Geopolymer is a generic name of the substance that is generated by kneading enough fly ash or meta-kaolin (processed kaolin by calcining and dehydrating) including aluminum silicate with amorphous phase and alkali water solution. Concretely speaking, the substance is made of aqueous solution of sodium hydroxide or potassium hydroxide and water glass (sodium and potassium system). It is casted by pouring the kneaded slurry in a mold-frame, in generally cures for a few days in around 80% of high relative humidity, and becomes to the hardened body. Whereas the hardening mechanism of usual known cement is depending on a hydration reaction, in the case of geopolymer it is said that the hardening is caused by the dehydration condensation reaction, namely, by polymerization reaction of four-coordinated silicon (silanol group) combined with a hydroxyl group. In Japan, it is often called by another name such as alkali activity cement or alkali stimulation hardening body. The term, geopolymer was named by a French scientist, Davidovits, and is a coined word created from the concepts that the formation of sedimentary rock of the surface of the earth might come from a geopolymer and that the chain reaction of inorganic polymer structure might be occurred on the basis of SiO₄ unit. A conceptual diagram of a simple formation mechanism of geopolymer is shown in Fig. 1. Institute of Geopolymer hosted by Davidovits is still continuing now and the books published by it are very interesting. Recently, a hypothesis was introduced in commercial broadcasting TV programs and it is attracting attention from an ordinary citizen very much. The hypothesis is that the huge stone building, Egyptian pyramid may be constructed using artificial stones made of the ancient cement including this geopolymer. The mechanical strength of geopolymer is approximately equal to the one of cement or more and it is said that the service life (time-related deterioration) of it is longer than the one of cement (the service life of the current OPC structure is said 30 to 50 years, while the service life of geopolymer calculated in the same weather condition with the natural sedimentary rock is said to be 10,000 years). Therefore it attracts attention as structural materials substituting for cement. In Europe and Australia, the fundamental researches are led and are performed, and geopolymer attracts attention also as a stabilized shielding material of the radioactive wastes. Geopolymer is presumed, as explained in the above paragraph, that some Si⁴⁺ of the connected ring structure of [SiO₄] are substituted for Al³⁺, and alkali ions such as Na⁺ are made come near for the electric charge compensation to keep an electrochemical stability, and therefore it is told that it has a similar chemical bond state with zeolite. Ikeda contributed a paper to bulletin the constitution of geopolymer before, titled "Carbon Dioxide Problem and Geopolymer Technique", and authors of this paper suggest to refer it, because an intelligible commentary was accomplished. Judging from the experiments that the authors went for, when considering the utilization of K-based geopolymer as refractories, they reached the conclusion that the formation of Kalsilite and Lucite having high melting point becomes a key factor. Well, the authors think to use this K-based geopolymer material as a new castable. As one idea of that case, the authors think to use Lucite crystal possessing of high thermal resistance as a main refractory aggregate of new developing castable, and to manufacture the Lucite crystal beforehand. But it is said that Lucite crystal does not provide a good crystal, unless passing through a process that starting raw material is heated up to high temperature more than 1500°C and then it is cooled off slowly and carefully. In other words, the production of the Lucite crystal takes considerable heating cost and time. By the way, the authors have succeeded at letting a Lucite crystal precipitate relatively at low temperature conveniently before now, using a relatively low price natural minerals such as kaolin and feldspar as raw materials and sulfate salts such as K₂SO₄ as fluxes. Therefore the authors would like to propose the development of new castable using geopolymer that was produced due to synthesizing the Lucite crystal particle produced at low price and abundantly by the authors' developed method, in particular K-based geopolymer technique as a bonding material. If the castables are able to be developed, which is superior to high temperature characteristic in low price by utilizing industrial waste and unused resources, the authors believe that it is possible to find new application value as the alkali-resistant refractories.