Volcanic glass properties

Ss noted earlier, the prime component of slag is silicates, and as early as 500 B.C. metal workers noticed that solidified slag had properties not unlike those of the highly prized volcanic glass obsidian. This prompted people to heat all sorts of rocks in various combinations. Slags of many colors were pro-... 

Sodium and borosilicate glasses are generally used, but some low-cost Sirasu brand microspheres have started to be manufactured in Japan using volcanic glass and ashes16 17). The properties of some glass microspheres manufactured in USA are given in Table 1 18). 

More than 10,000 years ago, the first developments in the field of metallurgy were made in the Near East. Until that time man had used tools made of wood, bone and stone and with these materials he was able to meet all his requirements for devices and tools. An example from that period is the knife made of obsidian, a shiny black, brown or grey magmatically formed rock. The material is very finely crystallized owing to rapid cooling and is therefore often called volcanic glass. By beating it with another rock, you can break off extremely sharp pieces, a property shared with the flint that was mined in the south of The Netherlands in prehistoric times (fig.2.1). 

Obsidian is a volcanic glass which has fracturing properties that make it eminently suitable for fashioning implements such as knives, scrapers, and arrowheads. Obsidian sources are not found in every region and its use as an early article of trade is evidenced by its appearance far removed... 

These rocks are suitable for use as aggregates in concrete due to their physical and mechanical properties. However, there are some engineering works where important damage has been detected as result of alkdi-silica reaction due to the presence of volcanic glass (fresh and devitrified) and cryptocrystalline silica (e.g. Shayan Quick 1988, Marfil Maiza 2001, Marfil et al. 2010). 

Another problem is the increased water absorption of the autoclaved aerated concrete elements, therefore water retarding additives, e.g., cellulose esters, are used in the mixture of plasters. Due to an insufficient amount of water in the mixture of plasters, the hydration of the cement minerals slows down. The influence of natural zeolite and clinoptilolite on the properties of plaster has been investigated (45). Clinoptilolite is a natural zeolite comprising a microporous arrangement of silica and alumina tetrahedra (46). It commonly occurs as a devitrification product of volcanic glass shards in tuff and as vesicle fillings in basalts, andesites and rhyolites. 

A clay mass is a mixture of clay and additives for a specific ceramic (processing) technique. The clay provides the plastic properties. Possible additives are a flux and a filler. Some examples of fluxes are potassium feldspar, bone meal, volcanic ashes and ground glass, all of which serve to affect the density and decrease the melting range. Fillers reduce the shrinkage as well as the sticky character of the clay. 

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