57 posts
Re: Bulk density
Sorry to say, but the remark of "Cement supplier" seems completely wrong to me.
The density of a packing of particles is generally independent of the average size of the particles. For example, the particle volume fraction of a random close pack of spheres of identical size is always ~0.64. This is totally independent of the size of the spheres, whether they are oranges or nanospheres does not change anything.
The random close pack density is slightly influenced by particle shape (unless the shape is very different from the sphere) but can be strongly influenced by particle size distribution. Broader size distributions tends to have higher densities as small particles can fill the gaps between larger ones.
The bulk density will be lower than the random close pack density by definition (the random close pack should be completely incompressible and is the lowest bulk achievable through tapping). For same (scaled) size distribution, finer powders will generally show lower bulk densities as air has more difficulty to escape the powder and electrostatic effects can appear, which will make approaching the random close pack more difficult.
In the case described, the difference probably lies in the size distribution.
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108 posts
Re: Bulk density
Dear ovancantfort, Here size does matter. The finer the particles, the more particles will be accommodated leaving less space to be occupied by voids.
Let me clear it more by following calculation for spherical particles -
Suppose we have a cube having side r. Now we put n spheres along each sides of this cube in such way that side and sum of the diameter of sphere are same.
Now total such spheres willbe n*n*n = n3 (cube of n)
Volume of the cube =r3
radius of sphere = d/2 = r/2n
Volume of each sphere = (4/3)*(22/7)*(r/2n)3
Volume of total spheres = (4/3)*(22/7)*(r/2n)3*n3
Therefore volume occupied by voids = r3 - (4/3)*(22/7)*(r/2n)3*n3
=r3 - (4/3)*(22/7)*(r3/8)
=r3*(0.48)
This shows that if r is greater than the volume occupied by the voids will be greater
and less will be the BD.
Regards,
Gulam Dastgir
57 posts
Re: Bulk density
Dear Dastgir,
In your example, to get the VOID FRACTION, you have to divide the the volume of voids by the volume of the cube (that is r3).
So we have: void fraction = 0.48*r3 / r3 = 0.48 and it is a constant, independent of the size of the spheres.
In your example, the particle volume fraction is then 1-0.48 = 0.52. And you can see this is lower than the particle volume fraction of the random close pack, which means your arrangement is in fact quite loose.
As the spheres become smaller, yes you can put more of them in a given volume, but you will also get more smaller voids and the total void remains constant.
Best regards