Porosity: The property of the refractory brick is the ratio of its pore volume to the total volume. The refractory material with high porosity is not preferred for furnace lining because the molten charge, gases, slags, etc. May enter through pores in appreciable amount, may react and decrease the life of refractory and may create problems. It may change the physical nature of inner side of the material and this may result in the development of internal stresses during heating. The tendency of being corroded by slag etc. So good refractory material with high porosity has less strength, poor strength to abrasion and greater porosity in the percentage of voids or pore space in the refractory. This can be found by measuring the volume of water or liquid absorbed by the specimen. This will be apparent porosity, since it does not include the pore space of sealed pores also. This can be calculated from the above measurements are as follows:
Apparent porosity, P = W-D/V*100 = W-D/W-S*100
From apparent specific gravity value and the true specific gravity (Powder density) as determined by specific gravity the true porosity can be obtained as follows:
True porosity = (1-apparent sp. Garr)*100/1-true Sp. Gr.)
The value of true porosity is higher (1-2% higher) than of apparent porosity, since the latter does not include the volume of the sealed pores. The difference between the two represents the percent volume of closed pores.
Porosity affects the strength of refractories. High porosity makes the refractories mechanically weak, whereas low porosity gives higher resistance to slag attack and better heat conductivity. Porosity in refractories can be controlled by the following:
(a) Control of texture, control of practical size
(b) Method of making
(c) Control of firing temperature