We have a VSK unit with 300tpd production. The raw mix contains limestone, clay, cokebreeze. We are getting continuously high free lime. Please advice the procedure through which we can reduce the free lime.

There are a number of reasons why you might be getting continuously high free lime in your clinker. The most likely ones are: (i) there is too much limestone in your raw mix, (ii) the raw mix is not ground to sufficient fineness, or (iii) the temperature in the kiln is not high enough to finally combine the free lime with the clay minerals. The problem might be a combination of all these factors.

We regulate our raw material pile at 105-110 LSF. However the LSF of the raw mill's sample becomes 85-90. How can we have such a large difference?

There might be a problem in the sampling station. More likely you have a problem of segregation of the material. This is most likely when you are reclaiming the end-cones of the pile or in the intermediate feed bin between the pile and the raw mill. One solution is to add the end-cone to the next pile rather than sending it to the raw mill. The intermediate bin should be kept at a high level by setting the reclaimer start signal to a high bin level.

We are carrying out research on blended cements with a view to increase the utilisation of blast furnace slag/fly ash. In this context, we are exploring use of high energy mills (Jet Mill, Vibratory Mill etc) together with traditional milling devices. I wish to ask:
1. How important is the strength of cement? There is considerable amount of literature on high performance cement/energetically modified cement ie cement with compressive strength in the excess of 90 Mpa and concrete with greater than 145 Mpa.
2. What could be the major limitations in using mills such as Jet Mill in cement industry? We understand these mills consumes lots of energy but it is also true that the efficiency of traditional mills decreases as we approach the target size. Should not the combination work better?
3. I am looking addresses of supplier for lab size jet mill and vibratory mill (2-10 kg/h) for the grinding of cement clinker, BF slag and fly ash.

The strength of the cement is important as the hydration of the cement is responsible for all the early strength development of blended cements. The hydration of the cement minerals also releases the calcium hydroxide which activates the pozzolanic reaction of the fly ash or slag. Jet mills are not used in the cement industry due to the operating costs and the throughput capabilities of the mills. Large cement plants employ mills capable of producing tens por hundreds of cement per hour. I don't think jet mills can reach that capacity? For a laboratory mill my first port of call would be ELE (Engineering Laboratory Equipment).

I am confused by some of the terminology associated with clinkerisation and what is meant by the Bogue method etc. Can you simplify a little please?

C₃S is the mineral providing the early strength of the cement. If it rises the one-, two-, three- and seven-day strength will rise and vice versa. C₂S is the mineral providing the later strength. If it rises then this should rise, however if it rises at the expense of C₃S then you will lose early strength.
C₃A causes the initial setting of cement. However hydrated C₃A is susceptible to attack by sulphates in the ground water. Sulphate resisting cement therefore limits the C₃A content to a maximum of around four per cent.
C₄AF is important for manufacturing cement clinker as this is the first mineral to form a liquid in the kiln and therefore flux the clinker formation.
The Bogue method of calculating the content of these minerals is only another way of mathematically representing the oxide content of the clinker or cement. LSF, SM and AM are also just further methods of representing the oxide composition of the clinker or cement. High LSF means high C₃S. High SM means high total C₃S and C₂S. Low AM means low C₃A.

I recently had a new driveway poured and we have encountered a "blistering/popping" problem in several areas of the concrete. An engineer from the cement delivery plant says the defects are caused by lime chips that contaminated several tanker loads of cement mix. My questions are: Should I be concerned with structural integrity of the concrete because of the defective spots caused by the lime chips? Is there any way of knowing how long the blistering/popping may continue?

This problem is a bit out of the ordinary nowadays, although it seems to have concerned people a century or so ago, when cement manufacturing methods were less sophisticated and coarse particles of various materials could end up in the product. The damage is caused by the contaminant material reacting with water and forming a hydration product which is of larger volume than the starting material and therefore needs to create space in which to expand.
Finely-ground high-calcium lime would not cause any problem, as it reacts rapidly with water and expands while the concrete is still plastic. Dolomitic lime (containing significant amounts of Magnesium) is well known to give problems because it reacts slowly (weeks rather than hours), and its acceptable level is therefore limited in national standards for cement. From what you describe, it appears that you have contamination from lime which is both coarse and relatively unreactive (perhaps overburnt, dense and highly crystalline calcium lime?). You should ask the supplier to confirm that the lime is not dolomitic. If it turns out that it is dolomitic, then you can expect defects to continue to develop during the coming months and you should insist that they keep the matter open and monitor developments.
For the more likely case of high-calcium lime, there are two extreme scenarios. (a) If there is not a great deal of it in any region of the concrete and it has now more or less finished reacting, mainly affecting surface appearance, then that will be the end of the matter. You can complain to the supplier about the sub-standard appearance and demand that it is either put right or you are given a rebate. (b) On the other hand, if there is a good deal of this material in at least part of the bulk of the concrete, then its expansion may weaken the structure and shorten its working life - not that a domestic driveway is a very demanding structure in terms of its duty and of strength requirements (not like an airport runway, for example).
You should seek reassurance from the supplier that they know whether scenario (a) or (b) is correct. You can ask if they have test data or experience from which you can be assured that your driveway will be fit for normal use for as long a period of years as you had originally expected. They should be able either to replace or repair the driveway now or - because they are confident about their product - to give you a written promise that they will put it right in the future if it continues to deteriorate, perhaps with a commitment to inspect it with you in (say) a year's time. Alternatively, you might get a rebate on the price due to the substandard quality.
(If you want to know more, you are best enquiring via a concrete information service, rather than through us, as our readership is mostly concerned with making and trading in cement, not concrete.)

There are two kinds of alkalis contained in clinker-soluble and insoluble alkalis. Please explain about the formation processes in the kiln and the effects of two alkalis respectively on clinker quality, especially on setting and compressive strength.

Alkalis in clinker will preferentially combine with chloride and sulphate, the alkali chloride and sulphate formed is volatile and leads to the build up of the alkali cycle in the kiln. Alkali sulphates are less volatile than chlorides and some pass out of the kiln in the clinker. These are the source of the soluble alkali in the clinker. They promote the hydration of the cement and lead to higher early strengths. If alkali is present in excess of the sulphates in clinker then it enters into the clinker minerals. These are the "insoluble" alkalis. They do affect the hydration behaviour of the cement as they affect the clinker mineralogy, however effects are not as clear as for the soluble alkalis.

We determinate free lime by X-ray, but when we make a program to determinate it. This program has a life period about one month, after that, the program starts to give wrong results and we can?t calibrate it, because the problem becomes bigger. What can we do to eliminate this problem and what can we to do to improve the program.

I presume you are using X-ray diffraction to determine the free lime but the problem is that the calibration drifts with time. I suggest that you measure the free lime by traditional titration once per day. The program can then be modified to continuously update the relationship between the free lime and the XRD intensity using the last 20 results. In this way the calibration will be continuously updated and there will be no need to make a special recalcibration once per month.

We're suffering from brown clinker. As the results of microscopic analyses, there surely should be reducing condition in the burning of the kiln. So we increased the cooling air flow into the kiln to supply sufficient oxygen. It's getting better but we still have brown clinker. We can't increase the cooling air rate anymore because we already operated the exhaust fan with fully opened condition. So we're considering reducing the coal flow rate. It means clinker production should be decreased. What's the next methods we can do except reduction of throughput production to avoid the brown clinker.