In this study, pure Al2O3-2SiO2 powders for a geopolymer were prepared by a sol-gel method, alkali-activation tests and alkali-dissolvability of the powders were carried out, and structure of the powders and alkali-activated products was investigated by 29Si and 27Al MAS NMR and SEM. Results showed that higher alkali-activation reactivity (higher compressive strengths of alkali-activated products) appeared in the powders heat-treated between 600-800 °C and alkali-dissolvability trend was different from that of alkali-activation tests. There were clear correlations between microstructure of alkali-activated products and alkali-activation reactivity and Al environment of the powders. It was found that high strength was related to a dense, fine grained microstructure. Such a structure was found in the alkali-activated products synthesized with the powders with high 5-coordinated Al contents. In addition, the peaks attributed to 5-coordinated Al were strengthened with the rise of heat-treated temperature of the powders.