High MgO in limestone

Hello Haliff,

Above 2% in the clinker, MgO will appear as the mineral periclase. If the particle size of this periclase is large, this could result in an increase of the expansion of the cement, causing unsoundness. The particle size of the periclase is dependent on the size and distribution of MgO within the limestone, and on the fineness of the clinker particles in the cement.

3% MgO isn't exceptionally high though and you should be able to make cement with acceptable expansion at this level. However, I would still try to reduce the MgO content of the limestone as much as practically possible, by selective mining and even wasting material which has very high MgO. Also, you may have to grind your cement a bit finer to help counteract magnesia expansion.

Therefore, it would be advisable to carefully monitor cement expansion by frequent testing of production cement samples once you start using clinker made with the higher MgO limestone.

On the positive side, a modest increase in MgO will slightly increase both the liquid phase quantity and the mobility of Ca ions within the liquid phase, promoting clinkerisation and formation of C3S.

Good luck,

Ted.

Thanks Ted,

Please, can you share with me the impact of high MgO in limestone used as additive.

In recent time i noticed high initial setting time in our cement and the major change in parameter was observed with MgO (the limestone additive contains higher MgO than usual).

Also, does the so3 content of additive limestone have influence on setting time (does high SO3 in additive limestone leads to reduction of gypsum usage). 

Please kindly give your expert opinion. (the chemistry responsible for the answer will help in my understanding)

Thanks All

Hello Haliff,

When limestone is used as a cement mineral additive, any MgO is normally present in the limestone as dolomite (CaCO3.MgCO3). Since MgCO3 has very similar chemical reactions to CaCO3, I would not expect that MgCO3 would have much impact on cement properties at all. (See http://www.sciencedirect.com/science/article/pii/S187770581300831X )

MgO could also be present in volcanic rock intrusions within the limestone deposit. However, in this form MgO is even less likely to have much effect on cement properties, since it is locked within the volcanic mineral chemical structure.

With respect to SO3 contained in the limestone, this should not have any effect on cement properties as long as the increased SO3 is not present as soluble calcium or alkali sulphates. You would have to determine the soluble sulphate content of the limestone to determine if there could potentially be any influence on cement setting times.

Regards,

Ted.