Building Materials, Third Edition

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Quarrying and cutting have a great bearing on the weathering properties of stones. Stone from
top ledges of limestone, granite and slate and from the exposed faces of the rock bed is likely
to be less hard and durable. Highly absorbent stone should not be quarried in freezing weather
since the rock is likely to split. The method of blasting and cutting also influences the strength
of the stone and its resistance to freezing and temperature changes. Small, uniformly distributed
charge of blasting powder has a lesser weakening effect than large concentrations of explosives.
A porous stone is less durable than a dense stone, since the former is less resistant to freezing.
Also, rocks with tortuous pores and tubes are more apt to be injured by freezing than those of
equal porosity having straight pores and tubes. Repeated hammering in cutting is likely to
injure the stone. Polished stone is more enduring than rough surfaced work, since the rain
slides off the former more easily. Stones from stratified rocks should be placed along the
natural bed in order to secure maximum weathering resistance. Pyrite, magnetite and iron
carbonate oxidize in weathering and cause discolouration of the stone in which they are present.
Since oxidation is accompanied by a change in volume, the surrounding structure is weakened.

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Preservation of stone is essential to prevent its decay. Different types of stones require different
treatments. But in general stones should be made dry with the help of blow lamp and then a
coating of paraffin, linseed oil, light paint, etc. is applied over the surface. This makes a
protective coating over the stone. However, this treatment is periodic and not permanent.
When treatment is done with the linseed oil, it is boiled and applied in three coats over the
stone. Thereafter, a coat of dilute ammonia in warm water is applied.
The structure to be preserved should be maintained by washing stones frequently with
water and steam so that dirt and salts deposited are removed from time to time. However, the
best way is to apply preservatives. Stones are washed with thin solution of silicate of soda or
potash. Then, on drying a solution of CaCl 2 is applied over it. These two solutions called
Szerelmy’s liquid, combine to form silicate of lime which fills the pores in stones. The common
salt formed in this process is washed afterwards. The silicate of lime forms an insoluble film
which helps to protect the stones.
Sometimes lead paint is also used to preserve the stones, but the natural colour of the stone
is spoilt. Painting stone with coal tar also helps in the preservation but it spoils the beauty of
the stone. Use of chemicals should be avoided as far as possible, especially the caustic alkalis.
Although cleaning is easy with chemicals, there is the risk of introducing salts which may
subsequently cause damage to the stone.
In industrial towns, stones are preserved by application of solution of baryta, Ba(OH) 2 —
Barium hydrate. The sulphur dioxide present in acid reacts on the calcium contents of stones
to form calcium sulphate. Soot and dust present in the atmosphere adhere to the calcium
sulphate and form a hard skin. In due course of time, the calcium sulphate so formed flakes off
and exposes fresh stone surface for further attack. This is known as sulphate attack. Baryta reacts
with calcium sulphate deposited on the stones and forms insoluble barium sulphate and calcium
hydroxide. The calcium hydroxide absorbs carbon dioxide from the air to form calcium carbonate.
Ba (OH) 2 + CaSO 4 BaSO 4 + Ca(OH) 2
(Barium sulphate) (Calcium hydroxide)