HI Ted,

Can you please dig deeper on the decomposition of C3S to C2S and free lime.

Another thing, by experience, how would you deal a customer complaining of a false set or quick setting cement delivered to them. What can be suggested to them?

Thanks.

Hello Norew,

The reaction C2S + CaO <--> C3S is temperature dependent and reversible. Pure C3S is not stable below about 1250, but the presence of other elements such as Al, Fe, Mg, Na and K, present in solid solution, stabilise C3S at lower temperatures as the impure form called Alite.

However, this stability is temperature dependent, and if  Alite is held in the temperature range 1250 - 700degC, it will partially decompose back into its constituents, C2S and free CaO.  This reaction happens fastest between 1250 and 1000degC but slows down significantly as the temperature drops below 1000degC, until it is almost stopped completely below 700degC.  Therefore to prevent any loss of C3S, clinker must be cooled as rapidly as possible from 1250 to 700DegC.

To answer your second question; Customer complaints are always difficult, especially when the customer does not provide detailed information about the problem. The best way to deal with such a complaint is to ask for a sample of the cement and perform your own tests to confirm what the customer is claiming.  (Also, if you have any 'point of sale' samples, stored in your laboratory, from the time of the customer deliveries, you can also test these.)

False set and flash set (or rapid set) have totally different causes, so you have to determine which one the customer is experiencing before you can solve the problem and offer any solution. False set is caused by an excess of hemihydrate (Plaster of Paris) as aresult of over-heating the cement during milling. (ie. mill exit temp. too hot)    Flash set is normally due to insufficient soluble sulphate available in the cement, either caused by too little gypsum addition or by insufficient dehydration of gypsum to hemihydrate during milling. (ie. mill exit temp. too cold)

(Flash set is normally accompanied by the release of an abnormal amount of heat, whereas false set is not.)

Regards,

Ted.

Thanks Ted.