It sounds to me that you must have combustion problems. I recommend that you should contact specialists and pay for a combustion audit. This will be the best way to find the solution. You might try increasing the primary air flow of the burner to maximum, or reducing the secondary air temperature by running the cooler grate with a lower under-grate pressure setpoint.

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.

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.

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)?

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.

We have a automatic Robolab with Pfaff divider, Hertzog mills & press with Phillips 2400 XRF. We are seeing a drop in our Fe₂O₃ in our raw meal &kilnfeed samples of 0.2 per cent, it occurs about one time in 10. It also occurs in a stock sample with similar frequency. If the low Fe₂O₃ pellet is analysed ten times the same drop can be seen with a SD of 0.01. This suggests xrf is not the problem! The problem never occurred in fused beads. We have tried to contaminate the sample with cement in the dividing station to replicate the problem, this also failed. Contamination of sample with limestone going to press had no significant effect. Using more or less grinding aid had no significant effect on Fe₂O₃. Using different weights of sample to both milland press had no effect on the iron.
This problem left for 12 months and is now back. The problem never occurred in 12 years with a Hertzog manual mill, manual press and phillips 1606XRF.Fls Has looked at the problem for us with no answers. Has anybody else had a similar drop in Fe₂O₃ in sample while other oxides remain constant.

This a question we are going to have to put on the web (anonymously) as I do not have an answer. It must be something systematic in the sample preparation or measurement, but you have tried everything obvious?

We intend to increase the strength of our pozzolan (from natural pozzolan) cement to 30MPa (28 days). But we could hardly reach it. What would the best thing to do?

There are two possible options. First you need to maximise the strength potential of the cement clinker you are blending with the natural pozzolan to make the pozzolanic cement. This means stabilising the clinker mineralogy and boosting the alite content. You can do that by lifting LSF or SM dependent on your current clinker chemistry. Another alternative would be to add calcium fluride to the raw mix. Secondly you could add a cement conditioner to activate the pozzolanic reaction in the cement. Possibly alkali carbonates or potassium sulphate.

Will an OPC concrete become less or more resistant to sulphates with addition of extra OPC.

I believe it will become more susceptible to sulphate attack if more cement is present as there will be more hydrated C₃A which can react with the sulphate. If you want to confer sulphate resistance to OPC then add ground granulated blast furnace slag.

I would like to know more about alkali silica reaction in the concrete. Kindly clarify the following. I am aware that alkalis in the cement will react with reactive silica of the aggregates and cause cracks. I would like to know how fast these cracks are likely to appear once the concrete is poured. Is it possible that within a day or two after pouring the concrete, cracks are likely to be surfaced due to ASR problem? Fly ash addition in cement is supposed to mitigate ASR problem in the concrete. Is there any reason due to which fly ash is likely to increase the ASR problem? What tests are to be conducted on fly ash, to find out the suitability for mixing in the cement? What tests are to be conducted on aggregates to understand the reactivity with alkalis present in the cement?

No, it is not possible for the alkali-silica reaction to cause cracking of concrete within a day or two of pouring the concrete. The expansion reaction caused by the alkali -silica reaction causes cracks to appear a number of years after the concrete. Fly ash addition should not increase the susceptibility to alkali-silica reaction. The pozzolanic reaction of the fly ash fills the pores of the concrete and reduces the tendency for cracking. However, the fly ash should be of low alkali content. You must test the type of silica present in the aggregates, reactive forms are opal, chalcedony, some types of quartz, cristobalite, tridymite and various siliceous glasses.

What is the significance of axial sampling for a finished cement mill ?

The principle of axial testing is to crash stop the mill, then go inside and take samples at 1m intervals along the axis of the mill. You need to dig down into the media to take the samples as the mill ventilation fan will pull some of the cement off the top of the media while the mill cools sufficiently for you to enter. The size distribution of the samples are then measured in the laboratory. You are looking for progressively finer material along the axis of the mill, reaching the desired fineness at the outlet diaphragm.