Characterization of CFBC fly and bottom ashes-based sodium carbonate-activated binders

J. Santos1, E. Cifrian1, J. Dacuba1, J. Fernandez-Ferreras1, C. Pesquera1, I. de Pedro2, A. Andres1.1Department of Chemistry and Process & Resource Engineering, ETSIIT, University of Cantabria, Cantabria, Spain 39005 2Department of CITIMAC, Faculty of Sciences, University of Cantabria, Cantabria, Spain 39005

Mayor oxides (wt%) Fly Ash Bottom AshCaO 45.83 30.54SiO2 5.17 1.75Al2O3 4.71 0.86Fe2O3 0.70 0.44MgO 0.57 0.21SO3 42.94 66.18K2O 0.02 0.01TiO2 0.06 0.01

Chemical compositions (expressed as oxides)

INTRODUCTION

CHARACTERIZATION OF CFBC ASHES

Circulating fluidized bed combustion (CFBC) has been considered more energy-efficient among availabletechnologies, and it is the most used technology in the world. The chemical composition andcharacteristics of coal ashes generated in CFBC boilers differ significantly from conventional boilers, sincecoal is fired at relatively lower temperatures and a large amount of limestone is used for desulfurization.CFBC process generates two types of ashes, fly and bottom ash, with very different mineralogicalcomposition, chemical composition, and morphology.

Sample Solid Liquid

Ratio

FA/BA

Ratio FA /

Na2SiO3

Ratio FA /

Na2CO3

WG0 1/0 0,33 0,27

WG2 1/0,2 0,396 0,324

WG4 1/0,4 0,462 0,378

WG6 1/0,6 0,528 0,432

WG8 1/0,8 0,594 0,486

WG10 1/1 0,66 0,54

WG20 1/2 0,99 0,81

WG30 1/3 1,32 1,08

WG40 1/4 1,65 1,35

Mixtures of alkali-activated binders developed:

ALKALI-ACTIVATED BINDERS

0

5

10

15

20

25

30

35

WG0 WG2 WG4 WG6 WG8 WG10 WG20 WG30 WG40

com

pres

sive

stre

nght

(MPa

)

Sample

14 días 28 días

METHODOLOGY

CHARACTERIZATION OF CFBC BINDERS

CONCLUSIONS

REFERENCES

This work was funded by the University of Cantabria“Proyecto Puente 2017” through a grant (PI A. Andres) underSODERCAN and ERDF Regional Operational Programme2014-2020.

ACKNOWLEDGEMENT• S. Siddique, H.Kim, J.G. Jang. Properties ah high-volume slag cement mortar incorporating circulating fluidezed bed combustion fly ash and bottom ash. Con.

Build. Mater. 289 (2021) 123150.• S.-H. Lee, G.-S. Kim, Self-Cementitious Hydration of Circulating Fluidized Bed Combustion Fly Ash. J. Korean Ceram. Soc. 54, 2(2017)128-136.• M. Chi, Synthesis and characterization of mortars with circulating fluidized bed combustion fly ash and ground granulated blast-furnace slag. Cons. Build. Mat.

123 (2016) 565–573

The aim of this paper is to analyse the possibilities of preparing sodium carbonate-activated binders fromblends of CFBC fly and bottom ashes including as additional binder sodium silicate to develop bettertechnical properties, as compressive strength.

Compressive strength

CaO + H2O -> Ca(OH)2 (1)

Ca(OH)2 + Na2CO3 -> 2NaOH + CaCO3 (2)

CaSO4 + 2H2O -> CaSO4.2H2O (3)

CaSO 4 + Na2CO3 -> CaCO3 + Na2SO4 (4)

xCa(OH)2 + SiO2 + (y-x)H2O -> CxSHy (C-S-H) (5)

X-ray diffraction-XRD

Thermogravimetric and Differential Thermalanalysis-TG-DTA

Scanning electron microscopy-SEM

Fourier-transform infrared spectroscopy-FTIR

1. CHARACTERIZATION OF CFBC ASHES- Chemical compositions (XRF)- Mineralogical Compositions X-ray diffraction (XRD)- Particle Size Distribution- Scanning electron microscopy-SEM

2. DEVELOPING CFBC BINDERS- Solids: Different FA/BA Ratios

- Liquids: Commercial Na2SiO3 + Na2CO3

3. CHARACTERIZATION OF CFBC BINDERS- Compressive strength- Mineralogical Compositions X-ray diffraction (XRD)- Scanning electron microscopy-SEM- Thermogravimetric and Differential Thermal analysis-

TG-DTA- Fourier-transform infrared spectroscopy-FTIR

Na2CO3

FA BA

- The chemical composition of the ashes, shows a very smallamount of Si and Al to perform an alkaline activation.

- FA is formed mainly by Ca and S, in the form of Anhydrite(CaSO4) and Lime (CaO).

- The composition of BA is mainly S and Ca, in the form ofAnhydrite (CaSO4).

- The particle size distribution shows a mean size of the BA of0.45mm while that of the FA is less than 0.05mm.

- This difference in particle size is also observed in the SEMimages.

- Both ashes show great heterogeneous morphology, farfrom spherical shape of traditional coal combustion flyashes.

FA BA

200µm20µm

- Liquid /Solid =0,6- Concentración Na2CO3 6M- Na2SiO3/Na2CO3=1,2- Total mass 300gr

- Samples WG0 without BA develop 25MPa of strenghtat 28 days

- Samples with FA/BA ratio around 1 present the bestbehaviour

- WG0, WG10 and WG40 were selected for furtheranalysis

- Presence of a broad peak between 25 and 35° (2θ)that corresponds to the amorphous phase, probablydue to C-S-H gels.

- WG10 and WG40 have more presence of anhydriteand thenardite, what can be explain because ofhigher amount of S in the bottom ashes.

- Equations 1,2,3,4 and 5 show the possible reactionshappening in the process.

3

- Moisture or dehydration products (20-260ºC): FA and BAnot present. From WG0 to WG40 increase in the interval(17-22.4%). This weight loss could include: dehydration ofgypsum and amorphous C-S-H hydrates.

- Dehydration of portlandite (400-550ºC)- Decomposition of CaCO3 (550 to 800ºC)- FA, present an important weight loss at 1000ºC, showing an

overlapping between CO2 and SO3 emissions. The presenceof two peaks for CO2 may differentiate the CaCO3 formed orthe CaCO3 initially present in the sorbent, that achieve alower or higher decomposition temperature, respectively.

- BA, present a weight loss at a higher temperatures (1000ºCto 1350ºC), attributed to CaSO4.

- WG0 to WG40 present two weight losses (800-1200ºC)corresponding to variable proportions of CaSO4 and Na2SO4

• The utilization of CFBC Bottom and Fly ashes in different proportions, mixed with Na2SiO3 and Na2CO3 to produce bindershas been investigated.

• The composition of these ashes differs greatly from common CFBC ashes, presenting very high amounts of S and an excessof Ca, that is used as a desulfurizing agent (limestone). These ashes contain a low amount of Si and Al that does not allowalkaline activation of these materials. BA contains mainly anhydrite, and FA consists mainly in lime and anhydrite.

• The compressive strength values of all of the samples mixtures developed with different proportions of FA/BA ranged from15 to 35 MPa after 28 days of curing. The samples that incorporated similar amount of FA and BA (WG8 and WG10)exhibited the maximum compressive strength value of all samples. This may be due to the fact that the FA provides thenecessary elements for the formation of the gel, and the BA, due to their larger particle size, act as an aggregate, improvingthe strength.

• Mixtures with high proportions of BA decrease the compressive strength due to the appearance of gypsum from thehydration of anhydrite.

• According to the results obtained using different analytical techniques, the main compounds found in the final productsare CaCO3, CaSO4, Na2SO4, Ca(OH)2 and Gel C-S-H.

• This work presents the feasibility of using CFBC BA, which are currently not valued in other uses, together with the FAfrom the same boiler to obtain materials with good resistance properties. Future works should analyse the replacementof commercial silicate by blast furnace slag to obtain a product entirely with secondary materials.

OBJECTIVE

PeaksSO4

Water absorbedCO3

-

CaOH CaOGel C-S-H

SO4 CaSO4 NaSO4 CO3- CaCO3 Si-O-Si H-O-H Si-O-T

cm-1 1120 594 1111 1600-1700 >3000 1442 1417 3638 3642 964-980 1645 70

The FT-IR results indicate that the hydration products mainlyinclude C–S–H, Na2S04, CaSO4 2H2O, CaCO3 and Ca(OH)2, whichare in agreement with those of SEM, TG and XRD. 0

20

40

60

Wei

ght (

%)

WG0

Na Si Ca O0

20

40

60

Wei

ght(

%)

WG10

Na S O0

20

40

60

Wei

ght(

%)

WG40

Na S Ca O