K2O & P2O5

CemNet Training - Course CKPC01

Module 2.6

Cement Kiln Process Chemistry Module 2. Cement clinker quality and composition. 2.6 K2O barrier, SO3, P2O5 decomposition.

Cement Kiln Chemistry

2.6 K2O Barrier

In the preceding session of the course we saw that the clinker mineral composition can deviate from that calculated according to Bogue due to the cooling rate of the clinker. We also introduced the possibility that the presence of minor components which prevent combination of the clinker minerals, or decompose the clinker minerals after they have formed can affect the potential mineral composition. In particular three species which are present in cement clinker with some regularity, K2O, SO3 and P2O5, were suggested to have these effects.

Cement Kiln Chemistry

2.6 K2O Barrier

The alkali metals, K2O and Na2O, are always present in cement clinker, usually derived from the clay components in the raw mix. We learned in Module 1 that K2O will preferentially combine with chloride in the hot meal, and that any excess will then preferentially combine with sulphate to form K2SO4. In clinker there will be very little chloride or KCl, due to the volatility of chlorides at the temperatures in the burning zone of a cement kiln, K2O in clinker will therefore be preferentially combined as sulphate, K2SO4. K2O levels are normally (~3x) higher than Na2O levels in clinker and provided sufficient sulphate is available, the alkalis will be combined in the double salt K3Na(SO4)2. in clinker.

This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com

CemNet Training - Course CKPC01

Cement Kiln Chemistry

Module 2.6

2.6 K2O Barrier

The alkali metals, K2O and Na2O, are always present in cement clinker, usually derived from the clay components in the raw mix. Complications arise when there is insufficient sulphate available to fully combine with the alkalis, K2O and Na2O…… ……in this case the alkalis enter the clinker minerals. Both alkalis can be taken into solid solution in the main clinker minerals, C3S, C2S, C3A and C4AF…… ……although, as we saw in session 1.6, they are preferentially taken into solid solution in the C2S rather than the C3S and this explains their inhibiting effects on C3S formation. Solid solution:

K2O

Na2O

C3S

0.1~0.3%

0.1~0.3%

C2S

0.2~1.0%

0.3~1.0%

Cement Kiln Chemistry

2.6 K2O Barrier

The alkali metals, K2O and Na2O, are always present in cement clinker, usually derived from the clay components in the raw mix. Complications arise when there is insufficient sulphate available to fully combine with the alkalis, K2O and Na2O…… ……in this case the alkalis enter the clinker minerals. Both alkalis can be taken into solid solution in the main clinker minerals, C3S, C2S, C3A and C4AF…… ……both alkalis are taken into solid solution in the C3A mineral and to a much lesser extent the C4AF. Solid solution:

K2O

Na2O

C3A

0.4~1.1%

0.3~1.7%

C4AF

0.0~0.1%

0.0~0.5%

Cement Kiln Chemistry

2.6 K2O Barrier

The alkali metals, K2O and Na2O, are always present in cement clinker, usually derived from the clay components in the raw mix. Complications arise when there is insufficient sulphate available to fully combine with the alkalis, K2O and Na2O…… ……in this case the alkalis enter the clinker minerals. Alkalis in excess of sulphate and that that can be taken into solid solution in the main clinker minerals, C3S, C2S, C3A and C4AF form modified clinker minerals, NaC8A3 and KC23S12. Essentially one unit of Na2O is substituting for 1 unit of CaO in ……and one unit of K2O is every third unit of C3A…… substituting for CaO in every 12th unit of C2S.

This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com

CemNet Training - Course CKPC01

Cement Kiln Chemistry

Module 2.6

2.6 K2O Barrier

The alkali metals, K2O and Na2O, are always present in cement clinker, usually derived from the clay components in the raw mix. Complications arise when there is insufficient sulphate available to fully combine with the alkalis, K2O and Na2O…… ……in this case the alkalis enter the clinker minerals. Alkalis in excess of sulphate and that that can be taken into solid solution in the main clinker minerals, C3S, C2S, C3A and C4AF form modified clinker minerals, NaC8A3 and KC23S12. It has been reported that NaC8A3 and KC23S12 have greater …and hydraulic reactivity that C3A and C2S…… therefore that setting time is shortened and strength …but other researchers report their development promoted……. hydraulic activity as essentially the same and suggest simply adding NaC8A3 to C3A and KC23S12 to C2S.

Cement Kiln Chemistry

2.6 K2O Barrier

The alkali metals, K2O and Na2O, are always present in cement clinker, usually derived from the clay components in the raw mix. Complications arise when there is insufficient sulphate available to fully combine with the alkalis, K2O and Na2O…… ……in this case the alkalis enter the clinker minerals. Alkalis in excess of sulphate and that that can be taken into solid solution in the main clinker minerals, C3S, C2S, C3A and C4AF form modified clinker minerals, NaC8A3 and KC23S12. Clearly formation of new clinker minerals NaC8A3 and KC23S12 …C3A is not taken into consideration in the Bogue calculation…… and C2S will be reduced by formation of these new …but if their hydraulic behaviour is minerals……. comparable and NaC8A3 is simply added to C3A and KC23S12 to C2S then this is perhaps not important.

Cement Kiln Chemistry

2.6 K2O Barrier

The alkali metals, K2O and Na2O, are always present in cement clinker, usually derived from the clay components in the raw mix. Complications arise when there is insufficient sulphate available to fully combine with the alkalis, K2O and Na2O…… ……in this case the alkalis enter the clinker minerals. The main problem is that the mineral KC23S12 is not available for combination with CaO in the final sintering reaction to form C3S. When K2O is substantially in excess of sulphate then a significant proportion of the potential C2S in clinker can be blocked from forming C3S in this way…… ……the silica which is tied up in KC23S12 is unavailable to combine with free CaO and the remainder of the clinker is significantly over-limed, possibly resulting in high free lime clinker.

This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com

CemNet Training - Course CKPC01

Cement Kiln Chemistry

Module 2.6

2.6 K2O Barrier

This effect can be illustrated by an example from a cement factory where the K2O content of clinker is substantially in excess of the SO3. The excess K2O in clinker is given by the formula: % Excess K2O = %K2O - %SO3 * 80/94 If the excess K2O is then presumed to combine as KC23S12 then the content of this mineral which will be present is given by: % KC23S12 = % Excess K2O * 2102/94

↑

A very large factor due to only one unit of K2O being present in the high molecular weight KC23S12.

Cement Kiln Chemistry

2.6 K2O Barrier

This effect can be illustrated by an example from a cement factory where the K2O content of clinker is substantially in excess of the SO3. SiO2 combined in the mineral KC23S12 is not available to combine with further free CaO and form C3S. % Available SiO2 = % SiO2 - %Excess K2O * 720/94 % Residual CaO = % CaO - %Excess K2O * 1288/94 By this methodology the effective lime saturation of the clinker was lifted by 5.5%.

Cement Kiln Chemistry

2.6 K2O Barrier

This effect can be illustrated by an example from a cement factory where the K2O content of clinker is substantially in excess of the SO3. Resulting in significant periods when the clinker was uncombinable due to real LSF well above 100%. Kiln 2 clinke r LSF correcte d for combination of K2O in KC23S12

1.15

1.1

Corrected LSF

1

Conventional LSF

0.95

0.9

Date

27

30

18

24

15

12

21

3

6

9

30

21

24

27

18

15

12

3

6

9

25

31

22

28

16

19

7

10 May

13

1

0.85

4

Lime Saturation

1.05

July

This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com

CemNet Training - Course CKPC01

Cement Kiln Chemistry

Module 2.6

2.6 K2O Barrier

This effect can be illustrated by an example from a cement factory where the K2O content of clinker is substantially in excess of the SO3. The excess K2O in clinker is given by the formula: % Excess K2O = %K2O - %SO3 * 80/94 If the excess K2O is then presumed to combine as KC23S12 then the content of this mineral which will be present is given by: % KC23S12 = % Excess K2O * 2102/94 This methodology presumes all excess K2O will form KC23S12 and ignores solid solution……… ….however it provides a real example of the problems KC23S12 formation can cause….. ….this kiln did suffer long periods with excessively high free lime content in the clinker produced.

Cement Kiln Chemistry

2.6 P2O5 Decomposition

Some limestones used for cement manufacture contain ….this has certainly been reported in Uganda phosphates….. and other central African limestones. The effects of phosphates on cement kiln performance and clinker composition have become topical in Europe with the widespread burning of meat and bonemeal following the BSE crisis. Bonemeal injection lance.

Cement Kiln Chemistry

2.6 P2O5 Decomposition

Some limestones used for cement manufacture contain ….this has certainly been reported in Uganda phosphates….. and other central African limestones. The effects of phosphates on cement kiln performance and clinker composition have become topical in Europe with the widespread burning of meat and bonemeal following the BSE crisis. Bones are formed from calcium phosphate.

This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com

CemNet Training - Course CKPC01

Cement Kiln Chemistry

Module 2.6

2.6 P2O5 Decomposition

Some limestones used for cement manufacture contain ….this has certainly been reported in Uganda phosphates….. and other central African limestones. C3S can take up to 0.5% P2O5 into solid solution with no loss of hydraulic properties. Beyond that level there is progressive decomposition of C3S in favour of a solid solution of C2S and tricalcium phosphate, C3P. If the C3P content rises beyond ~5% then C3S is no longer a stable phase and completely breaks down to C2S, C3P and CaO. Portland cement clinker must contain C3S and high free CaO content is also unacceptable.

Cement Kiln Chemistry

2.6 P2O5 Decomposition

Some limestones used for cement manufacture contain ….this has certainly been reported in Uganda phosphates….. and other central African limestones. C3S can take up to 0.5% P2O5 into solid solution with no loss of hydraulic properties. It is reported that adequate clinker can be produced with P2O5 content up to 2~2.5%..... ……..however, breakdown of C3S into C2S and C3P has to be accepted…... ……..C3S will be lower than calculated by Bogue and C2S will be higher. As the C3P content rises the β, α’ and α C2S are progressively stabilised.

Cement Kiln Chemistry

2.6 P2O5 Decomposition

Some limestones used for cement manufacture contain ….this has certainly been reported in Uganda phosphates….. and other central African limestones. C3S can take up to 0.5% P2O5 into solid solution with no loss of hydraulic properties. As the C3P content rises the β, α’ and α polymorphs of C2S are successively stabilised. The polymorphic forms of the clinker minerals will be discussed in the next session of the course, however β-C2S is the only form with appreciable hydraulic properties…… ……..avoiding the transition to α’ and α-C2S, with attendant loss of C2S hydraulic reactivity, are therefore further reasons to limit the P2O5 content of clinker.

This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com

CemNet Training - Course CKPC01

Cement Kiln Chemistry

Module 2.6

2.6 P2O5 Decomposition

Some limestones used for cement manufacture contain ….this has certainly been reported in Uganda phosphates….. and other central African limestones. These adverse affects of P2O5 are somewhat offset if fluorides are present, as can be the case with phosphate bearing limestones. Fluorapatite, C10P3.CaF2, then enters solid solution in the C3S allowing a higher proportion of P2O5 to be present before the progressive decomposition of C3S to C2S and C3P sets in… …..chloroapatite, C10P3.CaCl2 can also increase tolerance of P2O5 in clinker…….. …..and has the added advantage of binding chloride in C3S and reducing the cycles of chloride that build up in a cement kiln.

Cement Kiln Chemistry

2.6 SO3 Decomposition

We have already discussed the effects of excess SO3 over alkalis in session 1.9….. ….this balance between alkalis and sulphates is a topic we will keep returning to. One of the potential effects of excess sulphates is similar to the affect of P2O5 on clinker composition. We saw in session 1.6 that SO3 preferentially enters into solid solution in the C2S and for this reason is an inhibitor of C3S formation………. ……..this solid solution of SO3 in C2S stabilises the β-C2S polymorph and makes combination with CaO and conversion into C3S more difficult…………..the breakdown of C3S into β-C2S and CaO below 1250°C is also accelerated…….. ……..low C3S content and high C2S and free CaO content in clinker can be the result.

Cement Kiln Chemistry

2.6 SO3 Decomposition

We have already discussed the effects of excess SO3 over alkalis in session 1.9….. ….this balance between alkalis and sulphates is a topic we will keep returning to. One of the potential effects of excess sulphates is similar to the affect of P2O5 on clinker composition. We saw in session 1.6 that SO3 preferentially enters into solid solution in the C2S and for this reason is an inhibitor of C3S formation………. ……..this solid solution of SO3 in C2S stabilises the β-C2S polymorph and makes combination with CaO and conversion into C3S more difficult. Solid solution of minor components in the clinker minerals and polymorphic form have been recurrent themes ……..those will be throughout this session…….. the topic of the next session of the course

This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com

CemNet Training - Course CKPC01

Module 2.6

Cement Kiln Process Chemistry Module 2. Cement clinker quality and composition. 2.6 K2O barrier, SO3, P2O5 decomposition.

This document is for the sole use of students enrolled on course CKPC01 and cannot be reprinted, reproduced or distributed without prior written consent from Tradeship Publications Ltd © http://Training.CemNet.com