Building Materials, Third Edition

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testing machine. A 75 × 100 × 300 mm block is placed over the ridge of the tile and a load of
2.7 kN/min is applied on the block. The breaking load of individual tile is noted. It is divided by
the length of tile. The results are reported in N/mm.

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Fire-clay is a term, loosely applied, to include those sedimentary or residual clays which vitrify
at a very high temperature and which, when so burnt, possess great resistance to heat.
These are pure hydrated silicates of alumina and contain a large proportion of silica 55–75%,
alumina 20–35%, iron oxide 2–5% with about 1 per cent of lime, magnesia and alkalis. The
greater the percentage of alumina, the more refractory the clay will be. Fire clays are capable of
resisting very high temperatures up to 1700°C without melting or softening and resist spalling.
The presence of a small percentage of lime and magnesia and alkalis help to melt the clay
particles more firmly, whereas a large percentage of lime and magnesia tend to melt the clay at
low temperatures. Iron oxide or other alkalis reduce refractory qualities of fire clay. The fire
clay is used for manufacturing fire bricks used in furnance linings, hollow tiles, and crucibles.

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Fire-clay bricks are made from fire-clay. The process of manufacturing is as of an ordinary
brick, burnt at very high temperatures in special kilns (Hoffman’s kiln). The raw materials
used for the manufacture of fire bricks consist of flint clay and grog (burnt fire clay) as non-
plastic materials and soft fire clay as plastic material. Fire clay mortar is used to clay rerfractory
bricks.

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1. The colour is whitish yellow or light brown.


2. The water absorption of fire-clay bricks varies from 4–10%


3. The minimum average compressive strength of the bricks should be 3.5 N/mm^2.

… These are used for lining blast furnances, ovens, kilns, boilers and chimneys.
The principal varieties of fire-clay bricks are as follows:

e™ ‚ 6—™ f™4 consist of silica bricks (95–97% silica and 1–2% lime) and ganister bricks
(ganister—a hard coloured sand stone containing 10 per cent clay and 2 per cent of lime), used
in lining furnaces having siliceous and acidic slag, steel industry and coke oven. The softening
temperature ranges from 1700° to 1800°C. Silica bricks are hard and also possess good
refractoriness under load. But they have tendency to spall during rapid temperature change
therefore, these can not be used for lining of furnaces which have to be cooled and reheated
frequently.

f—™ ‚ 6—™ f™4 consist of magnesia bricks (magnesia minimum 85 per cent, calcium
oxide maximum 25 per cent and silica maximum 5.5 per cent) and bauxite bricks (minimum 85
per cent aluminium oxide and maximum 20 per cent clay). These are highly resistant to
corrosion and are used for lining furnances having basic slag. Due to high thermal expansion
and consequent poor resistance to spalling the use of these bricks is retricted to copper metallurgy
and basic open hearth.