PROPERTIES OF FLUIDIZED BED BURNT OIL SHALE ASHES
Lembi Merike Raado,
Ehitajate tee 5, 19086 Tallinn, Estonia
Tallinn University of Technology
lemera@staff.ttu.ee
Fax: 372 6202453
Phone: 372 6202452
Viive Nurm,
Ehitajate tee 5, 19086 Tallinn, Estonia
Tallinn University of Technology
Keywords: Cement, burnt oil shale, structure
Introduction
Oil shale is a major natural resource of Estonia. The power plants use oil shale as a
fuel despite its low calorific value of 8-14 MJ/kg /1/. Estonian oil shale consists
approximately of 55-65% of inorganic matter. The combustion process takes place in
the boilers of pulverized fuel at the temperature of 1300oC. Burnt oil shale has been
collected by dust collecting systems, the finest fraction by the electric precipitators.
In the renovation process, the high temperature system is being replaced with
fluidized bed firing. The firing temperature of a flying bed boiler is about 800oC. This
leads to a sustainable change in the quality of the resulting ash. The combustion
temperature of this boiler is about 800o C. Thus, the properties of the resulting fly ash
undergo serious changes.
This paper presents the results obtained from the testing of oil shale fly ashes from
fluidized bed combustion as well as their possible use as a constituent of Portland
cement.
Burnt at 1300oC oil shale and Portland oil shale cement
Since 1960 burnt at 1300oC oil shale has been used as the main constituent of
cement in the Kunda cement plant production. EN 197-1 sets requirements to the
Portland cement containing burnt oil shale described as CEM II strength classes
32.5; 42.5 and 52.5. Fig.1 presents the level of Portland cements production with
burnt oil shale as the main constituent.
Burnt oil shale is not only the main constituent that reduces the clinker content of
Portland cement, it also gives Portland cement some unique properties.
Besides clinker minerals C2S and CA (about 13%) and active glassy phase 33%,
the burnt oil shale contains free CaOfree 10%, CaSO4 17% and an insoluble residue
27%/2/. Minerals containing iron and alumina oxides melt in the extremely rapid
combustion process at temperature of about 1300oC. As a result, fine spherical
particles are covered with a melt glassy layer. As a consequence, burnt oil shale as
the main constituent of Portland cement reduces the water requirement of the cement
paste.
In a fresh mortar, the fine spherical particles of burnt oil shale covered with a glassy
layer have a so-called ball bearing effect: water requirement for the same standard
consistency is reduced. Thus, burnt oil shale has both plasticizing and puzzolanic
properties.
The content of clinker minerals formed in the high temperature firing process gives an
extra amount of tobermorite gel during the hydration process, reducing the amount of
the capillary pores of cement stone /3/.
1500000
1200000
900000
CEM I
600000
300000
CEM II
0
1960
1970
1980
1990
2000
Figure 1 Production of Portland burnt oil shale cement in Estonia since 1960
The burnt oil shale, as the main component of Portland cement, is characterized by
EVS 636:2002 /4/ as a fine product of combustion containing less than 12.5% of free
CaOfree, less than 0.42% of Cl´ and a specific surface by Blaine 350 m2/kg. Ground
burnt oil shale reaching specific surface about 500m2/kg, tested by EN 196-1, must
show a compressive strength higher than 25.0 N/mm2.
Thus, as the main component of CEM II has a lower water demand and it differs from
all of the main components described in EN 197-1. The high temperature burnt oil
shale has also a higher CaOfree content than provided by the recommendations. The
finest particles of CaOfree are evenly distributed in the mass of the material and
consequently, do not induce abnormal expansion. The numerical value of soundness
has to conform to EN 197-1.
60
Compressive strength 28days, N/mm
2
58
56
54
52
50
48
0
15
20
25
Content of burnt oil shale in cement%
Figure 2 Compressive strength of mortars made with various high temperature burnt
oil shale (PP) content in Portland oil shale cement
Fig. 2 shows the effect of the high temperature burnt oil shale constituent from 0 up
to 25% in cement on the compressive strength. Mortar prisms were made with an
equal water-cement ratio of 0.5. It will obviously increase the compressive strength
despite an accompanying excess of water. Standard consistency of the tested
cement paste was as follows:

CEM I - 27.9%

Cement CEM II containing 20%PP - 26.4%.
Tests with burnt oil shale from fluidized fuel boilers
The first pulverized combustion boiler has already been replaced with a fluidized
state combustion boiler. Both types of the burnt oil shale from the electric
precipitators were studied using the methods of chemical, x-ray and REM analysis.
Fig.3 shows the results of distribution by size of 400 particles of the studied burnt oil
shale material.
40
40
%
%
30
30
20
20
10
10
0
0,4 0,8 1,2 1,6
2
2,4 2,8 3,2 3,6
4
4,4 4,8 5,2 5,6
6
6,4 6,8 7,2 7,6
8
0
0,5
1
1,5
size,um
a
Figure 3 Distribution by particle size:
a - burnt at 800oC oil shale ash (F)
2
2,5
3
3,5
4
4,5
5
5,5
6
6,5
7
7,5
size,um
b
b - burnt at 1300-1400oC oil shale ash (PP)
Both of the analyzed materials had a particle size characteristic of an electric
precipitator material - 10m. Fig.3 a and b show almost a similar distribution of
particles by the size of burnt oil shale from electric precipitators of both boiler types.
It was found that about 67% of oil shale PP (b) burnt at high temperatures has a
particle size of 0.5-2m, whereas about 89% of low temperature oil shale (a) have a
size of 0.5-2m. High temperature burnt oil shale has more particles of size 2.56.5m.
a
b
Figure 4 Results of REM analyses:
a- burnt at 800oC oil shale ash (F); b- burnt at 1300oC oil shale ash (PP)
Figure 4 a and b show different shapes of pulverized oil shale burnt at high (PP) and
low combustion temperatures (F) gathered in an electric precipitator. Particles of
high temperature combustion are spherical and have a smooth surface, see Fig. 4 b.
Particles of oil shale burnt at 800oC have no clear geometrical form and no smooth
surface was detected (Fig. 4 a).
8
Fig. 5 shows the mineralogical composition of the material burnt at various
temperatures. The results of X-ray analyses show similarities and differences:

Both of them have high peaks of SiO2. Low temperature material obviously
has a higher peak of SiO2 (Fig.5A).

Low temperature material (Fig.5A) shows a higher peak of non-decomposed
CaCO3 than a high temperature material (Fig.5B).

PP(B) has clearly formed peaks of clinker minerals 2CaOxSiO2 and
mCaO xnAl2O3. The peak of 2CaOxSiO2 does not occur in a low temperature
material (Fig.5A).

The peak of mCaO xnAl2O3 occurs in the case of both materials.

The peak of CaSO4 has the same intensity in the case of both materials
studied.

Both of the materials have equal peaks of CaOfree.
The results of chemical analysis showed a high content of CaCO3 only in the case
of a low temperature material. Thus, the decomposition degree of CaCO3 of oil
shale from a fluidized bed boiler is lower. X-ray analysis results of a low
temperature material (Fig. A) show a higher peak of the non-decomposed CaCO3.
Chemical analysis shows that the decomposed CaCO3 content is almost three
times higher than in high temperature material. The amount of the decomposed
CaCO3 depends on the field of the electric precipitator and is about 12% CaCO3.
For a high temperature material it is about 5%. The content of CaOfree a in a low
temperature material is 9-2.7%. The last fields of electric precipitators have a very
low of CaOfee content - less than 3%. The content CaOfree in high temperature
material is about 9%, accordingly. The EVS 636:2002 Standard sets a limit value
of CaOfree ≤12.5% for the high temperature burnt oil shale (PP) used as the main
constituent of cement.
Fig.5 shows a higher content of clinker minerals in high temperature burnt oil
shale (PP). In our tests the content of the clinker minerals, 2CaOSiO2 and
CaOAl2O3, in PP was 11 and 2%, accordingly. The content of the same minerals
in the low temperature material F was 5 and 0.5%, accordingly.
2-Theta scale
Figure 5 Mineralogical composition of the burnt oil shale from
4040
3030

A - low temperature fluidized bed combustion boiler at 800oC (F) and

B - high temperature pulverized fuel combustion boiler at 1300oC (PP)
2020
1010
00
by x-ray analyses.
One of the positive properties of the used high temperature burnt oil shale PP is its
content of CaSO4 - 10-12%, which permits replacement of gypsum as a constituent of
Portland cement. The low temperature material F has a variable content of CaSO4: in
the first fields, it is as low as 7-8%. In the last, fourth field, it is equal to the PP
content of CaSO4, at about 12-15%.
Importantly, the content of glassy phase varies, thus the glassy phase content of PP
is about 35%, in F of the first three fields up to 25%, in the fourth field - about 32%.
Pozzolanic properties of various burnt oil shale were tested using the method of
reaction of SiO2 with OH- from a saturated Ca(OH)2 solution.
A decrease of the OH –concentration in a saturated lime solution during 30 days was
observed. Reaction ability was measured in milligrams of CaO to the mass of burnt
oil shale in grams. Table 1 shows higher pozzolanic reaction ability of low
temperature materials (F). The strong reaction ability of a low temperature material
can be explained by a higher content of reactive SiO2. Fig. 5A demonstrates that an
obviously higher peak of SiO2 occurs in the case of a low temperature material F.
Table 1 Pozzolanic properties of various ashes as a reaction with a saturated
Ca(OH)2 solution
Burnt oil shale
Fluidized bed equipment
800o (F)
 I field
 II
 III
 IV
Pulverized fuel
combustion at
>1300oC (PP)
Bound from saturated
solution as CaO, mg/g
CaO free of tested ash,%
19.2
25.1
46.7
91.7
14.85
8.7
7.9
5.0
2.4
9.1
Table 2 gives the test results of the standard consistency and the setting times of
cements made with the studied materials conducted in compliance with the Standard
EN 196-3. The standard consistency of common Portland cement CEM I is higher
than that of cement CEM II, containing 20% of a high temperature material PP,
whereas cement consisting of 20% of a low temperature material F reveals a
relevant rise in water requirement. Results of initial and final setting times show a
decrease in the velocity of the setting process in the case of a low temperature
material.
Table 2 Standard consistency and setting times of studied cements
Portland cement
CEM II/A
CEM II/A
CEM I
(20%F)
(20%PP)
Standard consistency,
140
170
132
ml
Initial setting time , h :min 2:58
3:16
3:07
Final setting time, h :min
3:58
4:03
3:48
The compressive strength of Portland and Portland burnt oil shale cements with a
20% content of both of the studied burnt oil shale was tested by the preparation of
standard mortar bars using the methods of EN 196-1.
Results in Fig.6 show no significant differences in the alteration of compressive
strength of the tested cements despite an equal water cement ratio of 0.5 used.
Tests confirm that the low temperature burnt oil shale tested might be characterized
as a typical pozzolanic constituent for cement:
Compressive strength, N/mm
2
60
50
40
30
2
7
14
28
Time of hardening,days
CEM I
CEMII/A-T800
CEM II/A-T
Figure 6.Kinetic alteration of compressive strength of various cements.

It has a high content of SiO2, i.e, it has a pozzolanic reaction ability,
simultaneously.

It contains a large amount of non-decomposed CaCO3.

The water requirement of cement is increased when oil shale is burnt at 800o
C.

Further studies are needed to analyze the usage of oil shale burnt at 800o C
as the main component of Portland cement or material for other building
materials.
References
/1/ Kikas, W. Composition and binder properties of Estonian kukersite oil shale ash,
ZKG International, vol.50, 1997, No.2,pp 112-125,
/2/ Kikas, W., Ojaste,K., Raado,L, Cause and mode of action of the silica reaction in
concretes made with Estonian Portland oil shale cement, ZKG International, vol.552,
1999, No.2,pp 106-111
/3/ Raado,L, Laur,T., Corrosion resistance of Portland cement CEM II/B-T stone and
structural characteristics based on the requirements of the European standard,
Modern Building Materials, Structures and Techniques, Proceedings of the 7th
International Conference, 2001, Vilnius Lithuania, CD ROM pp.1-6;
/4/ EVS 636:2002, Burnt oil shale for production of Portland burnt oil shale cement,
Portland composite cement and masonry cement
X-ray and REM analysis of the combustion products are
made by The Centre for Materials Research of TUT