Silicate liquid phases dominate the primary particle bonding or sintering mechanisms during boiler slagging. Clay minerals, classified as phyllosilicates in the silicate system, are the main inorganic matter in coal, with the aluminosilicates forming from them in coal ash. The high NBO/T (the ratio of nonbridge oxygens to tetrahedrally coordinated network formers) matter, rich with network modifiers resulting from the further reactions of dehydroxylated kaolinite and illite at high temperatures, is of special interest because they cause slagging. In this study, fly ash samples were prepared from two coals in a drop tube furnace system (referred as DTF ash samples) at 1050,1150, and 1250 degrees C and then chemically separated using hydrochloric acid. The products of the DTF ash samples before and after chemical separation were analyzed by elemental and phase analyses to characterize the high NBO/T matter in the ash samples and to investigate the relationship between their formation and the combustion temperatures. The results show that the dissolved aluminum in the boiling acid separation could represent the soluble substances of the clay mineral reaction products. The aluminum dissolved share of 1250 degrees C DTF ash had a positive correlation with the content of the high NBO/T matter generated from the clay minerals, which could be used to develop a method to estimate the slagging characteristics of coals based on the silicate sintering mechanisms.