My studies on clays and clay minerals in the Clay Science Society of Japan(CSSJ)were summarized in this presentation. My activity in the CSSJ started at the 14th Annual Meeting and the topic was structure analysis of allophane by X-ray radial distribution analysis. Since then my main topics shifted gradually from allophane to halloysite and further to kaolinite. In my study on halloysite, formation of clay minerals from volcanic ash by weathering, especially in the Kanto area, was focused on the viewpoint of their chemical composition and clay mineral changes. On kaolinite, thermal phase change of kaolin group minerals was investigated and a new model for the phase change different from the Brindley and Nakahira model was proposed. During this study, a good hint to develop porous materials was obtained. Microporous silica was able to prepare by selective leaching of metakaolinite using an acid solution and was very good starting material to synthesize zeolites with wide range of Al/Si ratios and mesoporous silica. The mesoporous silica synthesized using this silica source showed very high specific surface area owing to the unique character of the starting material. Mesoporous γ-alumina was possible to prepare by the selective leaching method using an alkali solution for kaolinite heated at about 1000℃ , which had a nanotexture consisted of amorphous silica matrix with dispersing nano γ-alumina grains formed by phase separation and partial crystallization. The resulting γ-alumina has a uniform size of mesopores in several nm by a unique microstructure consisted of fine γ-alumina grains several nm in size connected by amorphous silica pillars with maintaining the original platy particle shape. Due to this unique microstructure, the resulting γ-alumina showed excellent thermal stability and humidity control ability.

Recycling of coal fly ash is an emergency issue to be addressed by the international community. The coal fly ash is mainly composed of anhydrous oxides of Si and/or Al, and contains much Si-O-Si and Si-O-Al bondings in its structure. We treated the coal fly ash with up to 0.5 M LiOH, NaOH and KOH aqueous solutions at up to 573 K within 3 h to generate hydroxyl groups on the surface of the ash. Cation exchange capacity values of the products measured at equilibrium pH of 10 tended to increase with increasing alkali concentration and temperature of the treatment. Weight losses of the products between 473 K and 773 K measured by thermal analyses were more than 1 %, while that of the untreated coal fly ash was almost zero. These observations indicated that hydroxyl groups, presumably Si-OH, were formed by the sub-critical water treatments. In case of the treatments with NaOH, zeolite species, Na-P1 and analcime, were formed at higher NaOH concentrations and temperatures. When LiOH and KOH were used, no crystals were formed, however by scanning electron micrograph, amorphous materials were observed on the surface of the coal fly ash particles. The amorphous materials were suggested to be formed with dissolution of Si and Al from the coal fly ash, followed by precipitation composed of Si, Al and alkali metal cations on the surface of the coal fly ash particles. In conclusion, the treatment with sub-critical water can generate hydroxyl groups on the surface of coal fly ash, and the treated coal fly ash is possibly available as adsorbents with cation exchange ability.