1156 posts
Quality control Question 9
We have a problem in determining which residue is correct, either the raw meal or the kiln feed. In fact we do get about 13 per cent residue on 90 micron for raw meal and when it comes down to kiln feed it is about two per cent higher. Could you be kind enough to explain why this phenomenon takes place and what actions could we attempt to resolve the differences.
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1156 posts
Re: Quality control
The kiln feed normally contains the dust from the precipitator in addition to the raw meal and therefore the two samples are quite distinct. The kiln feed is normally finer than the raw meal but this depends on the exact circuit used on the particular factory. Your situation is different and might be explained by drying the raw materials in the preheater exhaust prior to precipitation. In terms of quality control the raw meal is the residue you should be controlling. The kiln feed sample is within the external dust recirculation circuit. If you send more details of the circuit I can explain better.
1156 posts
Quality control Question 10
We have got two sulphate resistant cement types (CEM I and a CEM II using fly ash), and our clinker has got: C3S: 57-64 per cent, C2S: 10-18 per cent,C3A: 0-1 per cent, free CaO less than 1 per cent. Our fly ash has: SiO2: 48 per cent, Al2O3: 19 per cent, Fe2O3: 10 per cent, CaO: 15-18 per cent (free:2-4 per cent), MgO: 4 per cent, SO3:2 per cent.How much (in percentage terms do you recommend to use of fly ash? How will it effect sulphate resistance? How should the fly ash be ground and what is the most important limit in being able to use the fly ash (I mean e.g. CaO, free CaO, Al2O3, residue on a 90 micron sieve or other else)?
1156 posts
Re: Quality control
1. The percent fly ash addition depends on the cement standards in your country and also on the expectations of your customers. You must expect a drop in early strength and the maximum fly ash will be determined by the reduction in early strength the market will accept.
2. Addition of fly ash should increase the sulphate resistance of concrete made with the cement.
3. It is often best to add the fly ash to the separator of the cement mill circuit. The material that is fine enough already will pass straight to the finished product without grinding. This will preserve the spherical shapes of the ash and promote workability of the cement and reduce water demand.
4. The most important limit is the reduction in early compressive strength.