artons
Name *) Gas attacks stonework of buildings and metals
Answers
The natural durability of stone can be affected by a number of factors; probably the most significant is pore structure. Pore structure is significant because it affects the amount of water entering and moving through the stone. Also, salts that may damage the stone can be transported by water and accumulate in the pores. The critical factor is not the total amount of space created by the pores but how they are structured.
Stones with low porosity will not allow much water penetration and are therefore less likely to suffer salt and/or frost attack. Stones with low porosity are therefore generally more durable.
Stone with high porosity will allow more water in but, if the pores are large, the water will tend to be able to evaporate reasonably quickly. However, if there is a large network of fine pores, capillary action will be high, but evaporation will be relatively low. In addition, a stone with large pores is less likely to suffer salt damage than one with small pores. This is because the larger space is more likely to be able to accommodate the expansion pressures of salt crystallisation.
Incorrect bedding
Sedimentary rocks are laid down in beds. Defects can arise if a stone block is incorrectly placed in a building in relation to its bedding plane.
When placed in a wall the stone should generally lie in its natural bedding position. This is, the layers should run horizontally, in the manner in which the stone was originally formed. The stone is stronger in this position and is also less vulnerable to defects. If the stone is face bedded (the layers are vertical), it is more vulnerable to damage through crystallisation of salts and/or frost action. This is because the mechanical actions involved find it relatively easy to push off the bedding layers as there is no restraint from the adjoining stones.
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