Apparent Porosity and True Porosity of Refractory Samples

Pores in Refractory Bricks (Porosity)

Porosity is the percentage relationship between the volume of the pore space and the total volume of the refractory sample. Apparent Porosity does not include the volume of the sealed pores. The True Porosity includes the volume of the sealed pores also. The usual difference between the apparent porosity and the true porosity is of the order of 1 to 2 percent unless the proportion of the sealed pores is high. The true porosity figure is the higher than the apparent porosity figure as the true porosity includes the volume of the sealed or closed pores also. The difference between the two values represents the percent volume of closed pores. Porosity can be controlled by the following:

=> By controlling the texture of the brick i.e. by controlling the size distribution of the particles.

=> By the methods of green manufacturing and composition.

=> By controlling the firing temperature, soaking time etc.

=> Quality of raw materials i.e. the inherent grain porosity of the raw materials used.

Higher the porosity, lower will be the strength of the brick. Bricks with lower porosity will have greater resistance to slag attack and more sensitiveness to fluctuations in temperature. Their thermal conductivity will be more.

Suggested Article: Bulk Density of Refractory Samples 

Hence, apparent porosity is the percentage ratio of the void space in the refractory specimen to the total bulk volume of the same. There are two methods used for the determination of apparent porosity of refractory materials which are:

=> Boiling point method, and

=> Evacuation method.

Refractory Samples (Refractory Specimen) measuring 6.5 cm x 6.5 cm x 4 cm is cut from burnt refractory bricks by a cut off wheel from within its core and cleaned any dust or loose particles adhering to its surface and are dried in an oven at 110^OC to a constant weight. For graded materials take 3 to 5 mm size grains and dry at 110^OC. It can be determined by following the steps given in any of the Standard Methods for Refractory Testing like - ASTM, Indian Standards (IS), Ghost, DIN etc.

True Porosity (%) = {1 - (Apparent Sp. Gr. ÷ True Sp. Gr.)} x 100

Apparent Porosity (%) = {(Soaked Wt - Dry Wt) ÷ (Soaked Wt - Suspended Wt)} x 100

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Permeability of Refractory Bricks and Monolithics

5-July-2020

Permeability of any refractory material is defined as the volume of a gas or air which will pass through a cubic centimeter of the material under a pressure of 1 cm of water per second. 

Fig - Refractory Bricks being stacked in stock yard

Permeability is calculated by the following formula:

Permeability = (Vol. of gas/air x thickness) ÷ (Area x Time of flow x Pressure difference)

It is determined by forcing a known volume air or gas through a cube. Time of flow, pressure difference and dimensions of specimens are noted. However, there are ready-made apparatuses and systems available in the market for testing permeability of refractory bricks and monolithics. There is no direct dependence permeability has on porosity however, permeability depends upon the existence of closed pores or channel pores and is a measure of these, whereas porosity measures the total pore volume including closed pores.

Suggested Article: Pore Size Distribution in Refractories 

However, this property permeability for refractory bricks or monolithics become important wherever molten liquid like metal, slag, glass etc. come in contact with the refractory lining i.e. wall of the furnace or container especially, when gas under pressure is present. Due to the anisotropic nature of refractories, the result will depend upon on factors such as the direction of flow and presence or absence of the original skin on the test specimen.

Low permeability is more important than low porosity from the point of view of slag resistance of the refractory. Uniform permeability is an indication of absence of cracks in the refractory.

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• Different tests and procedures to check slag corrosion resistance of refractory products
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Slag Corrosion Test of Refractories | Refractory Industry Guru

Slag attack is particularly important. The structural strength of the refractory may be critically reduced by the solvent action of liquid slags. The slag attack on the refractories in contact may be in the two ways:

Slag Corrosion - It is the wear and tear of refractories caused by static chemical attack of slag.

Slag Erosion - It is wear caused my mechanical action i.e. the process of breaking and washing away of refractory materials by molten slag.

The conditions of operation are variable and complex. Hence the standardization of this test is difficult. However, there are various test methods, viz. Crucible test, Solid cube test, Suspended rod test, Model wall test, Cone test, Powder impact test, but none is exact simulative test. It is done in various ways to suit the working conditions. The following are an outline of different methods of Slag Corrosion Test  or Slag Resistance Test:

=> This method is called ‘Pill test’ is used when the quantity of slag is less as compared to the quantity of refractories. The slag, more often in the form of a pill, is placed on the refractory body or in a cavity made in it and heated. The depth of penetration of the slag inside the refractory, the spread of the molten mass and also the corrosion or bloating is observed. Theses factors form the measures of the attack.

=> This is another method known as immersion method and is used when the quantity of the slag is far in excess of refractory. Here the refractory is subjected to attack of the slag by immersing a small piece of refractory in the molten slag. The depth of penetration of the slag inside the refractory is the measures of the attack. 

=> Another test also known as Impingement method or Powder impact test consists of letting the slag fall on the refractory bricks at high temperature. Many a time a spray of solid powdered slag is directed against the hot refractory brick at an angle of 45^O for a certain period and at a certain temperature. The extent of corrosion under gone by the refractory is the measure of the slag attack. Several types of furnaces have been designed for this test.

=> This is Fusion test and consists of making a mixture of different quantities of powdered slag and refractory material and studying the fusion material of the mixture. The interval between softening and flattening of the cone is supposed to indicate the critical range of deformation of refractories in contact with slag.

The extent of penetration of slag is to be carefully studied. The bore diameter, depth in the refractory test specimen, the overall specimen size, fineness, as also the quantity of slag to be tested should be equal in every case for obtaining comparative test results. Overall slag corrosion / erosion will depend on so many factors such as porosity of the refractory brick, the composition, nature of the brick and of the slag, the temperature and duration of the attack, load on the brick at during slag attack, the products of the reaction formed and the rapidity with which they are removed, etc. Thus various refractories are affected variously and therefore it is difficult to simulate the exact conditions encountered in service. Still one can get an approximate an approximate idea by doing the chemical analysis and studying the various phases developed at the slag-refractory interface through Microstructural and XRD (X-ray diffraction) analysis. Out of all the slag corrosion tests described above, most of which give a qualitative and comparative result only, there is one method which has been somehow accepted as standard is the German DIN 1069 based on crucible test.

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How to check Alkali Resistance (AR) of Refractory Lining - Bricks and Castables

23-Jan-2010

BACKGROUND

Refractory lining materials such as bricks and castables etc. are susceptible to alkali attacks. As we already know that alkalies (Na₂O and K₂O) are very damaging to refractories, and can reach them either in liquids or in gases. The resistance of Zircon and Zirconia refractories to their attack (at glass-making temperatures) comes mainly from the non-wettability quality of these compositions with respect to alkalies. We have also seen that carbon resists wetting by silicate slags, and graphite is also resistant to wetting by many liquid metals as well as by slags and fluxes. In cement or lime rotary kilns attacks from alkali vapours or alkali salts take the form of an infiltration at the surface of refractory lining, with consequential adverse impacts on the bonding (bricks, castables or mortars).

Such damage may already occur at temperatures in the range of 800 - 900^OC. Refractory lining, the alkali resistance (AR) of which is unknown, can be tested according to the following instructions (based on DIN 51069 standards).

Preapring Test Specimen  

Refractory Bricks (Lining)

Cut the sample from a standard refractory brick. The size must be half of a standard brick, i.e. approximately 114 x 114 x 65 mm, with a hole having a diameter of 35, 40, 45, or 50 mm and a depth of 40 mm. Then dry the sample at 110^OC.

Refractory Castable

Cast the test specimen (piece) on a vibrating table. The size of the test specimen must be approximately 80 mm in outside diameter, with a height of approximately 65 mm and a hole which is 35 or 40 mm in diameter and 40 mm deep. After casting, the specimen must be dried at 110^OC and burnt at 1200^OC for five hours in an electric furnace.

Testing of Refractories

Put the specified quantity of anhydrous potassium carbonate (K₂CO₃) into the hole of the test piece or specimen. See table below -

Hole diameter (mm)	Potassium Carbonate (gm)
35 40 45 50	32 38 44 50

Make a lid of firebricks, approximately 80 x 25 mm, or approximately 114 x 114 x 25 mm. Use the lid to cover the hole in the test piece and seal with air setting refractory mortar between lid and the specimen.

Burn (fire) the specimen at a temperature of 1100^OC for 5 hours in an electric furnace. Then allow the specimen to cool off. Remove the lid and cut the specimen into two halves for visual inspection.

Assessment of Alkali Resistance

The test described will subject the specimen to an environment which is more hostile than the one normally encountered by refractory lining materials, but it provides an excellent basis for comparison.

The assessment of the alkali resistance is mainly based on the depth of penetration. If the test reveals a penetration depth of less than 3 mm, without expansion and cracks (alkali bursting), the test result is considered to be satisfactory.      

Usually the specification of Alkali Resistance (AR) in connection with the material designation on refractory lining drawings if any, or specifications that are required in regard to the properties of the various refractory materials concerned to resist chemical attacks of alkali salts in the form of vapour or liquid etc. are provided by the customer to the refractory supplier (vendor). Manufacturers of refractories generally furnish conventional information on their materials (Bricks, Castables, and Mortars etc.) such as, compressive and tensile strength, modulus of rupture, chemical analysis, thermal conductivity, density, porosity, refractoriness, resistance to creep and gas permeability etc. In addition, there are some special properties, determined by certain tests that have become standardized in the refractory industry. Results obtained from these tests, while not 100% conclusive, do furnish a good indication of the properties of the refractory and its resistance to various exposures within the kiln, and are the basis for the selection of a refractory particularly suited to any given area of the kiln.          

Related Article:

Calculation of Permeability and why Permeability, Porosity are important in refractories