Bulk Density of Refractory Samples

19-July-2020

Bulk Density of Refractories

This property is important for both insulating and dense refractories (shaped and unshaped). Bulk density (B.D.) is the ratio of the mass of the refractory specimen to the bulk volume of the same or in other words it is the weight per unit volume the refractory (including the volume of the pore space present in that refractory sample).

There are two methods for determination of Bulk Density in case of refractories:

1. Direct measurement method

B.D.= (Weight of the specimen in gm.) ÷ (Volume of the specimen in cc)

2. Direct volume determination method

B.D (gm/cc) = (Dry weight) ÷ (Soaked weight - Suspended weight)

Related Article: Green BD (Bulk Density), Burned BD and Moisture Content of Green Mixture (Powder) of Alumina Refractory Bricks

Fig: Refractory Shapes (PCPF)

Out of these two methods the ‘direct volume determination method’, which is generally used for irregular refractories, gives more accurate results. The nearer the Bulk density approaches the Specific Gravity the lower is the Porosity. Lower B.D indicates higher porosity, lower strength of the refractory material. The BD will also affect other properties of the refractory such as the load bearing capacity and thermal conductivity etc. Unless there is any specific reason, the aim of a refractory manufacturer is to produce products of higher Bulk Density. The ultimate Bulk Density of the product will depend on a number of factors like - type of raw materials used and their processing, process control at every step during green manufacturing and firing etc.

Manipulating the Test Results of Apparent Porosity (AP) During Testing of Refractory Bricks

Apparent porosity (AP) is the percentage ratio of the void space in the refractory specimen to the total bulk volume of the same. AP is one of the most important physical properties for any type of refractory brick that will be certainly mentioned in its specifications. Hence determination of Apparent Porosity is almost a compulsory part of Inspection / Testing of a Refractory Lot. This property becomes more stringent when the refractory brick is of any complicated shape like, checker bricks or nozzle bricks having tongue - groove etc. In such cases often, it remains a cause of worries for the laboratory person conducting inspection.

The results of AP can be manipulated to show less AP% than what the brick has actually and thus, an inspector may be deceived by a laboratory person. But such actions or manipulation of results are undeniably wrong and liable to be penalized or even the whole Refractory Lot may get rejected, if caught red-handed.

Nevertheless, this article is not to discuss the merits-demerits or right-wrong of the action. That is for you to decide. Here we assume that you have made a conscious decision to learn the trick as how to manipulate the results of Apparent Porosity in order to show a better result of a porous brick. Having so decided, below is a guide (trick) on how you can do it:

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

=>> To show less AP% we need to increase Dry wt as much as possible.

=>> Make 30% conc. salt solution (i.e. 30gm salt in 100gm water).

=>> Example: To bring down 75 mm std. Brick having actually AP-25% to AP-18%, soak the brick in approximately 30ml quantity of above salt solution so that its Dry Wt. is increased by approx 9 gm. After applying solution from different sides and its complete soaking put the brick in the drier & properly clean itssurfaces after drying.

Better try to know the actual AP before applying/manipulating.

Pore Size Distribution in Refractories

It is highly essential to measure pore size and its distribution to get the desired optimum properties in any refractory brick. Actually ‘pore size’ and ‘pore size distribution’ are two different things. First, let us discuss the difference between these two terms. The ‘pore size’ is a measure of the diameter of the largest pore whereas; the ‘pore size distribution’ is a measure of the range of pore sizes. The range of pore sizes can be normally distributed, and the spread can be quite narrow (e.g. the ratio of largest to smallest may be less than 2). On the other hand, pore size distribution can be very heterogeneous. In the case of large spreads and heterogeneity, which is not desired, the pore size will be far less predictive.

It is measured by analytical instrument known as Porosimeter based on mercury intrusion and capillary flow methods. The volume of mercury penetrating the pores measured directly as function of applied pressure. Pore Size information is best described graphically, rather than with a single number. An alternative method of describing the distribution curve is to use a collection of values that better describe the distribution curve in more detail, i.e., upper limit, lower limit, mean, standard distribution, etc. This P-V information serves as unique characterization of pore structure. This data together with Microstructural analysis and few other test reports of the refractory sample can be of great help in optimizing the properties as well as for any failure analysis.

Suggested Articles: 

• Permeability of Refractory Bricks and Monolithics
• Effects of Compacting Pressure on Sintering and other Properties of Refractory Bricks
• Bulk Density of Refractory Samples

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

 Related Articles 

• How to manipulate test results of Apparent Porosity (AP) during Testing of Refractory Bricks (Refractory Inspection)
• Permeability of Refractory Bricks and Monolithics
• How to check Alkali Resistance (AR) of Refractory Lining - Bricks and Castables