Lime-silica bricks

Hydraulic limes (84) may be used for mortar, stucco, or the scratch coat for plaster. They harden slowly under water, whereas high calcium limes, after slaking with water, harden in air to form the carbonate but not under water at ordinary temperatures. However, at elevated temperatures achieved with steam curing, lime—silica sand mixtures do react to produce durable products such as sand—lime bricks. 

Silica bricks are manufactured from crushed ganister rock containing about 97 to 98 percent silica. A bond consisting of 2 percent lime is used, and the bricks are fired in periodic kilns at temperatures of 1,500 to 1,540°C (2,700 to 2,800°F) for several days until a stable volume is obtained. They are especially valuable when good strength is required at high temperatures. Superduty silica bricks are finding some use in the steel industry. They have a lowered alumina content and often a lowered porosity. 

As previously mentioned, silica brick can withstand temperatures close to its melting point, this may be attributed to silica s ability to form immiscible liquids in the presence of lime or iron oxide. No immiscibility, however, is present with silica in the Na20-Si02-Al203 system. With small additions of alumina or alkali oxides to silicate systems in which immiscibility occurs, the miscibility gap is narrowed and ultimately eliminated. A range of from 0.3% to 0.5% alkali or 2.5% to 3.0% A1203 is sufficient to cause this effect, specifically alkalies form low melting eutectics with silica. As a result, the presence of these oxides even in small... 

Whether an acidic or a basic slag will be needed is a factor that must be considered when a furnace is constructed so that its refractory linings will be compatible with the slag. Silica bricks would deteriorate quickly in the presence of basic slag, and magnesia or lime bricks would dissolve in acid slag. 

Refractory silica bricks, containing 96 to 98 % of Si02, are bonded using 1 to 3 % of CaO added as a milk of lime or as hydrate, together with a small quantity of a finely divided sodium-iron-silicate flux [32.3]. 

The oldest and most common are soda-lime-silica glasses, which contain 12-16% alkali (mainly Na20), 10-12% alkaline-earth oxides (CaO, MgO), and 71% Si02. This type includes window glasses, container glasses, glass bricks, and certain types of glass fibers. 

Cement and concrete are used in construction on an enormous scale, equalled only by structural steel, brick and wood. Cement is a mixture of a combination of lime (CaO), silica (SiOj) and alumina (AI2O3), which sets when mixed with water. Concrete is sand and stones (aggregate) held together by a cement. Table 15.4 summarises the most important facts. 

All silica glasses are particularly susceptible to dissolution by solutions above pH 9 due to the nucleophilic attack of the hydroxyl ion on the silicon-oxygen bond J), This is relevant in our particular discussion on bricks, as these are set in masonry with mortar, from which lime may be leached out such that solutions reaching the neighbouring pores could well reach pH 11 or 12. 

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. 

If a brick with a high-lime-to-silica ratio were placed into a glass tank checker in an oil-fired furnace where V2O5 deposition occurred, there would be a progression of reactions resulting in formation of low-melting phases that would contribute to deterioration of the strength and creep resistance of the brick. This would be influenced by the total amount of CaO... 

The ancient Greeks and Romans developed pozzolanic cement by mixing lime mortar with a source of reactive silica such as volcanic ash, tuff or crushed bricks and tiles. The silica reacts slowly but continuously with calcium hydroxide after initial hydration to form large amounts of CSH gel. 

Silica refractory bricks are manufactured from groimd ganister rock (quartzite) containing 98% Si02, to which 2% CaO is added as milk of lime. 

Incinerator designers have used 70% AI2O3 brick for years in the primary and secondary combustion chambers of the kilns. For a long time, the 70% AI2O3 bricks would experience rapid wear in the fireball regions of the secondary combustion chamber where conditions reach in excess of 1750°C. This is also an area where corrosive lime-alumina-iron oxide-silica slags coat the refractory walls. 

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