A hydrothermal hot-pressing method: apparatus and application

Abstract

Loose particles occurring in sediment are transformed into sedimentary rock by a process termed lithifi-cation, which is a chemical process that reduces the original porosity by compaction and cementation [I]. A hydrothermal hot-pressing method is intended for artificial lithification. In this study, silica powder including mineralizer solution is compressed under hydrothermal conditions from outside an autoclave for hydrothermal hot-pressing and converted to a solid body. Few studies have been carried out on hot-pressing under hydrothermal conditions. Roy et al. [2, 3] prepared very high strength materials by hot-pressing cement paste under conditions up to 250 0 C and 345 MPa. A small amount of water (0.093 g water per I g cement) was added to anhydrous cement and the degree of hydration was around 30% for hardened materials [3]. In this case, vapour pressure was very small, because water was consumed for hydration of cement. The hydrothermal reaction sintering method [4-6] has been used to produce sintered oxide materials from metals. In this method, water reacts with metals to prepare oxides and is completely consumed. The hydrothermal hot-pressing method has two characteristics: continuous compression of samples under hydrothermal conditions and space for water retreat. Compression accelerates compaction of starting powder and prevents development of cracks by shrinkage. Compaction depends on the rate at which water can be expelled from the starting powder. The hydrothermal hot-pressing method requires the space for water retreat, into which water included in the starting powder is released. Without the space, water exists in pore space of the starting powder and hinders compaction of the powder. Alkaline solution, used as a mineralizer, increases solubility of silica and is easily saturated under hydro-thermal conditions. Under such conditions, cemen-tation of silica is accelerated by means of dissolution and deposition. Furthermore, solubility of many inorganic compounds increases under hydrothermal conditions and the compounds react with l: