Synthesis of Calcium Sulfoaluminate Cements from Blends of Coal

Combustion Ashes with Flue Gas Desulfurization Gypsum

M. Marroccoli1, F. Montagnaro

2, M. L. Pace

1, A. Telesca

1, G. L. Valenti

1,

1. Dipartimento di Ingegneria e Fisica dell’Ambiente

- Università degli Studi della Basilicata, Potenza - ITALY

2. Dipartimento di Chimica - Università degli Studi Federico II, Napoli - ITALY

1. Introduction

Calcium sulfoaluminate (CSA) cements are special hydraulic binders, very interesting from

both technical and environmental point of view. They contain calcium sulfoaluminate

(4CaO∙3Al2O3∙SO3), dicalcium silicate (2CaO∙SiO2) and calcium sulfates (CaSO4∙2H2O and/or

CaSO4) as main components together with tetracalcium-iron aluminate (4CaO∙Al2O3∙Fe2O3),

calcium sulfosilicate (5CaO∙2SiO2∙SO3), calcium-aluminates (3CaO∙Al2O3, CaO∙Al2O3,

12CaO∙7Al2O3) and -silicoaluminates (2CaO∙Al2O3∙SiO2, CaO∙Al2O3∙2SiO2). Upon hydration,

calcium sulfates, belonging or added to CSA clinker, react with 4CaO∙3Al2O3∙SO3 and generate

ettringite (6CaO∙Al2O3∙3SO3∙32H2O) which, depending on the conditions of its formation,

regulates the technical properties of CSA cements (shrinkage compensation or self stressing

behaviour or rapid-hardening associated with dimensional stability) [1-11]. 2CaO∙SiO2 can add

strength and durability at medium and long ages, while 4CaO∙Al2O3∙Fe2O3 and calcium

aluminates contribute to ettringite formation; on the other hand, 5CaO∙2SiO2∙SO3 and

calcium-silicoaluminates display a poor hydraulic activity. The distribution of the secondary

components is mainly influenced by the synthesis temperature as well as the nature and

proportioning of raw materials.

Compared to Portland cement production, the manufacturing process of CSA cements has a

pronounced environmentally friendly character [4; 12]. In this regard important features are: 1)

synthesis temperatures 200°-300°C lower than those required by ordinary Portland cement

clinkers; 2) clinkers easier to grind; 3) reduced amount of limestone in the kiln raw mix and,

consequently, reduced thermal input and CO2 generation; 4) greater usability of wastes and

by-products.

Several industrial residues were successfully experienced as raw materials for the synthesis of

CSA cements [13-23]. The industrial by-products generated by coal-fired power plants can play

a very important role [22]; in particular, pulverized fly ash (PFA, as a source of SiO2 and

Al2O3), fluidized bed combustion (FBC) waste (as a source of lime, calcium sulfate, silica and

alumina) and flue gas desulfurization (FGD) gypsum (as a source of calcium sulfate) are

worthy of consideration because their present utilization degree is still unsatisfactory.

PFA generally has a good pozzolanic behaviour and other useful characteristics which can be

exploited in a variety of applications, but its unburnt carbon content (generally expressed as

loss on ignition, l.o.i.) must be relatively low in order to meet the requirements of the ordinary

cement and concrete industry. Ashes originated from either old, poorly efficient plants or

modern, environmentally friendly pulverized coal combustors (operating at reduced

temperatures) can display unacceptably high l.o.i. values.

The utilization of FBC waste, mainly composed by exhausted sulfur sorbent and coal ash, is

generally made difficult by its chemical and mineralogical composition. The fairly high amount

of lime and calcium sulfate is responsible for exothermal and expansive phenomena during

hydration; moreover, the pozzolanic activity of FBC ash is poor, due to its reduced glass

content [18].

FGD gypsum can replace natural gypsum in its main application fields (plaster and cement

1

PTSE 2010

industries, manufacture of preformed building elements). However its widespread use is

generally hindered by the large availability of the natural mineral.

Limestone, bauxite and gypsum are the natural materials involved in the manufacture of CSA

cements. A typical raw mix composition consists of about 35% limestone, 38% bauxite and

27% natural gypsum [24].

In the present study, blends of limestone and bauxite with a high l.o.i. PFA, a FBC waste and a

FGD gypsum (in full replacement of natural gypsum) were investigated and their suitability to

be used as CSA cement generating raw mixes was assessed. In particular, two mixtures, X and

Y, containing about 44% limestone, 13% bauxite, 19% PFA, 9% FGD gypsum and 15% FBC

bottom- and/or fly-ash were heated in a laboratory electric oven. Their burnt products were

submitted to X-ray diffraction (XRD) analysis in order to determine both conversion of

reactants and selectivity towards the expected hydraulic phases.

2. Experimental

Table 1 shows the chemical composition of natural materials (limestone, bauxite) and industrial

wastes (PFA, FBC waste, FGD gypsum) used in this investigation and respectively given by

BUZZI UNICEM SpA – Casale Monferrato and ENEL Area Tecnica Ricerca – Brindisi. It was

evaluated through X-ray fluorescence analysis by means of a BRUKER Explorer S4 apparatus.

Table 1: Chemical composition of natural materials and industrial wastes, mass %.

Limestone Bauxite PFA FBC fly ash FBC bottom ash FGD gypsum

CaO 54.70 1.69 4.30 24.20 43.12 32.04

SO3 - 0.03 0.04 12.80 25.89 45.77

Al2O3 - 55.22 22.80 13.71 5.85 0.08

SiO2 - 6.48 35.08 23.23 18.45 0.10

MgO 0.30 - 1.13 1.04 1.00 0.37

SrO - 0.03 0.11 - - -

P2O5 - 0.01 0.10 - - -

TiO2 - 2.34 1.52 0.82 0.48 -

Fe2O3 - 6.25 8.20 6.74 3.15 -

Mn3O4 - - 0.10 0.07 0.08 -

Na2O - - - - - 0.03

l.o.i.* 42.61 27.68 25.85 16.26 1.39 20.59

Total 97.61 99.73 99.23 98.87 99.41 98.98

*loss on ignition at 950°C

The thermal treatment of the CSA cement generating raw mixtures was carried out for 2 hours

at temperatures ranging from 1150°C to 1300°C. The synthetic clinkers were submitted to

X-ray diffraction (XRD) analysis by means of a PHILIPS PW1710 instrument, operating

between 5° and 60°2 (Cu K radiation).

2

Italian Section of the Combustion Institute

3. Results and discussion

3.1 Proportioning of raw mixtures

The composition of the mixtures X and Y is shown in Table 2. The mass ratio between FBC fly

and bottom ashes in the mixture Y is the same as that between the corresponding industrial flow

rates (1.5).

Table 2: Composition of raw mixtures, mass %.

The proportioning of the raw mixtures was made by assuming that SO3 and Al2O3 on the one

hand, and SiO2, on the other, reacted to give only 4CaO∙3Al2O3∙SO3 and 2CaO∙SiO2,

respectively, and supposing also that solid solution effects were absent. The alumina and silica

contents were the stoichiometric amounts needed for the synthesis of the above mentioned

phases. The SO3 content was twice the stoichiometric amount required by the formation of

4CaO∙3Al2O3∙SO3, in order to avoid considerable decreases of 4CaO∙3Al2O3∙SO3 concentration

associated with sulfur dioxide losses occurring at high burning temperatures. Table 3 shows the

potential concentration values of 4CaO∙3Al2O3∙SO3, 2CaO∙SiO2 and CaSO4 (estimated for zero

sulfur dioxide emission) in the burning products of the two mixtures.

Table 3: Potential concentration of 4CaO∙3Al2O3∙SO3, 2CaO∙SiO2 and CaSO4 in the burning

products of mixtures X and Y, mass %.

3.2 Burning of raw mixtures

Figure 1 shows the XRD patterns (peak intensity-counts per second vs diffraction angle-2) of

the mixtures X and Y, respectively, both heated at 1250°C. Reactants were absent and the

presence of 4CaO∙3Al2O3∙SO3, 2CaO∙SiO2 and CaSO4 was observed. Furthermore, weak

signals related to 4CaO∙Al2O3∙Fe2O3, 3CaO∙Al2O3 and 5CaO∙2SiO2∙SO3 were detected. From

the qualitative point of view, similar results were obtained at the other burning temperatures.

Mixture X Y

Limestone 44.0 43.8

Bauxite 12.5 13.1

FBC fly ash 14.8 9.2

FBC bottom ash - 6.1

PFA 18.8 19.6

FGD gypsum 9.9 8.2

Total 100.0 100.0

Mixture X Y

4CaO∙3Al2O3∙SO3 38.2 38.0

2CaO∙SiO2 45.0 45.4

CaSO4 4.3 4.3

3

PTSE 2010

Angle 2, Cu k

10 20 30 40 50 60

Pea

k i

nte

nsi

ty,

cp

s

0

200

400

600

800

1000

1200

1400

1600

&

§

*/#

B#

§§/#

#/*

& B/§

B

B/#

§/#&

B*

#

§ &*

B

A

§

B&

&

*

Angle 2, Cu k

10 20 30 40 50 60

Peak

in

ten

sity

, cp

s

0

200

400

600

800

1000

1200

1400

1600

&

§

*/#

B#

§§/#

#/*

§ &/§

B

B/#

§&

B*

#

§*

B

§

§

BB

*

Fig. 1 XRD patterns of mixtures X (left) and Y (right) burnt at 1250°C:

*=4CaO∙3Al2O3∙SO3, A=CaSO4, #=5CaO∙2SiO2∙SO3, §=2CaO∙SiO2, &=3CaO∙Al2O3,

B=4CaO∙Al2O3∙Fe2O3.

Figures 2 and 3 show the XRD intensity of the main peak related to 4CaO∙3Al2O3∙SO3 and

2CaO∙SiO2, respectively, for the burning products of both mixtures.

Fig. 2 XRD intensity of the 4CaO∙3Al2O3∙SO3 main peak for the burning products

of mixtures X and Y.

4

Italian Section of the Combustion Institute

Fig. 3 XRD intensity of the 2CaO∙SiO2 main peak for the burning products

of mixtures X and Y.

The best selectivity towards calcium sulfoaluminate and dicalcium silicate was practically

attained at 1250°C, thus highlighting the negative influence exerted by too high temperatures.

4. Conclusions

Three by-products of coal-fired power plants (a high l.o.i. pulverized fly ash, a fluidized bed

combustion waste and a flue gas desulfurization gypsum), blended with limestone and bauxite,

proved to be useful sources of silica, alumina, lime and calcium sulfate in the raw mixes

generating special cements based on calcium sulfoaluminate.

It has been found that two mixtures (containing about 44% limestone, 13% bauxite, 19% PFA,

9% FGD gypsum and 15% FBC bottom- and/or fly-ash), burnt for two hours in a laboratory

electric oven at temperatures ranging from 1150°C to 1300°C, show a good conversion and a

high selectivity towards 4CaO∙3Al2O3∙SO3. No significant differences in thermal behaviour

were observed between the mixtures which gave the best results when heated at 1250°C.

5. Acknowledgements

The research activity was performed under the Collaboration Agreement between CNR/DET

(Consiglio Nazionale delle Ricerche/Dipartimento Energia e Trasporti) and DIFA

(Dipartimento di Ingegneria e Fisica dell’Ambiente – Università degli Studi della Basilicata)

within the Project “New technologies for enhancing the environmental performance of

pulverised-coal fired power plants”, according to the Programme Agreement MSE (Ministero

dello Sviluppo Economico) – CNR (Gruppo Tematico: Carbone Pulito).

6. References

1. Kurdowski, W., George, C.M., Sorrentino, F.P.: Proceedings of the 8th

International

Congress on the Chemistry of Cement, Rio de Janeiro, Brazil, September, 1:292 (1986).

2. Mudbhatkal, G.A., Parmeswaran, P.S., Heble, A.S., Pai, B.V.B., Chatterjee, A.K.:

Proceedings of the 8th

International Congress on the Chemistry of Cement, Rio de Janeiro,

Brazil, September, 4:364 (1986).

3. Muzhen, S., Kurdowski, W., Sorrentino, F.P.: Proceedings of the 9th

International

5

PTSE 2010

Congress on the Chemistry of Cement, New Delhi, India, November, 1:317 (1992).

4. Mehta, P.K.: World Cement Technology, 11:166 (1980).

5. Wang, L., Glasser, F.P.: Advances in Cement Research, 8:127 (1996).

6. Muzhen, S., Yanmou, W., Zhang, L., Dedong, L.: Proceedings of the 10th

International

Congress on the Chemistry of Cement, Goteborg, Sweden, June, 4:4iv029 (1997).

7. Glasser, F.P., Zhang, L.: Cement and Concrete Research, 31:1881 (1999).

8. Glasser, F.P.: Proceedings of the 5th

International Symposium on the Cement and

Concrete, Shanghai, China, 1:14 (2002).

9. Bernardo, G., Telesca, A., Valenti, G.L.: Cement and Concrete Research, 36:1042 (2006).

10. Bernardo, G., Buzzi, L., Canonico, F., Paris, M., Telesca, A., Valenti, G.L.: 12th

International Congress on the Chemistry of Cement, Montreal, Canada, July, W3 11.4

(2007).

11. Marroccoli, M., Nobili, M., Telesca, A., Valenti, G.L.: Proceedings of the International

Conference on Sustainable Construction Materials and Technologies, Coventry, UK,

June, 1:389 (2007).

12. Gartner, E.: Cement and Concrete Research, 34:1489 (2004).

13. Majling, J., Sahu, S., Vlna, M., Roy, D.M.: Cement and Concrete Research, 23:1351

(1993).

14. Belz, G., Beretka, J., Marroccoli, M., Santoro, L., Sherman, N., Valenti, G.L.: Proceedings

of the 5th

CANMET/ACI International Conference on Fly Ash, Silica Fume, Slag and

Natural Pozzolans in Concrete, Milwaukee, USA, June, 1:513 (1995).

15. Beretka, J., Cioffi, R., Marroccoli, M., Valenti, G.L.: Waste Management, 16:231 (1996).

16. Ikeda, K., Fukuda, K., Shima, H.: Proceedings of the 10th

International Congress on the

Chemistry of Cement, Goteborg, Sweden, June, 1:1i025 (1997).

17. Arjunan, P., Silsbee, M.R., Roy, D.M.: Cement and Concrete Research, 29:1305 (1999).

18. Bernardo, G., Marroccoli, M., Montagnaro, F., Valenti, G.L.: Proceedings of the 11th

International Congress on the Chemistry of Cement, Durban, South Africa, May, 3:1227

(2003).

19. Belz, G., Bernardo, G., Caramuscio, P., Montagnaro, F., Telesca, A., Valenti, G.L.: 28th

Italian Meeting on Combustion, Napoli, Italy, June, (2005).

20. Belz, G., Caramuscio, P., Marroccoli, M., Montagnaro, F., Nobili, M., Telesca, A., Valenti,

G.L.: 29th

Italian Meeting on Combustion, Pisa, Italy, June, (2006).

21. Marroccoli, M., Montagnaro, F., Nobili, M., Telesca, A., Valenti, G.L.: 12th

International

Congress on the Chemistry of Cement, Montreal, Canada, July, W3 11.2 (2007).

22. Marroccoli, M., Nobili, M., Telesca, A., Valenti, G.L.: Proceedings of the 7th

International

Congress on Concrete, Construction's Sustainable Option, Dundee, Scotland, UK, June,

Role for concrete in global development:299 (2008).

23. Marroccoli, M., Montagnaro, F., Pace, M.L., Telesca, A., Valenti, G.L.: Proceedings of the

20th

International Conference on Fluidized Bed Combustion, Xi’an, China, May, 2:1072

(2009).

24. Marroccoli, M., Pace, M.L., Telesca, A., Valenti, G.L.: Proceedings of the 2nd

International Conference on the Sustainable Construction Materials and Technologies,

Ancona, Italy, June, (2010), accepted for publication.

6