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
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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
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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.
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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).
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