Autoclave Materials

structural materials and products that are obtained from a mixture of lime and quartz sand and that harden at raised temperatures and pressures.

In the fabrication process, autoclave materials are subjected to heat treatment (“steaming”) in autoclaves at 175–200°C, using saturated steam under 0.9–1.6 MN/m² pressure (9–16 kgf/cm²) for 8–16 hrs. As a result of the physical and chemical interaction of the components—lime, sand, and water—calcium hydrosilicates form and force the materials to harden and form a monolithic body. The method used to fabricate autoclave silica brick from a mixture of lime (8–10 percent by mass) and quartz sand (90–92 percent by mass) was first proposed by the German scientist W. Michaelis in 1880.

In Russia the production of silica brick began in the late 19th century. The production of autoclave masonry lime sandstone and lime-slag-sandstone blocks and stones (both solid and hollow), limestone-tripoli fibrolite facing slabs, and other products began in the USSR in the 1930’s. In those years, fabrication technology was developed and the production of portland cement-based concrete blocks was organized—thus marking the start of the use of cement in the production of autoclave materials—and the production of cellular concrete from a mixture of ground unslaked lime, ground quartz sand, and foaming agents and gasifiers (in the form of foam silicate and gasified silicate), and products made from them, with bulk densities from 400 to 1,200 kgf/m³ and higher. In the 1950’s the fabrication technology of coarse, large-size silica concrete autoclave-cured wares with compressive strengths to 50 MN/m² (500kgf/cm²) and higher was developed in the USSR; these products are equivalent, in their properties, to reinforced concrete products, and their prime cost is 10–20 percent lower. This work was awarded the Lenin Prize in 1962. Soviet scientists also discovered the possibility of replacing lime and portland cement in the production of autoclave-cured products with ground slag (metallurgical slags, fuel slags, and so forth), nepheline sludge, and some ashes, containing as much as 20 to 50 percent calcium oxide in free form and also in the form of silicates and aluminates susceptible to hydration when heat-treated in autoclaves. Vast production of large components (wall blocks and panels) of heavy, light, and cellular concretes with bulk weights from 300–500 to 2,000–2,400 kg/m³, heat-insulating materials, facing materials, and other similar products and materials have been organized on the basis of autoclaving treatment in the USSR.