Effects of Al/Na and heat treatment on the structure and properties of glass ceramics from molten blast furnace slag

Abstract Glass ceramics with different Al/Na molar ratio from blast furnace slag were prepared using conventional melting-casting method. The structure and properties of glasses or glass ceramics were investigated by DSC, Raman, MAS NMR, XRD, and SEM. The DSC results indicated that the thermal stability (ΔT = Tc-Tg) and crystallization temperature (Tc) of the parent glass firstly increased and then decreased when Al/Na exceeded 1.21. The Raman and 27Al MAS NMR spectra analysis revealed that [AlO6] increased positively with Al2O3/Na2O. The calculation of Qn ([SiO4] units with bridging oxygen atoms number of n) suggested an obvious decline of (Q0+Q2)/(Q1+Q3) and that [SiO4] mainly existed in the form of Q1 when Al/Na exceeded 1.21, which accorded closely with Tc variation. The crystallization results determined by XRD showed that as Al/Na increased, the main crystal phase was transformed from akermanite to gehlenite and nepheline disappeared. Glass ceramics with Al/Na of 1.48 nucleated at 780 °C for 2 h and crystallized at 880 °C for 3 h exhibited the maximum value of flexural strength. Orthogonal experiment (L9(34)) were carried out to investigated the optimum heat treatment of glass ceramics with a Al/Na of 1.48. The analyses indicated that nucleation time variation has little influence on the flexural strength, and the optimum heat treatment was determined as 760 °C – 1 h–900 °C – 1 h and the flexural strength was characterized as 81.310 MPa.