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User’s Guide to the
10 Basic Facts on Clinker
1 Raw mix rejects ................................................................................................................................ 1 1.1 ................................................................................................... 1 1.2 .......................................................................................................... 3 1.3 ................................................................................................... 4
2 Thermal profile ................................................................................................................................. 5 2.1 Port la Nouvelle ......................................................................................................................... 5 2.2 Le Teil ........................................................................................................................................ 6
3 Burning atmosphere and volatilization .............................................................................................. 7 3.1 Saint Constant ........................................................................................................................... 7 3.2 Brookfield .................................................................................................................................. 7 3.3 Le Teil ........................................................................................................................................ 8
4 Free lime content and setting time ................................................................................................... 9 4.1 Saint Constant ........................................................................................................................... 9 4.2 Bath ........................................................................................................................................... 9 4.3 Val d’Azergues ........................................................................................................................ 10 4.4 Woodstock .............................................................................................................................. 10
5 Clinker C3S content ........................................................................................................................ 11 5.1 Villaluenga kiln 2302................................................................................................................ 11 5.2 Villaluenga kiln 1501................................................................................................................ 13 5.3 Whitehall.................................................................................................................................. 13
6 Clinker C2S content ........................................................................................................................ 14 6.1 Ocumare.................................................................................................................................. 14 6.2 Karsdorf ................................................................................................................................... 14
7 Alkalies and 28-day strength .......................................................................................................... 16 7.1 Cements from ................................................................ 16 7.2 ....................................................................................................... 17
8 Alkalies and short term strengths ................................................................................................... 18 8.1 .................................................................................................................. 18
8.1.1 ......................................................................................... 18 8.1.2 .................................................................................. 18
8.2 ................................................................................................................... 19
9 Alkali saturation .............................................................................................................................. 20 9.1 ................................................................................................................................. 20
9.1.1 . .................................................................................................................. 20 9.1.2 Alkalies in solid solution. .................................................................................................. 20
9.2 Ranteil ..................................................................................................................................... 21 9.3 Sète ......................................................................................................................................... 21
10 Excess of sulfate with respect to alkalies ....................................................................................... 22 10.1 ..................................................................................................... 22
10.1.1 Sète 1971 ......................................................................................................................... 23 10.2 ..................................................................................................... 23
10.2.1 Meknès............................................................................................................................. 23 10.2.2 Sète .................................................................................................................................. 24 10.2.3 La Couronne .................................................................................................................... 24 10.2.4 ......................................................................................................................... 25
1-Raw mix rejects
______________________________________ 1 ________________________________
1
Reducing raw mix rejects lowers burning temperature and grinding energy. This is particularly the
case with siliceous rejects.
This action is also beneficial to strength properties.
Example: When the amount of 100µm rejects is reduced from 20 to 10%, the global « raw mix + cement » energy consumption is lowered by about 4 kWh per tonne of cement at a fineness of 350 m
2/kg.
Raw mix fineness is generally characterized by the weight of rejects in one or several sieves (a 100µm sieve is often used).
The following examples illustrate these three points: 1.1 Effect on burning temperature
Observed: in 1991, the purchase of a precrusher allowed the Contes plant to improve raw mix fineness. The burning temperature (pyrometer reading or kiln outlet temp.), whose measurement integrates all changes occurring in the burning zone, shows a poor relationship with rejects, whereas fuel consumption shows a good relationship, as demonstrated in Figure 1, taken from four typical periods.
55.5
56
56.5
57
57.5
58
58.5
8 12 16 20
Oil
(l/t Raw mix)
>100m residue (%)
Figure 1: Contes plant 1991
Several industrial raw mixes were characterized in lab burnability tests at temperatures between 1400 and 1550°C (1000C°/h heating rate and a 30 minutes hold point at final temperature), both « as is » and with regrinding of the rejects.
The two graphs that follow (Figures 2 and 3), which give the observed free lime as a function of temperature, show the effect of rejects regrinding. We can see that: :
the Martres raw mix, which is rich in quartz (> 10%), is harder to burn and more susceptible to fineness than the Karsdorf raw mix.
1-Raw mix rejects
______________________________________ 2 ________________________________
0.00
2.00
4.00
6.00
8.00
1400 1450 1500 1550 1600
% free CaO
temperature °C
Figure 2: Free lime after burning for different raw mix finenesses (Martres lab test)
0.00
2.00
4.00
6.00
8.00
1350 1400 1450 1500 1550 1600
16% > 100m industrial as is
8% > 100m lab re grind
% free CaO
temperature °C
Raw mix
Figure 3: Karsdorf lab test
the % rejects criterion is not sufficient, of itself, to determine a given raw mix’s burnability (different results for Martres according to whether the entire sample or only the rejects are reground).
1-Raw mix rejects
______________________________________ 3 ________________________________
1.2 Effect on grinding energy
In 1990, the Lexos plant had the chance to grind its raw mix to different finenesses (13% to 21% rejects at 100µm) over a long enough period of time so that the plant’s monthly averages could be considered as meaningful.
Figure 4 shows the increase in the raw mill’s power consumption with the increased fineness.
y = -0.4267x + 29.759
20
21
22
23
24
25
12 14 16 18 20 22
kWh/t
% > 100 µm residue
y = - 0.42 x + 29.7
r2 = 0.91
Figure 4: Raw mix grinding energy vs. fineness
A study1 was done to determine the cement grinding power consumption for clinkers that correspond to different raw mix finenesses and for various manufactured products. Figure 5 shows the rise in power consumption for cement grinding corresponding to increasing raw mix rejects for a CEM I 52.5 R cement.
55
60
65
70
75
12 14 16 18 20 22
kWh/t
y = 1.38 x + 37.8
r2 = 0,58
% > 100 µm residue
Figure 5: Clinker grinding energy vs. raw mix fineness
If we look at the end result, for the production of a CEM I 52.5 R, an additional 1% of 100µm rejects in the raw mix causes an increase in grinding energy of more than 0.5 kWh/t.
1 H. Geesen : Influence of raw meal fineness on cement grinding energy (Lexos plant)
1-Raw mix rejects
______________________________________ 4 ________________________________
1.3 Effect on mechanical strength
When raw meal fineness was changed at Contes2 (Figure 6), a small increase in strength at constant Blaine fineness was observed. This allowed the plant to slightly lower the Blaine (and increase mill production) while maintaining strength.
Raw CPA HPR
Month >100m >200m Blaine
cm2/g
t/hmill
1d 2d 28d
February 20.0 2.0 3850 49.0 22.5 35.7 70.0
March 13.0 0.8 3850 52.5 24.0 37.0 71.0
April - May 13.0 0.8 3680 55.5 22.6 37.0 70.0
MPa MPa MPa
Figure 6: Contes 1991
2 R. Dupont : Contes plant 1992
2-Thermal profile
______________________________________ 5 ________________________________
2 Thermal profile
A short profile promotes grindability and strength development.
Note: The optimum is achieved when the kiln torque is at the minimum value compatible with stable kiln operation.
By thermal profile, we mean the rate of heating and cooling of the product in the kiln and cooler. The « burning zone length » can also be assimilated to this concept.
The thermal profile is affected by a number of factors:
the raw mix burnability and the kiln’s heat consumption
the type of fuel and its preparation
the burner and its settings
cooler operation (via secondary air temperature)
kiln operation, especially the draught and fuel settings, but also
rotational speed
Generally speaking, it is somewhat difficult to compare a thermal profile from one kiln to the next. On the other hand, for a given system, several sensors provide readings as to the burning zone length: amps or torque of the drive motors, clinker temperature measured at the kiln outlet (or the NO in the kiln exit gases), temperatures in the preheater cyclones, pressure drop through the Lepol grate, shell scanners, etc.
For each kiln, the most representative indicator should to be determined and analyzed.
Two recent examples can be given :
2.1 Port la Nouvelle
Cooling zone temperature
45
50
55
60
65
70
75
1050 1100 1150 1200 1250 1300 1350 1400 1450
°C
kWh / t
BB
10
Figure 7: Grindability vs ; kiln cooling zone temperature (Port-la-Nouvelle)
2-Thermal profile
______________________________________ 6 ________________________________
45
50
55
60
65
70
75
600 700 800 900 1000 1100 1200 1300 1400 1500
Kiln drive torque
BB
10
(k
Wh
/t)
Nm
Figure 8: Grindability vs. kiln drive torque (Port-la-Nouvelle)
Weekly spot clinker sampling over a period of approximately one year. Simultaneous recording of kiln operating parameters. Clinker grindability measured by BB10.
A short burning zone, characterized by a low kiln torque value and high burning zone temperature, affords the best grindability.
2.2 Le Teil
30/05/1995 04:00 30/05/1995 12:00 30/05/1995 20:00 31/05/1995 04:00 31/05/1995 12:00
200
250
300
350
400
450
500
50
55
60
65
70
75
80
kWh / t
Kiln amps decrease
Grindability
improvement
Kiln amps
Figure 9: Grindability vs. kiln amps (Le Teil)
Spot clinker sampling during a SHTS (~CEM I 52.5 R) production test over a two-day period. Simultaneous recording of kiln operating parameters. Clinker grindability measured by BB10.
A lengthening of the burning zone early in test leads to a decrease in clinker grindability. A shorter burning zone (low kiln amp values) affords the best grindability.
The technical literature and lab studies point to the favorable impact of a short thermal profile on strength, as well as, the beneficial effect of rapid quenching for strength and workability. This information has not been verified industrially.
3-Burning atmosphere
______________________________________ 7 ________________________________
3 Burning atmosphere and volatilization
Steady production requires an oxidizing atmosphere because a reducing atmosphere increases
volatilization, causing both « cyclical » operations and sulfate and alkali fluctuations, hence
producing a non-uniform clinker.
Numerous industrial trials have demonstrated the effect of the burning atmosphere on volatilization, particularly for sulfur.
We can mention the correlation seen in the Sulfur Synthesis3 which was derived using the industrial data from 73 volatile balances:
vSO3 = K(K2O, % liquid phase) - 9.3 * O2%
Therefore, one %of oxygen equals roughly ten points of sulfur volatilization coefficient.
The standardization of high impulse burners and the overall increase in clinker sulfur levels has changed this relationship.
3.1 Saint Constant
The following results have been observed at the Saint Constant plant, where unground fluid coke with up to 2% rejects at 5mm is burned:
Fuel Kiln exit oxygen (%)
v SO3 (%)
Clinker SO3 (%)
Clinker K2O (%)
Emissions SO2 ppmV
Emissions SO3 ppmV
Fuel oil 100% 1.8 47.1 1.39 0.94 2 25
Coke 23.8% 2.4 59.6 1.6 1.04 7 19
Coke 47.7% 1.1 83 1.2 0.98 1278 43
Coke 47.7% 2.1 67 1.96 1.15 200 58
Figure 10: Sulfur emissions results at Saint Constant
It was observed that the kiln remained stable at all coke percentages. It is evident that the coarse particles in the coke burn on the clinker load: the reducing atmosphere that results shows no effect on kiln stability (because, in the case of a long kiln, the volatilized sulfur has an « exit » via the stack), but has an important effect on sulfur volatilization and increased SO2 emissions at the stack.
In this particular case, a 1% increase in the oxygen exiting the kiln, at constant coke input, translates into a 16 point decrease in the volatilization coefficient. The level of sulfates in the clinker is increased by 60%.
3.2 Brookfield
At Brookfield (long dry kiln), where raw mix SO3 is 1.5%, the kiln went through many cycles that were recognizable by:
the cyclical variation of kiln torque
the cyclical variation of clinker production, with surges every eight hours
variation in the clinker SO3 and K2O content : SO3 between 1% and 4%
Cyclical operations, with a period of about two hours, were observed on other long kilns such as Bath and Exshaw (kiln 4), where the raw mixes are rich in volatile elements.
One solution used to reduce the sulfur cycle is to divert part of the electrostatic precipitator dust (the finest and highest in sulfur content) away from the kiln circuit.
At Brookfield, some tests were carried out on the kiln exit oxygen values. An increase in the draught augmented the dust pick-up, which lead to a redesign of the chain section. A setting of 3.2% oxygen was decided upon instead of 1.9%. Of course, this had a negative impact on heat consumption, but a positive effect on the quantity of by-passed dust.
Kiln exit oxygen (%) 1.9 3.2 4
volatilization SO3 (%) 78 32
Dust eliminated (t/d) 90 40
Figure 11: Brookfield kiln 1
3 J.C. Guerche, Viviers, 1985.
3-Burning atmosphere
______________________________________ 8 ________________________________
3.3 Le Teil
The figure below shows the impact of oxygen levels on the volatilization of sulfates (clinker SO3 in the C4 material).
3
3,5
4
4,5
5
5,5
17/sep
09:00
17/sep
13:00
17/sep
17:00
17/sep
21:00
18/sep
01:00
18/sep
05:00
18/sep
09:00
18/sep
13:00
18/sep
17:00
18/sep
21:00
% O2 sortie tour
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
% SO3 C4
% SO3 clinker
O2 sortie tour
SO3 C4
SO3 KK
% O2 Preheater exit
% SO3 C4
% SO3 Clinker
% O2 preheater exit
% SO3 Clinker
% SO3 C4
Figure 12: Influence of the oxygen level at preheater exit on sulfate volatilization.
Figure 12 shows the switch from 60% to 100% coke on September 17 at 9 a.m. (beginning of chart).
In cyclone 4, one can see the increase in SO3 attaining 4% at 1400h. The clinker SO3 remains low. The sulfur introduced as a result of the additional coke does not leave the system: there is a risk of plugging.
At 1400h the preheater exit O2 was raised from 4.4% to 4.9%; the C4 material SO3 content goes down from 4% to 2.5%, the clinker SO3 content increases from 0.7% to 1.3%. The oxygen increase allows an acceptable sulfur balance to be reached with 100% coke thereby avoiding plugging in the cyclones, ring formation, etc.
4-Free lime
______________________________________ 9 ________________________________
4 Free lime content and setting time
Increasing the clinker free lime content reduces both initial and final setting
times in the same proportions.
Adding lime also accelerates both initial and final setting times.
Order of magnitude: When free CaO increases from 0.5 to 1.5%, initial set decreases by about 40 to
50 minutes. This impact may vary greatly from clinker to clinker4 (-10 to 100 minutes).
4.1 Saint-Constant
A burning intensity study undertaken with St. Constant’s long dry kiln, allowed the plant to compare the relationship between clinker free lime and initial setting time, as shown in the table below:
100
120
140
160
180
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Clinker free lime
Initial setting time (mortar)
( min. )
%
Figure 13: Setting time as a function of free lime
The correlation is non-linear: the effect setting time is a lot stronger between 0.4 and 0.8% free lime than beyond 0.8%.
4.2 Bath
At Bath, another long dry process, numerous tests were carried out to reduce initial and final setting times in response to a customer request. These tests were all aimed at increasing the free lime content to 0.9%: fineness, mix composition, kiln speed/feed ratio, etc. None of these tests gave the desired results.
The solution applied today consists of adding limestone at the kiln outlet, in the nose ring area. This addition could technically be done at the cooler inlet, however this would not be in accordance with ASTM standard which apply to part of the plant’s sales.
The results are as follows:
4 Lime quality (specifically its burning temperature, its hydration level, etc.) and clinker quality have an influence on the results obtained.
4-Free lime
______________________________________ 10
_______________________________
Sample SSB ( / kg)
free lime (%)
Setting time 23°C
(plant)
Setting time 23°C (CTS)
Concrete setting time
(h:min)
Concrete setting time
24°C / 2%CaCl2
Concrete setting time
10°C / 2%CaCl2
Con- crete set. time 24°C
Con- crete set. time 10°C
Reference 360 0.35 165 180 5:45 3:30 4:45 5:10 8:30
Test 1 357 0.7 125 110 4:55
Test 2 371 0.9 120 115 4:40 3:10 4:25 4:30 7:15
Figure 14: Injection of limestone at kiln outlet (Bath)
Tests 1 and 2 were done with an injection of different addition rates of limestone at the kiln outlet, as reflected in the different levels of free lime. The final setting times were not recorded (they were estimated based on initial setting times). The gain in concrete setting time at 10°C is especially noteworthy.
.
4.3 Val d’Azergues
Following a client’s request (BDI), in order to improve the reactivity of Val d’Azergues’ CEM I 52.5 R, particularly in heat-cured (HC) concrete, the level of free lime was increased by adding lime from different sources. The increase in short-term HC concrete strength is partially related to the initial setting time. The gain in short-term HC concrete strength (2h30) slightly penalizes the « long-term » (5h30).
Two types of lime sources were tested industrially:
The lime contained in the ash from Gardanne
The co-grinding of 5% ash containing 30% free lime and clinker results in a better performing cement with a setting time that decreases by 60 minutes.
Cooler limestone
Some trials with limestone injection (6/10 mm) were done in the cooler throat (at a temperature of 1250 to 1350 °C). By introducing 2.5% limestone, 0.95% added free lime was obtained and, for the most part, incorporated in the clinker. The gain in initial setting time was 40 to 60 minutes.
Normal + 5 % Limestone
Ash 1 2
Free lime % 1,10 2,10 2,90 2,50
Blaine cm2 /g 4320 4480 4600 4420
Setting time initial 175 110 110 135
(min) final 245 180 170 255
Strength EN 1d 24,7 25,3 24,5 24,0
(MPa) 28d 69,1 67,2 66,9 67,2
Heated concrete 2h30 0,5 3,8 6,0 3,8
80°C (MPa) 5h30 24,1 29,4 25,3 26,7
4.4 Woodstock
At Woodstock, a comparable test was carried out at the kiln outlet, where the effect was minimal. This could be related to the level of lime saturation, which is lower in this plant. This conclusion was also reached in the Delta-free lime study carried out by LCR: the decrease in setting time through an increase in free lime is most effective when the degree of lime saturation in the raw mix is high.
5-Clinker C3S
______________________________________ 11
_______________________________
5 Clinker C3S content
Increasing clinker C3S (to the detriment of C2S) improves strength at 1, 2, 3 and 7 days. After 28
days, the gain may be less because of the C2S’ contribution.
Order of magnitude: +10% C3S +2 to +5 MPa in the short and medium terms
5.1 Villaluenga kiln 2302
Villaluenga plant. Production period from February to June 1992.
Start up of kiln 2302 (kiln w/ AS precalciner): during this stage, there was a progressive increase in C3S (see Figure 15) which generated enough data to allow us to study its influence on strength.
40.00
45.00
50.00
55.00
60.00
65.00
7
/02/9
2
8
/02/9
2
9/0
2/9
2
1
2/0
2/9
2
3/0
3/9
2
1
0/0
3/9
2
1
2/0
3/9
2
1
7/0
3/9
2
2
4/0
3/9
2
3
1/0
3/9
2
0
7/0
4/9
2
2
1/0
4/9
2
2
9/0
4/9
2
0
5/0
5/9
2
1
3/0
5/9
2
1
9/0
5/9
2
26/0
5/9
2
0
3/0
6/9
2
1
0/0
6/9
2
% C
3S
Figure 15: Evolution of C3S (Villaluenga plant)
Characteristics of clinker produced:
The average clinker values are: C3S = 53.2, SR = 2.8, A/F = 1.75, SO3/alkalies molar ratio = 1.06. In addition to the variation of C3S, an increasing consumption of coke during the start up caused clinker SO3 to rise. This increase in SO3 resulted in an increase of the SO3/alkalies molar ratio from 0.8 to 1.24, and at the same time, a higher soluble SO3 content.
5-Clinker C3S
______________________________________ 12
_______________________________
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
7
/02/9
2
8
/02/9
2
9/0
2/9
2
1
2/0
2/9
2
3/0
3/9
2
1
0/0
3/9
2
1
2/0
3/9
2
1
7/0
3/9
2
2
4/0
3/9
2
3
1/0
3/9
2
0
7/0
4/9
2
2
1/0
4/9
2
2
9/0
4/9
2
0
5/0
5/9
2
1
3/0
5/9
2
1
9/0
5/9
2
26/0
5/9
2
0
3/0
6/9
2
1
0/0
6/9
2
% S
O3
Figure 16: Evolution of clinker SO3 (Villaluenga plant)
The strength increase with increasing C3S, at all ages is clearly shown in Figure 17 (clinker sample lab ground at 3600 SSB with total sulfates held constant).
50.0 55.0 60.0
C3 S
40.0
N/m
m2
15.0
20.0
25.0
45.0
30.0
35.0
45.0
50.0
55.0
60.0
y = 0.45x - 2.59
r2 = 0.431d
y = 0.55 + 15.51
r2 = 0.587d
y = 0.58x + 1.42
r2 = 0.672d
y = 0.42 x + 32.24
r2 = 0.3128d
Figure 17: Evolution of strengths as a function of C3S (Villaluenga plant)
1 day: gives a moderate correlation (r2=0.43) which improves in the case of multiple regression
using the proportion of sulfate in the clinker. Result: 4.5 MPa for 10% C3S
2 days: good correlation (r2=0.67). Result: 5.8 MPa for 10% C3S
7 days: good correlation (r2=0.58). Result: 5.5 MPa for 10% C3S
28 days: poor correlation (r2=0.31). Result: 4.1 MPa for 10% C3S
It must be noted, however, that the SO3 and C3S evolved in the same manner during those four months and it is therefore difficult to distinguish their individual effects and the impact of optimum sulfate addition. This may explain why the slopes of the linear regressions (1 day and 28 days in Figure 17) are very similar.
5-Clinker C3S
______________________________________ 13
_______________________________
5.2 Villaluenga 1501
Production in normal conditions for kiln 1501 (kiln w/ AS precalciner). This corresponds to the same period as the start up of kiln 2302 but the C3S is less variable.
Clinker characteristics: the average values are C3S =57.9, SR=2.7, A/F=1.73, SO3/alkalies molar ratio = 0.92.
The compressive strengths vs. C3S content for different ages are show in Figure 18:
15.00
20.00
25.00
30.00
35.00
40.00
45.00
50.00
55.00
60.00
50.0 55.0 60.0 65.0
C3 S
N/m
m2
y = 0.28x + 4.88
r 2 = 0.221d
y = 0.54x + 0.771
r 2 = 0.592d
y = 0.68x + 5.61
r 2 = 0.667d
y = 0.59x + 20.84
r 2 = 0.59
28d
Figure 18: Evolution of strengths as a function of C3S (Villaluenga plant, kiln 1501)
1 day: very weak correlation (r2=0.22). Result: 2.8 MPa for 10% C3S
2 days: good correlation (r2=0.59). Result: 5.4 MPa for 10% C3S
7 days: good correlation (r2=0.66). Result: 6.8 MPa for 10% C3S
28 days: poor correlation (r2=0.59). Result: 5.9 MPa for 10% C3S
Note the weak evolution of 1 day strength.
5.3 Whitehall
For reasons that have to do with the quarry, the Whitehall plant had to perform some clinker production tests with low C3S levels. The industrially produced clinkers were ground in the lab at constant sulfate addition. The table on the following page shows the test results.
0
5
10
15
20
25
30
35
40
10 20 30 40 50 60
C3 S
MP
a
1 d
3 d
7 d
28 d
Figure 19: Evolution of strengths as a function of C3S
6-Clinker C2S
______________________________________ 14
_______________________________
6 Clinker C2S content
For a given Blaine specific surface (SSB), grinding energy increases with C2S content. Conversely,
it decreases with increasing C3S.
Order of magnitude: +10% C2S, (or -10% C3S) +5 kWh/t (for 3500 c /g).
6.1 Ocumare
The Ocumare plant5 (FNC - Venezuela) recently modified its raw mix composition (February 1997) by increasing the lime saturation in order to improve clinker reactivity. All other parameters including the clinker free lime remained constant. The table below summarizes the situation over a consecutive two-month period, before and after changing the mix.
% January 1997 February 1997 Feb. - Jan.
LSF Clinker 94,50 98,60 + 4,10
C3 S 56 64 + 8
C2 S 22 15 - 7
C3 A 10,20 9,60 =
C4 A F 8,20 8,00 =
Free CaO 1,27 1,30 =
kWh/t BB10 46,70 44,40 - 2,30
SSB BB10 ( cm2/g ) 3680 3710 =
kWh/t Industrial 53,10 51,60 - 1,50
28 d. Industrial
(ASTM standards)
32,0 35,5 + 3,5
Figure 20: Evolution of grindability as a function of chemistry at Ocumare
The effect on grinding energy is lesser in the plant than in the lab, although it has the same tendency to decrease. If we use the figures obtained in the lab (calculated for 350 /kg), we can evaluate the drop in power consumption at 46.7 x (350/368)
1.5 - 44.4 x (350/371)
1.5 = 2.6 kWh/t for 7% less C2S, or:
- 3.8 kWh/t for - 10% C2S
Note that during the same period, the increase in C3S content resulted in a gain of 3.5 MPa at 28 days on industrial cement.
6.2 Karsdorf
Until 1994, only one clinker with high lime saturation (KST = 98) was used for the entire cement product range. At that time, the new European standards lowered the upper limit of 28-day strength of CEM I 32.5 to 52.5 MPa from its previous 55 MPa. Meeting this demand was not easy. The solution, which consisted of reducing cement fineness, caused problems with bleeding, which was unacceptable for the users.
5 Presentation given by L. Corda, J.A. Sbardella from Gerencia Desarollo y Procesos, Cementos La Vega, TYTP Combustion Meeting on April 15-16, 1997 at Yozgat.
6-Clinker C2S
______________________________________ 15
_______________________________
It was therefore decided to produce a second clinker less saturated in CaO (KST = 90), which gives less 28 day strength because of lower C3S content.
The results obtained6 are as follows:
KST = 90/92 KST = 96/98 Difference
% C3S 45 55 -10
% C2S 25 15 +10
KWh/t BB10 @ 3500 SSB 45 41 +4
The increase in the percentage of Belite causes the mill power consumption to increase by:
+ 4 kWh/t for + 10% C2S
It must be stated that this solution is not satisfactory firstly because of increased energy cost and secondly because the strong reactivity of the Belite at Karsdorf doesn’t allow for a significant reduction of 28-day strength in the cement considering the clinker’s high lime saturation. Figure 21 below shows the results of laboratory-ground cement made from two industrial clinkers with two different sulfate addition rates (2/3 gypsum 1/3 SH). Another method is being studied, which consists of modifying the sulfate addition of KST 98 clinker. The trouble here lies with the risk of rheological disturbances.
Figure 21: Laboratory cements made from Karsdorf clinkers
6 Presentation given by G. Cochet and G. Staupendahl at the CTI/CTEC Technical Days in Madrid, October 1996. « Study of clinker grindability at Karsdorf » by G. Cochet, CTI, September 1995.
7-Alkalies and 28-day strength
______________________________________ 16
_______________________________
7 Alkalies and 28-day strength
Alkalies7, whatever their form, are never favorable to 28-day compressive strength.
Order of magnitude: + 0.1 % Eq Na2O total -1 N/m at 28 days
It is usually very difficult to change the alkali content in a given plant without greatly altering other parameters, because the content in the individual raw materials tend to be relatively constant.
7.1 Cements from Lafarge Ciments and Lafarge Corp.
The measurements performed on 29 industrial cements8 (from Lafarge Ciments and Lafarge Corp. as well as French competitors) confirm the above relationship.
When the TOTAL alkalies increase from 0.2% to 1.8%, one notices that the mechanical strength at 28
days9 decreases from 66 MPa to approximately 45 MPa.
The regression equation indicates a loss of 1.3 MPa for an increase of 0.1% in total alkalies, with a correlation coefficient of 0.88.
The results are shown in the diagram below:
45
50
55
60
65
70
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
% Total Alkalies
28
-d
ay S
tren
gth
(M
Pa)
Figure 22: Influence of total alkalies on 28-day strength
7 The clinker alkalies may occur in different forms depending upon the degree of alkali saturation by sulfates. The total alkalies Na2O+K20 are present
in either or both of the following forms: Soluble alkalies
The alkalies combined with sulfate are Na2SO4, K2SO4, K2SO4(CaSO4)2. These compounds can make their appearance during industrial production when the fuel is changed from a natural gas to a sulfur-bearing fuel like bunker oil or petroleum coke or when gypsum is added to the raw mix. In this case, there will be more soluble alkalies at the expense of alkalies in solid solution. Alkalies in solid solution
The alkalies that are not combined with sulfates will enter the aluminate and silicate crystal lattices, modifying their reactivity. This can be a problem for the C3A (loss of workability) as mentioned in the ninth basic fact.
8 R. Guyot, R. Ranc, B. Cariou: « Sulfates Synthesis Report », June 1983.
9 at a constant Blaine and with sulfate addition optimized for 28-day strength.
7-Alkalies and 28-day strength
______________________________________ 17
_______________________________
7.2 Effects on a single clinker
Laboratory testing10 was carried out on a single clinker to measure the effects of alkaline sulfate addition in the water:
0
10
20
30
40
50
60
70
80
25 30 35 40 45 50 55 60 65
R 28j
R 28j + K2O
R 7j + K2O
R 7j
R 2j + K2O
R 2j
R 1j + K2O
R 1j
Rc MPa
0
10
20
30
40
50
60
70
80
25 30 35 40 45 50 55 60 65
R 28j
R 28j + K2O
R 7j + K2O
R 7j
R 2j + K2O
R 2j
R 1j + K2O
R 1j
Rc MPa
% C3S
7 d
2 d + K2O
7d + K2O
28 d + K2O
28 d
2 d
1d+K2O
1 d
Figure 23: Influence of alkalies on strengths
10 M. Debos, G. Chaudouard
8-Alkalies and short-term strengths
______________________________________ 18
_______________________________
8 Alkalies and short-term strengths
At optimum sulfate addition for early ages, soluble alkalies in the form of alkali sulfates improve
early strength.
Order of magnitude: + 0.1 % Eq. Na2O soluble + 0.5 to 1.5 N/m at 1 day
The soluble alkalies [Na2SO4, K2SO4, K2SO4(CaSO4)2] in the clinker are, as mentioned above, mainly in the form of K2SO4 although they are calculated on the basis of Na2SO4 equivalent.
There are two ways to increase soluble alkalies:
Increase the sulfates insofar as the alkalies are not yet saturated. In this case, the soluble alkalies will
increase. This can be done either via the fuel (ex. : gas bunker oil or low sulfur content oil high sulfur content oil) or via the raw mix (adding sulfates to the mix).
Increase the alkalies in the raw mix insofar as there are available sulfates. This can be done by using a siliceous sand that is rich in alkalies, for example (sea sand).
These two examples will be further investigated.
8.1 Increasing sulfates
8.1.1 Adding gypsum to the raw mix
Two industrial cases from the Ranteil and Sète plants can be cited as examples.
Plant % SO3 Clinker % K2OTotal
%Na2OTotal
SSB (m2/kg) 1d (MPa)
Ranteil As is 0.2 1.15 0.1 400 11.5
Gypsum addition 1.1 1.15 0.1 375 18.0
Sète As is 0.1 0.6 0.2 402 11.5
Gypsum addition 1.3 0.6 0.2 413 17.0
Figure 24: Sulfate addition to raw mixes at Ranteil and Sète
In both cases, it is very clear that an increase in the saturation of alkalies with sulfates (increase in soluble alkalies) leads to a very slight increase in 1-day strength.
In both cases with the gypsum addition to the raw mix, the C3A takes on a cubic form, and no longer the orthorhombic form which is the case in the presence of alkalies in the C3A crystal structure.
8.1.2 Increasing the fuel’s sulfur content
Shown in the table below are results from the Martres and La Malle plants during a switch from a fuel with a low sulfur content to a sulfur-rich fuel.
Plant % SO3 Clinker % K2O Total % Na2O Total SSB (m2/kg)
Martres Gaz 0.20 0.40 0.06 370 12.5
Fuel oil 1.00 350 15.5
La Malle low-s oil 0.60 0.95 0.10 360 18.0
Hi-s oil 1.00 340 21.0
1d (MPa)
Figure 25: Increase in fuel sulfur at Martres and La Malle
8-Alkalies and short-term strengths
______________________________________ 19
_______________________________
For both plants, when one operated with gas and the other with low-sulfur oil, the alkalies were not totally saturated. There would have to have been 0.40% and 0.93% of SO3 in the clinker to saturate the alkalies at the Martres and La Malle plants, respectively.
The switch to higher sulfur content oils in both cases produced the saturation of the alkalies and, therefore, an increase in the amount of soluble alkalies, with a positive impact on 1-day compressive strengths.
In the lab, the increase in the portion of soluble alkalies was also tested, confirming industrial results. In these laboratory tests, 1.85% of K2SO4 (=1% K20) was added to clinker, equivalent to a 1.85% addition of K2SO4 to the cement during the mixing with water.
The table below confirms once more the positive effect on early strength and the negative effect on long-term strength.
Term As is 1.85 % K2SO4 clinker 1.85 % K2SO4 cement
1 day 20.3 28.1 28.2
7 days 54.3 53.0 53.8
28 days 74.3 66.2 66.3
Figure 26: Strength of cements spiked with K2SO4 in the lab
8.2 Increasing alkalies
At the Retznei plant, in the past, we used an additional source of SiO2 containing alkalies with high amounts of Na2O between 2.2 and 2.5%, and K2O between 1.2 and 1.4%
Analysis of the raw mix at that time showed that the alkali content was the highest of all cement plants in Austria, with 0.45% of Na2O eq. (in the raw mix). Today, with a source of silica that is poorer in alkalies (Na2O = O and K2O = 0.5%), we have brought the Na2O eq. values down to around 0.3%.
The consequences of this drop in raw mix alkali content can be seen in terms of both short (lesson 8) and long-term (lesson 7) mechanical performances in the table below.
Rmeq Na2O=0.45% Rmeq Na2O=0.3%
Measured
(addition 16%)
Caldulated at
O% addition1
Measured
(addition 20%)
Calculated at
0% addition
1 day 17.1 0.1 21.1 0.1 13.9 0.12 18.5 0.12
28 days 46.8 0.19 57.8 0.19 47 0.23 62.7 0.23
Number of tests 143 100
Figure 27: Cement strength with varying levels of alkalies (Retznei)
Despite slight variability in the alkalies and the difference in cement composition (additive ratios of 16 and 20%), it remains possible to compare the results.
9-Alkali saturation
______________________________________ 20
_______________________________
9 Alkali saturation
The molar saturation of alkalies by SO3 in the clinker facilitates workability control.
9.1 Reminder
SO3 molar saturation: % SO3 = 1.29 (% total Eq. Na2O).
A low SO3/alkali ratio results in a small percentage of soluble alkalies and the presence of orthorhombic C3A.
All Portland clinkers11 contain alkalies in greater or lesser quantities. But depending on the nature of the fuel used (oil or petcoke rich in sulfur or low-sulfur coal), the alkalies can be found in two different forms.
When the clinker contains sulfates that come from the raw mix and/or fuel, a significant part of the alkalies are in the form of alkali sulfates. These alkalies are referred to as « soluble ».
Non-soluble alkalies are incorporated in the silicate or aluminate crystals structure
The sum of « soluble » alkalies and alkalies in the crystal strucutres is called « total alkalies ».
9.1.1 Alkali sulfates
Alkali sulfates improve initial strengths (1 and 2 days), but reduce long-term strength (28 days or more). There is no negative effect on the main cement properties.
9.1.2 Alkalies in solid solution
If the clinker does not contain enough sulfates to combine all the alkalies, then the alkalies enter the aluminate and silicate crystal structures.
The defects created in the C3A crystals modify the lattice and its morphology assumes an orthorhombic rather than cubic form. This transformation is accompanied by an increase in its reactivity with water.
The presence of alkalies in the crystal structure has numerous unfavorable effects :
change in burnability
rheological disturbances due to the slow formation of ettringites brought about by the hydration of very reactive orthorhombic C3A
strong sensitivity to weathering effects, which accentuates the rheological defects
expansion
increased shrinkage during the plastic state (24 hours)
increased shrinkage during drying at 28 days
inferior 28-day strength without any improvement in initial strengths
Given the fact that, in general, we are not able to control clinker alkali content, and given that alkali sulfates
present many advantages, it is imperative12 that the SO3/alkali molar ratio be >1.0.
There are very few recent examples because of:
the nearly-generalized use of sulfur-rich fuels for many years now (thus limiting the cases of orthorhombic C3A).
a lack of information on complaints or disputes from the plants that have SO3-poor clinkers (gas burning for example: Venezuela) for which the rheological control of cement and concrete is done either sporadically or not at all.
some tests for raw mix sulfate addition were not carried out under well controlled conditions (Gabon).
11 M. Debos, G; CHAUDOUARD. « Portland Cement: chemistry - mineralogy - properties of phases - reactivity » (June 1991).
12 R. Ranc. « Influence of alkalies on the physical and mechanical properties of Portland cements ». Sept. ‘93.
9-Alkali saturation
______________________________________ 21
_______________________________
The above observations are taken from examples of industrial and lab testing which compare the properties of normal clinkers without sulfates and clinkers made from a sulfate-rich raw mix as the result of the addition of gypsum.
These tests were all carried out for the purpose of improving the rheological properties of cement (by eliminating the causes of false set).
9.2 Ranteil
The table below presents the laboratory results13 (burning and grinding) due to the addition of gypsum to the raw mix at Ranteil (non-saturated alkalies).
% raw mix% gypsum SO3 kk
SSB
( cm2/ g ) SO3 cem. W / C
Fluidity (%)
1dMPa
Clinker as is 1000
0,35 3830 2,0 28,6 88 6,5
Sulf. raw mix 97,952,05
1,40 3770 2,50 26,0 112 10,5
96,03,95
2,40 3920 2,90 26,2 125 12,5Sulf. raw mix
Figure 28: Lab testing (burning and grinding)
The cements prepared from the clinker made from a raw mix with gypsum addition are more fluid than the control clinkers. The higher the SO3 clinker content, the better the fluidity appears to be.
.
Crystal form C3A
As is Orthorhombic ( alkalies in crystal )
Cubic ( pure ) Sulf. raw mix
9.3 Sète
Industrial tests14 were carried out in 1970:
SO3kk K20 tot. Na2O tot. SO3SSB Résidue17(mm) 1d
after 2 min after 30 min MPa
As is 0,1 0,37 0,10 2,80 3510 14 29 11,5
Cement (sulfated kk) 2,9 0,56 0,09 2,90 3500 10 19 16,0
(cm²/g)
Figure 29: Results of raw mixes with gypsum addition at Sète
Probe penetration tests show an improvement in the rheological characteristics of pure pastes and mortars that come from clinker where gypsum has been added to the raw mix.
13 Ray. Allègre. Study on the influence of the addition of gypsum to the raw mix at Ranteil (1972)
14 Ray. Allègre. CB N.20 « IDSG industrial trial at the Sète plant » - OS 11445 March 1971.
10-Excess of sulfates
______________________________________ 22
_______________________________
10 Excess of sulfates with respect to alkalies
If clinker SO3 is increased beyond the molar saturation of alkalies, an increase in both clinker
fineness and grinding energy is noted.
Order of magnitude: +1% Excess SO315 + 5 kWh/t at 350 /kg
If the clinker excess SO3 is increased beyond the molar saturation of alkalies, the following are observed:
an increase in clinker fineness
an increase in grinding energy
Each of these points will be dealt with separately.
10.1 Increase in clinker fineness
This observation has been reported time and time again, but has never been really quantified through granulometric analysis (see 1995 raw mix sulfate addition tests for Gabon). Nevertheless, the industrial testing done at Ciments Lafarge in the 1970s showed that the presence of sulfates in the clinker beyond the saturation of alkalies leads to a dustier clinker.
Below are shown three examples from industrial testing done with raw mix sulfate addition in the Sète and Ranteil plants.
0
20
40
60
80
100
0 5 10 15 20 25
mm
% p
ass
ing
0,1 % S03
1,3 % SO3
Figure 30: Comparison of clinker granulometries (Ranteil plant 72-73)
15 excess SO3 = SO3 clinker - 1.29 (% total Eq. Na2O)
10-Excess of sulfates
______________________________________ 23
_______________________________
10.1.1 Sète 1971
0
20
40
60
80
100
0 5 10 15 20 25 30 35
mm
% p
as
sin
g
0,1 % SO3
2,9 % SO3
0
20
40
60
80
100
0 10 20 30 40 50 60
mm
% p
ass
ing 0,1 % SO3
1,2 % SO3
Figure 31: Comparison of clinker granulometries (Sète plant 1971)
Figure 32: Comparison of clinker granulometries (Sète plant 197116)
It is likely17 that in these tests, there was a significant volatilization factor due to a poorly controlled burning atmosphere and that the increase in fines could (partially) be the result of alkali sulfate volatilization. Recent tests have shown that the burning zone length has a significant effect on clinker particle size and that the level of SO3 in the kiln load influences burnability.
10.2 Increase in grinding energy
« An excess of SO3 in the clinker beyond the saturation of alkalies worsens grindability. »
This is a fact that has been observed and reported time and again in numerous documents.
The attempts to quantify the impact of clinker SO3 (total SO3, excess SO3, etc.) on grindability have been numerous, both industrially and through laboratory and statistical studies.
For the statistical studies, the results are influenced by the parameters taken into consideration. Some results are reported below.
For the studies carried out in the plants, the recent increase in high sulfur fuel usage should give us more data. These data are difficult to exploit, however, insofar as the clinker SO3 parameter was not the only one to fluctuate (burner, combustion, optimized mill operations, fineness changes, changes in cement additive ratios, etc.).
10.2.1 Meknès
At the Meknes18 plant, a gradual switch to coke was made without any major changes to equipment, raw mix or products.
16 Ray. Allègre / CB – 20 OS n° 11445 – « Industrial test JDSG at the Sète plant » March 1971
17 M. Debos « Sulfates conference (L'Isle d'Abeau) : Influence of sulfates on kiln operations » Nov. 1993
18 Meknes plant. « Market reports » 1994/95
10-Excess of sulfates
______________________________________ 24
_______________________________
coke coal fuel SO3 kkK2OT Na2OT C/K Mill kW
CPJ 45F2 F3
1994 47,37 51,01 4,62 0,45 0,75 ~ 0,35 # 0 1,222 38,52
1995 74,14 21,30 3,56 0,95 1,18 ~ 0,35 # 0 1,237 40,29
Figure 33: Results of an increase in sulfur (Meknès plant)
* CPJ 45 = CEM II 32.5
These results show that despite a slight increase in the additive ratio (limestone, easier to grind than clinker), cement grindability (CPJ 45) decreases with an increase in the percentage of coke.
10.2.2 Sète
In the industrial study16 on sulfate addition to the raw mix at the Sète plant (1971), cement grinding trials were carried out both in the lab and in the plant.
The results are expressed in terms of:
output for the industrial trials
mill rotations and SSB (Blaine specific surface) for the lab trials.
SO3 kk K2OT Na2OT SO3 cem Prod. SSB # of mill rev
Industrial testskk normal 0,1 0,37kk sulfated 2,9 0,56
0,10 2,8 21 3150 -0,09 2,9 17 2700 -
Lab.testskk normal 0,1 0,37
kk sulfated 2,9 0,56
0,10 - - 2880 2000
0,09 - - 2580 2000
t/h cm²/g
Figure 34: Cement grinding results for raw mixes with gypsum addition (Sète plant)
10.2.3 La Couronne
The use of petcoke at the La Couronne19 plant produced an increase in clinker SO3 content and a reduction in terms of pure cement production rate from the various mills.
1991 1992
SO3 kk 0,80 1,45
K2 O 0,89 0,92
Production t / h
(B0) CPA 55
(B3) CPA 55
(B3) CPA HPR
21,6
19,3
14,2
20,3
18,2
13,2
Figure 35: Results of sulfur increase at La Couronne
The study done on 12 clinkers at La Couronne shows that the SO3/alkalies ratio correlates with clinker grinding energy.
W4000 SSB = 5.44 SO3 /alkal. + 55.7 r2
=0.67
with: SO3 /alkalies between 1 and 4
19 M. Debos « Sulfates conference Nov. 18-19, 1993. : Grindability » Nov. 1993
10-Excess of sulfates
______________________________________ 25
_______________________________
10.2.4 Cantagalo Clinker
On a series of 17 industrial clinkers20 taken from the Cantagalo plant, it was determined that lab grinding energy is correlated with clinker SO3 (or with the excess of SO3), according to the equation:
W (# of mill rotations BB 10) = 1008 SO3 kk + 3250 r2
=0.66
Figure 36
3400
3600
3800
4000
4200
4400
4600
4800
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
SO3 kk
rota
tion
s B
B10
y = 1008 x + 3255r 2 = 0.66
Figure 36: Grindability as a function of SO3 (Cantagalo plant)
20 P. Barriac : « Cantagalo clinker », May 1995
