Autoclaved calcium silicate hydrates

The well crystallized, autoclaved calcium silicate hydrates show low caibon-ation rate in spite of the higher lime content, than the amorphous forms of tober-morite, poorer in calcium [298]. However, Roy [80] found that the C-S-H richer in calcium than tobermorite, was readily carbonated and the eqnilibrinm CO2 partial pressure was lower. 

Figure 36. Differential thermograms of calcium silicate hydrates formed under autoclaving conditions.

After foaming and pre-ciuring, the aerated concrete is dimensionally stable and stiff. In this state the foamed mass is cut into blocks or elements, after which the final curing is performed with steam in autoclave at approximately 180°C. At this temperature the added lime CaO reacts with quartz Si02 to form calcium silicate hydrates, which increase the strength significantly. 

Sandlime bricks, also called calcium silicate bricks, are produced by moulding, under high pressure, a moist mixture of silica sand (or crushed siliceous stone, or flint) and hydrated lime. The green bricks are then autoclaved using steam pressures of at least 11 atmospheres. Under these conditions, the hydrated time reacts with the silica to form hydrated calcium silicate, which bonds the aggregate particles into a strong and durable brick [26.47]. Other shapes, such as blocks and building elements are also produced. 

Because hydrated calcium silicate is white, the autoclaved products are an attractive near-white colour. This enables pigments to be used to produce a wide range of coloured products. 

High density calcium silicate concrete products are made, particularly in Eastern Europe [26.46], by co-grinding silica sand and lime, adding controlled amounts of water and unground sand, and casting the mix into moulds. The products are then autoclaved at elevated temperature and steam pressure to produce hydrated calcium silicate concrete. A range of products, including slabs, beams and lintels, are produced, with and without metal reinforcement. 

Afwillite. A hydrated calcium silicate, 3CaO. 2Si02.3H2O it is formed when Portland cement is hydrated under special conditions and when calcium silicate is autoclaved (as in sand-lime brick manufacture). 

In the manufacture of autoclave products, cement, lime, or both, and silica-bearing materials form the raw mix. Various hydration products are identified when cement or calcium silicate is autoclaved. The compounds include C-S-H gel, a-C2SH, tricalcium silicate hydrate, CH, C3AHg, and a modified form of 7-C2S hydrate. In the cement-aggregate system,products that may form are C-S-H(I), C-S-H(II), 11 Atobermorite,... 

CSH, tobermorite, xonotlite and related materials are being widely utilized in recent years. It is produced on autoclaved lightweight concrete, autoclaved concrete, sand-lime blocks, hydrous calcium silicate he at-insu1 ation materials, hydration products of cement and other building or construction materials. 

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