Alkali-resistant oxynitride glasses
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T. El-Korchi Department of Civil Engineering, Worcester Polytechnic Institute, Worcester, Massachusetts 01609 (Received 7 June 1990; accepted 4 October 1990)
Oxynitride glasses in the Si-Zr-Na-Li-K-B-O-N system have been produced via incorporation of Si3N4 into the glass structure. This system is the oxynitride analogue of commercially available alkali resistant (AR) glasses used for concrete reinforcement. Glasses with nitrogen contents up to approximately 4 at. % have been obtained. Hardness, fracture toughness, and chemical durability were found to increase with increasing nitrogen content. Fibers were drawn from the glasses containing approximately 4 at. % nitrogen and used to produce reinforced cement composites. The microstructure of the fiber-matrix interface was examined in these samples after aging and compared to that of oxide AR glass fiber-matrix interface. Measurement of the wetting behavior of aqueous solution as a function of the nitrogen content of the glass suggests that this difference in microstructure is the result of changes in physiochemical properties of the glass surface due to the incorporation of nitrogen into the glass structure.
I. INTRODUCTION
H 2 SiO 3 = 2H + (aq)
Alkali-resistant (AR) glass fibers are being used as reinforcement in portland cement concretes to improve their tensile strength, flexural strength, and strain to failure. There exists, however, one major drawback hindering the use of glass fiber reinforced cement (GFRC) composites as load bearing materials. Aging results in embrittlement and loss of tensile strength, especially in a continuously wet environment.1"3 Degradation of the mechanical properties of GFRC composites over time appears to be due to a combination of the development of excessive fiber-matrix bond strength (a consequence of the formation of cement hydration products) and glass weakening due to mild chemical attack. This condition may cause the fiber to fracture prior to pullout, causing a decrease in both composite strength and strain to failure. Chemical attack of glass fibers is caused by hydration and dissolution reactions resulting in the formation of an H2SiO3 gel coating on the glass fiber. This process is represented by Eqs. (1) and (2). At room temperature and pH > 9, a rapid dissolution of salic acid occurs and thus drives the chemical attack of the glass network structure.4 This is a serious concern for GFRC applications where the hydration of cement produces a pore solution pH between 12 and 14.
C?298 = -36.50 kcal/mole
SiO2(gl) + H2O(1) = H2SiO3 G298 = -1.21 kcal/mole log Kx = log a(H2SiO3) 152
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SiO3"2
log K2 = log a(SiO3-2) - log a(HSiO3-) - pH (2) AR glasses containing ZrO2 have been shown to have greatly improved chemical durability over E-glass. As presented in Table I, zirconia silicates are more thermodynamically stable in aqueous solutions, making it more difficult for alkali attack of the glass network. Although an improvement over E-glass, the use of high zirconia AR glass has not eliminated the
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