sulfate in clinker

Good day all.

In the pass, our raw meal and hence clinker had a negligible amount of alkali sulfate in it. The SO3 in the clinker was in the range of 0%-0.2%.

Recently, the SO3 in our clinker is now as high as 0.8%. We have wet kilns, how does this extra SO3 affects burnability of the kiln feed and the quality of the clinker?

Thanks.

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re sulfate in clinker

Hello Stephen,

A change from 0.2 to 0.8% SO3 in clinker should not affect the burnability of the clinker too much. If anything, it should slightly improve the burnability, as long as the additional SO3 becomes part of the liquid phase, but that would depend on the amount of alkalis present.

Normally, sulphur will first combine with K2O and Na2O to form alkali sulphates, then as the molar SO3/(K2O + Na2O) ratio starts to rise significantly above ~1, excess sulphur combines with CaO and K2SO4 to form calcium langbeinite, Ca2K2(SO4)3.  In the past, it was thought that any sulphur in excess of that required to form alkali sulphates and calcium langbeinite was present in clinker as anhydrite (CaSO4), but numerous studies have shown this to be incorrect. In fact, as the SO3/(K2O + Na2O) ratio rises higher than that necessary to form calcium langbeinite, sulphur begins to be incorporated into the clinker minerals instead, especially belite (C2S). (Belite can contain up to about 2% of SO3 in solid solution, while alite only about 0.5 - 0.8%.)

Only when alkali sulphate/calcium langbeinite formation and clinker mineral SO3 saturation have  occurred would any excess sulphur present form calcium sulphate anhydrite. In addition, this anhydrite is not normally in a sufficiently soluble form to be of much use in controlling set times.

The effect of this increase in SO3 on your clinker quality therefore depends on the alkali content, more specifically the molar SO3/(K2O + Na2O) ratio.

If the excess sulphur is present as soluble sulphates, then you would require proportionately less gypsum in your cement. However, if the excess sulphur is incorporated into belite, then it might have the following effects;-

1. Reduction in alite content of the clinker, since SO3 stabilises the belite crystal structure hindering the uptake of CaO during clinkerisation and thus retarding the conversion of belite to alite. (NOTE: This effect is counteracted by the addition of a proportional amount of fluoride)

2. Since belite is stabilised by SO3 it also retards the hydration of belite.

The result of both of these effects is that your early cement strengths might be lower. Whether this reduction in strengths is significant or not is another thing.

If you let me know the SO3, Na2O, K2O, C3S and C2S content of your clinker I might be able give you an estimate of the theoretical distribution of the sulphur phases.

Regards,

Ted.

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re sulfate in clinker

Clinker analysis with SO3:

• SO3 - 0.40%
• Na2O - 0.10%
• K2O - 0.61%
• C3S - 61%
• C2S - 14%
• Alite and belite based on bogue calculations

Clinker with very little SO3

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re sulfate in clinker

Hello Stephen,

Thanks for your clinker info. Based on these data I have estimated the distribution of sulphur in your two clinker types as follows;-

1. Clinker with very little SO3.

                                                                      (0.01 / 80.1)
Molar SO3/(K2O + Na2O) ratio =  ----------------------------------      = 0.02
                                                            (0.42 / 94.2) + (0.1 / 62.0)

With such a low SO3/(K2O+Na2O) ratio this clinker contains negligible alkali sulphates, meaning that the alkalis are almost totally present in solid solution with alite, belite and aluminate according to the following table;-

Percentage of non-sulphate alkalis in individual clinker phases.

ALKALI      ALITE        BELITE     ALUMINATE      FERRITE      TOTAL
Na2O          44%           17%            36%                   3%             100%
 K2O            29%            41%           27%                   3%             100% 

2. Clinker with increased S03 content.

                                                                       (0.4 / 80.1)
Molar SO3/(K2O + Na2O) ratio =  ----------------------------------      = 0.62
                                                            (0.61 / 94.2) + (0.1 / 62.0)

Although the SO3/(K2O+Na2O) ratio in this clinker has increased substantially, the ratio is still well  below 1.0 which means that the alkalis are predominantly present in the clinker as separate alkali sulphate phases. Therefore there would be very little SO3 incorporated into either belite or alite.

Based on this, I would say that the increased SO3 content is beneficial in your case, because of increased sulphatisation of the alkalis. This would promote early strengths and may also mean that you might be able to reduce gypsum content in your cement slightly. (by about 0.5 to 1%

On the negative side, there might be two issues;-

1. Increased soluble alkalis in the clinker might cause 28d strengths to be slightly lower.

2. Since the alkalis are now preferably combined with sulphur, rather than incorporated in the clinker minerals, the type of C3A in your clinker may have changed. Alkalis incorporated into C3A result in the formation of alkali aluminate, or orthorhombic C3A, which is the more reactive  form (faster hydrating) compared with cubic C3A, the low alkali form. This might result in faster setting times.

Have you noticed any significant changes to your cement properties with the higher SO3 clinker?

Regards,

Ted.

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