Re: Impact of free lime on grinding efficiency
All other factors being equal, low free lime clinker (hard burned) is generally harder to grind than clinker with higher free lime. This is due to physical changes in clinker mineralogy and porosity caused by over-burning rather than the presence of the free lime itself.
Hard burning causes the following mineralogical changes which have a detrimental effect on clinker grindability;-
1. An increase in both Alite and Belite crystal sizes.
2. An increase in clinker density associated with a reduction in clinker porosity. In particular, a reduction in the percentage of open pores (ie microscopic pores that penetrate from the surface of a clinker nodule well into the interior.)
There are a couple of other secondary microstructural properties of clinker that are variously mentioned in the literature as having a negative effect on clinker grindability;-
(a) An increase in the quantity of Belite and its distribution (ie an over-abundance of clusters, particularly large dense clusters caused by coarse silica.)
(b) The presence of coarsely crystalline C3A, caused by very slow cooling.
(c) An increase in the quantity of fine clinker particles. (<5mm).