Re: Clinker Hanrdess
Litre weight is a measure of bulk density, not necessarily clinker hardness . In any case, with respect to clinker grindability, "brittleness", rather than "hardness" is the governing factor.
The brittleness coefficients of the main three clinker minerals have been measured by researchers and found to be as follows;- C2S =2.0 < C3A= 2.9 < C3S = 4.7 (the numbers represent average length of cracks radiating from micro-indentations, so a high number means the material is more brittle, or easier to grind).
These figures show that C2S is over twice as difficult to grind as C3S, and when C2S occurs in numerous dense clusters, due to oversized silica, the grindability is made significantly worse. Grindability is also related to the packing density of the mineral crystals within the melt matrix upon cooling. Since C2S crystals are round, they can pack closer together than the elongated crystals of C3S and C3A, all of which makes a clinker high in C2S harder to grind than one which is high in C3S and/or C3A.
Clinker grindability basically depends on the following;-
- Clinker composition and mineralogy, particularly the amount of C2S present, the degree of C2S clustering and the crystal sizes and degree of micro-fracturing of the clinker minerals.
- The thermal history of the clinker eg. rate of sintering, sintering temperature, time at sintering temperature and cooling rate.
So, while litre weight may indirectly give you some information about the sintering temperature and sintering time, it cannot give you any information about the clinker's mineralogy or cooling rate.
The definitive method of estimating clinker grindability is the Bond Work Index which uses a laboratory ball mill to measure the time taken to grind clinker to a certain fineness.
For more information on clinker grindability see;-
"Clinker Microstructure and Grindability", A literature review by Linda M. Hills, PCA (Portland Cement Association), 2007.