Determining the glass content in fly ash is crucial for characterizing its compositional properties (i.e., glass content, crystal types and quantities, and glass phase composition) and serves as a fundamental parameter for regulating and optimizing its engineering performance (i.e., fresh workability, volume stability, hydration process and products, anti-permeability, and mechanical performance). The glass content of four low-calcium and four high-calcium fly ash samples can be quantitatively analyzed by three methods, i.e., the alkaline-acid two-step dissolution method (NaOH followed by HNO₃), the acid dissolution method (HF), and quantitative X-ray diffraction (QXRD) method. A comparison of the three methods demonstrates that QXRD offers superior testing precision and characterizes the full components of fly ash, unlike the two dissolution methods. Although the two dissolution methods can provide a rough estimation of glass content, their precision is relatively poor. This low precision is attributed to the dissolution of crystalline phases (like quartz and mullite), the formation of new crystalline phases (such as hydromagnesite and tobermorite) during the dissolution process, and the residual undissolved glass phases. Consequently, QXRD can be recommended for the quantitative analysis of fly ash glass content.

The glass content of eight different fly ash samples was measured by three characterization methods, i.e., the alkaline-acid two-step dissolution method (NaOH followed by HNO₃), the acid dissolution method (HF), and quantitative X-ray diffraction (QXRD). A comparison of the results revealed a significant discrepancy between these three testing methods, highlighting a need for further research. To investigate the dissolution power of the NaOH and HNO₃ solutions, two additional methods (i.e., the alkaline dissolution method (NaOH) and the acid dissolution method (HNO₃) were adopted. The respective dissolving capabilities toward both the crystalline and glassy phases of the eight fly ash samples were ultimately disclosed via using QXRD to analyze the dissolving effect of these four dissolution methods.

The dissolving characteristics of the four dissolution methods (i.e., alkaline, HF acid, HNO₃ acid, and two-step) on the crystalline and glass phases of fly ash are analyzed by linear regression and the Pearson correlation method, based on the measured glass and crystal contents remaining after each treatment. The linear regression between the crystal content of the raw fly ash and the residual crystal content after dissolution demonstrates that the crystalline phases of fly ash are dissolvable by both alkaline and/or acid solutions. Furthermore, the results confirm that glass phase residue remains after all alkaline and/or acid treatments, indicating that none of the dissolution methods can achieve a precise selective dissolution of the fly ash's glassy phase. The Pearson correlation method is employed to investigate the dissolving tendency of the crystalline and glass phases in the different solutions. The resulting fly ash component-Pearson correlation coefficient diagram effectively reveals the dissolution characteristics of fly ash components in various alkaline and/or acid solution, thereby clearly explaining the underlying dissolution mechanism and demonstrating the infeasibility of using these alkaline and/or acid dissolution methods for accurately determining the fly ash's glass content.

The quantitative analysis of the glass content in four low-calcium and four high-calcium fly ash samples was conducted by three methods, i.e., the alkaline-acid two-step dissolution method (NaOH followed by HNO₃), the acid dissolution method (HF), and quantitative X-ray diffraction (QXRD). For the three methods, QXRD demonstrated superior precision and comprehensiveness, determining the glass content and the types and quantities of crystalline phases, whereas the two dissolution methods showed a lower precision. The QXRD analysis of the dissolution residues revealed that using alkaline (NaOH) and/or acid (HNO₃, HF) solutions failed to achieve an ideal selective dissolution of the fly ash's glass and crystalline phases. Furthermore, new crystalline phases were sometimes generated during the dissolution process, fundamentally limiting the accuracy of using these dissolution methods for quantitative analysis of glass content. Based on the results of QXRD tests on both the raw fly ash and the alkaline/acid dissolution residues, the error sources were associated with using alkaline and/or acid dissolution methods for determining fly ash glass content. The application of the Pearson correlation method and the resulting interpretation of the reaction mechanisms and products between fly ash components and the NaOH, HNO₃, and HF solutions mechanistically revealed the dissolution process and mechanism of fly ash, thereby fundamentally explaining the infeasibility of accurately quantifying fly ash glass content by alkaline and/or acid dissolution methods.