zirconia films formed by the sol-gel process
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Basalt fibers were dip-coated in zirconium-n-propoxide, unstabilized or stabilized by chelation with ethyl acetoacetate. The thermal transformations of the hydrated zirconia coatings formed were investigated by dynamic x-ray diffraction and differential thermal analysis. The changes 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 after immersion in 0.1 M NaOH for different durations. It was found that the transition of the amorphous zirconia coating to the tetragonal crystalline phase is shifted to higher temperatures by chelation of the zirconium alkoxide. Alkali corrosion of the uncoated basalt fibers results in dissolution of the oxides of Si, Al, and Ca, and the formation of unsoluble hydroxides of Fe, Mg, and Ti from the chemical constituents of basalt. These reactions are suppressed by the protective zirconia coating on basalt fibers formed by the unstabilized zirconium alkoxide. However, the coating formed from zirconium propoxide stabilized by ethyl acetoacetate does not form an effective barrier against alkali attack since it is easily detached from the fiber surface during alkali immersion. The tensile strength of uncoated basalt fibers is drastically reduced by alkali attack. But the strength of zirconia-coated basalt fibers is maintained even after 90 days of alkali immersion. The vastly improved alkaline durability of the coated fibers shows the potential of zirconia-coated basalt fibers for cement reinforcement.
I. INTRODUCTION Basalt fiber has the strength and stiffness needed for application as a reinforcing material for polymers.1^1 In a series of investigations from this laboratory it was shown that continuous fibers drawn from the naturally occurring basaltic lava of this region possess a Young's modulus ranging from 80-90 GPa, depending upon drawing conditions. The tensile strength of pristine basalt fibers, about 3.0 GPa, can be further improved by appropriate modification of the fiberizing process5 or chemical composition.6 Both strength measurements and surface morphology studies by scanning electron microscopy (SEM) have indicated considerable resistance of basalt fibers against alkali attack and, thereby, their potential for application as reinforcement in cement matrix composites.7-8 However, a detailed study of the durability of basaltcement composites9 revealed the poor retention of fiber strength in the alkaline matrix and, consequently, considerable decrease in composite strength also. In this respect, basalt fibers are similar to E-glass fibers.
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J. Mater. Res., Vol. 9, No. 4, Apr 1994 Downloaded: 11 Mar 2015
The unsatisfactory performance of E-glass fibers as reinforcement in cement is well established by the work of Majumdar and Nurse10 and Proctor and Yale.11 In order to overcome this disadvantage, zirconia is incorporated into the gla
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