Mai Uibu, P. Somelar, L-M. Raado, N. Irha, A. Koroljova, R. Kuusik

Participation is made possible by funding from the Baltic-American Freedom Foundation (BAFF).

For more information about BAFF scholarships and speaker support, visit

www.balticamericanfreedomfoundation.org.

» The Estonian kukersite oil shale (OS) is the largest industrially exploitable OS

resource in the world (annual mining output 14-16 Mt)

˃ Combustion of OS in electric power plants → CO2 emissions, ca 13 Mt

→ waste ash (OSA), ca 5-7 Mt

˃ OSA could be used as a readily available and cheap Ca-source for CO2

sequestration.

» Industrial wastes as well as CO2 emissions could be reduced by increasing the use

of supplementary cementitious materials (fly ashes, silica fume and steel slag) in

concrete

˃ - Slower strength development;

˃ - Optimal replacement percentage is typically 20-50%;

˃ + could be used as controlled low-strength material (<8.3 MPa) for structural

filling applications.

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» For the oil shale mining room and pillar method is used. This technology assures

acceptable stability of the ground and minimum subsidence but causes ca 30% loss.

» Oil shale ash can be used as binder and oil shale enrichment waste can be used as

aggregate material to backfill the mined chambers.

» The load on a pillar does not exceed 4 MPa at the excavation depth of 60 – 80 m in

case of 50-80 vol-% of backfill in a room.

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The hydrous transformation of free lime (CaO), anhydrite (CaSO4), secondary Ca(Mg)-silicate

minerals and amorphous Al–Si glass phases control the cementation of OSA, mainly by the

formation of secondary Ca-rich hydrate phases and by the carbonation of portlandite .

» Hydration of lime: CaO + H2O → Ca(OH)2

» Formation of carbonates: Ca(OH)2 + CO2 →CaCO3 + H2O

» Formation of gypsum: CaSO4 + 2H2O → CaSO4 2H2O

» Formation of ettringite as the initial strength giving phase (stable pH=10.7-13.0):

» 3Ca(OH)2 + 2Al(OH)3 + 3CaSO4 + 26H2O → Ca6Al2(SO4)3(OH)12 26H2O

» Formation of CSH as the main strength giving phase:

» 2Ca2SiO4 + 4H2O → 3CaO ·2SiO2 ⋅⋅⋅⋅3H2O + Ca(OH)2

» SiO2 + Ca(OH)2 + H2O = CaO ·SiO2 ·2H2O

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The study was focused on

» elaborating the strength development, mineralogy and

leaching characteristics of different types of OSA-based

concretes;

» optimizing the composite blends in order to utilize the

OSA flows from PF and CFB boilers as well as the solid

residues from accelerated carbonation process for mine

backfilling.

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Composite mix OSA based binder: type and content, % Aggregate

content, %PFEF1 PFCA CFBEF1 gPFCA cPFA cCFBA

PFEF1 50 50

PFCA 50 50

CFBEF1 50 50

MIX 25 25 50

MIXg 25 25 50

CARB1 12.5 25 12.5 50

CARB2 12.5 25 12.5 50

CARB3 12.5 12.5 25 50

ASH

Gas output

pH,

EC

cPFA,

cCFBALiquid phase recycle

Flue gas input

(15%CO2)

pH=11,5-12 pH=7,5pH=9pH=11-11,5

Uibu, M., Velts, O., Kuusik, R.

Developments in CO2 mineral

carbonation of oil shale ash. 2010.

Journal of Hazardous Materials

174, 209-214.

» Concrete constituents were mixed in a laboratory concrete mixer.

» Water was added to gain equal consistency, with a slump of 10-12 cm.

» After 48 hours concrete cubes were demoulded and further hardening was carried out at 95±5%RH and 20±2oC.

Phase composition, %

PFEF1 PFCA/gPFCA CFBEF1 cCFBA cPFA

Amorphous content 59.7 32.2 23.6

Quartz SiO2 3.8 4.9 17.4

Calcite CaCO3 6.1 11.9 9.4

Anhydrite CaSO4 6.9 7.6 8.7

Lime CaO 11.3 21.6 8.5

Adularia KAlSi3O8, 1.2 1.7 13.9

C2S Ca2SiO4 4.1 7.7 2.9

Also: Hematite , Periclase, C4AF4CaO*Al2O3*Fe2O3, Merwinite , Illite , Melilite

Chemical composition, %

CaOfree 17.9 22.5 9.1 0.9 4.5

CO2 4.3 5.3 3.4 19.9 18.1

Physical parameters

BET SSA m2/g 1.1 0.9/1.5 3.7 19.4 11.3

Particle size dmean, µm 17.0 55.8/40.3 32.5 32.8 50.2

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» Hydration, setting and hardening processes are influenced by the OS firing temperature

(1400oC in PF and 800oC in CFB) and dust cooler type;

» Water resistance is initially low, but in later stages the pozzolanic reactions are promoted

by leaching;

» The PFCA as a partly dead burnt air binder has no durability in water, but ground PFCA

could be used as a lime activator in mixed binders;

» Carbonated OSA as a neutral binder constituent in combination with PF ashes.

hydraulic

binder

air

binder

pozzo-

lanic

binder

Improved

by grinding

EC → 300–950 µS/cm

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21,7

14,6

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CARB3

Ettringite

Hydrated Ca-

Mg silicate crust

Ettringite

CARB3

PFEF1 leached 180d

PFEF1 leached 24h

PFEF1 leached 180d

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In order to synthesize the strength giving phases (CSH)

» Slaked lime is added to the pozzolanic material

» or high-Ca ash is added to the low-Ca ash

In our case ground PFCA is added to CFBEF1:

pH↑→H2SiO42-→ CSH → compressive strength↑

Tobermorite gel,

Pore size ∅∅∅∅↓Pore size ∅∅∅∅↑

» The major ash flows from PF and CFB boilers could be utilized as controlled low-

strength materials for mine backfilling;

» Carbonated OSA → neutral binder constituent in combination with PF ash;

» OSA-based concretes → initially low water resistance

→in later stages strength development promoted by leaching;

» PFCA composite →no durability in leaching conditions, but ground PFCA could be used

as a lime activator in mixed binders;

» Compressive strength →1–5 MPa after 7 days

→maximum >25 MPa after 28 days;

» OSA-based concretes → bind CO2 in curing and leaching conditions to form calcite at

the expense of portlandite and ettringite;

» The pH and EC of the leachates crossed or were close to the limits set for general

wastes in the earlier stage of curing but decreased considerably after 28 days

(pH<11.5; EC<2500µs/cm);

» In order to utilize the major ash flows from PF and CFB boilers and carbonated ashes

as concrete constituents, the composite blends must be optimized.

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» The financial support of the Archimedes foundation

(project 3.2.0501. 10.0002), Estonian Ministry of

Education and Research (SF0690001s09,

SF0140082s08, IUT33-19) and the Estonian Science

Foundation (Grant No 9334) are gratefully

acknowledged.

» Participation is made possible by funding from the

Baltic-American Freedom Foundation (BAFF). For more

information about BAFF scholarships and speaker

support, visit

www.balticamericanfreedomfoundation.org.

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