There is no definite interval between the initial and the final set of cement that denotes good or bad quality or workability. You must bear in mind that the test for setting time is conducted at a constant temperature in the laboratory. This is not the case when the cement is used in practice. In cold countries the setting time extends in practical applications and therefore a short setting time is best in these conditions. In hot countries (such as the Sudan) the setting time is shortened in practice and therefore a longer set in the laboratory conditions would be desirable.

Please advise the chemical and physical properties of:
1. Oil well cement
2. Sulphate resistant cement
3. Slag cement
4. Fly ash cement
Can sulphate resistant cement be replaced with any of the other three cements mentioned above?

1. Oil well cement comes in different grades (API Grades A to J inclusive).They are designed to be slow setting and therefore have very low C₃A contents and are coarsely ground. Some grades also contain added retarders. As they have low C₃A content they are sulphate resisting and could be substituted for SR cement, however the setting time would be long.
2. Sulphate Resisting cement has a limit of four per cent on the C₃A content to prevent attack by sulphates on the hydrated C₃A.
3. Slag cement has up to 65 per cent granulated blast furnace slag added to the cement clinker and gypsum. The slag content undergoes the pozzolanic reaction and this enhances the impermeability and durability of concrete made from the cement. This makes the concrete resistant to sulphate attack and these cements can be substituted for SR cements.
4. Fly ash cement contains up to 35 per cent fly ash added to the cement clinker and gypsum. The fly ash undergoes the pozzolanic reaction and this enhances the impermeability and durability of concrete made from the cement. This makes the concrete resistant to sulphate attack and these cements can be substituted for SR cements.

How does one reduce P₂O₅ in phosphogypsum to make it useful for application in the cement industry?

I'm sorry but I don't know how to reduce the P₂O₅ in phosphogypsum. It is inherent in the material and therefore I cannot envisage a means to do that which would economic for its subsequent use in the cement industry.

I would be very interested to hear any pointers you might have about where how best to get started on using microscopy as a further tool for examining clinker ? Where training might be obtained ? Anyone you might recommend thatI could contact and who could possibly come on site and provide this training ?

An article describing the work we have been doing in Slovakia will appear in the February 2003 issue of International Cement Review. In part this describes how microscopy is used in combination with pattern recognition in hour-to-hour control of the real mineralogy of clinker.

I own a 400tpd cement plant. Because of a recent reshuffle in government, the state is now asking us prove our pollution-free state of running. How can I prove the technology is a pollution-free one.

You need to begin the process of obtaining ISO 14001 certification for environmental management systems. There are various consulting companies who should be able to help you with this process.

I wish to know if the test of consistency of cement pastes has any significance as regards the quality of cement, eg a cement of lower consistency (<25 per cent) is better than a cement of higher consistency (> 25 per cent water). Secondly: it is observed that most of international cement standards set limits only to mortar compressive strengths although in practice the main usage of cement is in concrete applications. May I know why is that?

To calculate the velocity of the gases at the mill fan inlet you need the static pressure in addition to the dynamic pressure. If the mill fan has a capacity of 133m³/s then you need to divide this by the cross-sectional area of the inlet ducting to arrive at the design gas velocity of the fan

I've been working with analysers for the cement plants for last nine years and have still a little doubt about the actual use of measurement of NO_x in the kiln inlet. If you can kindly send me some literature about the firing in the kiln and the advantages of measuring various gases at the kiln inlet.

The NO_x level at the kiln inlet is a very good indication of the burning zone temperature in the kiln. In turn this is one of the most important considerations in the control and optimisation of the kiln. NO_x arises from two sources, (i) organic nitrogen in the fuel, and (ii) oxidation of the atmospheric nitrogen in the combustion air drawn into the kiln. This oxidation of the atmospheric nitrogen rises in direct proportion to the temperature in the flame and the burning zone.

I want to know what is Roslin Ramler slope and characteristic value for cements? Do they have any relationships and how do you understand the fineness by using them

The Rosin-Rammler-Sperling-Bennett particle size distribution for cement is found by measuring the residue on various sieves (25,32,45,63,90 microns etc.) Alternatively you can use laser granulometry or some other method of measuring the residues. You then plot the natural log of the sieve sizes on the x axis (ln(sieve)), against the double natural log of 100 divided by the residue on the y axis (ln(ln(100/Residue))). The reason for using these axes is that for fine powders such as cement the plot approximates to a straight line. The slope of the line gives a measure of the "tightness" of the particle size distribution and varies from 0.8 to 1.1 for cements, dependent on the type of equipment used for grinding. The characteristic grain size is the sieve size where 36.8 per cent of the cement would be retained. This can be calculated from the slope and intercept of the RRSB line and is typically between 20 and 30 microns for cement.