Fired Hydraulic Binder Based on Fluidized Combustion Fly Ash

Available online at www.sciencedirect.com

ScienceDirect Procedia Engineering 172 (2017) 319 – 324

Modern Building Materials, Structures and Techniques, MBMST 2016

Fired hydraulic binder based on fluidized combustion fly ash Dominik Gazdiča*, Karel Kulíseka, Marcela Fridrichováa, Karel Dvořáka a

Brno University of Technology, Faculty of Civil Engineering, 602 00 Brno, Czech Republic

Abstract This study is focused on possibilities of utilization of fluidized combustion fly ash for manufacturing of fired hydraulic binders as one of the principal raw materials for production of binders close to traditional cements. As a basic raw material high-calcium limestone was used, which was subsequently mixed with fluidized combustion filter fly ash (FCFFA) and fluidized combustion bed fly ash (FCBFA). The selected value of the hydraulic module was 1.7. Total two two-component raw material mixture were prepared which were burnt in a laboratory furnace under conditions at temperature of 1200 °C and 1250 °C. The evaluation was done on the basis of mineralogical composition using X-Ray diffraction (XRD) analysis, determination of the chemical composition of clinker and achieved basic physical properties. The aim was to produce a binder which could be very similar to strongly hydraulic lime or to cements of lower strength classes. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2016 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of MBMST 2016. Peer-review under responsibility of the organizing committee of MBMST 2016 Keywords: fluidized combustion fly ash, hydraulic binder, lime, limestone, cement, clinker.

1. Introducing In general, inorganic binders can be distinguish as air binders and hydraulic binders. After setting in the air, the hydraulic binders form rigid framework even under damp conditions and in water and are stable in these environments. Among these substances count hydraulic lime, roman cement, Portland clinker cement and special binders. Hydraulic lime is produced from a mixture containing calcareous constituents, respectively calcium magnesium carbonates with specific amount of hydraulic oxides such as SiO 2, Fe2O3, Al2O3 by firing under sintering point. Other procedure involves intergrinding of air lime with hydraulic admixtures.

* Corresponding author. Tel.: +420 54114 7511; fax: +420 54114 7502. E-mail address: [email protected]

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of MBMST 2016

doi:10.1016/j.proeng.2017.02.019

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Dominik Gazdič et al. / Procedia Engineering 172 (2017) 319 – 324

Due to its properties and composition, stands this binder on the border between air lime and Portland cement. It differs with Portland cement in amount of free lime CaO. In addition, in hydraulic lime is tricalcium silicate 3CaO.SiO2, which is formed at temperature above the sintering point of 1350 °C, absent, but Portland cement contains it. Hydraulic lime is characterised by hydraulic module, i.e. ratio of amount of CaO to amount of hydraulic oxides according to (1). According to the value of hydraulic module, nomenclature of hydraulic lime is as follows: MH = 1.7 – 3 strongly hydraulic lime; MH = 3 – 6 moderately hydraulic lime; MH = 6 – 9 feebly hydraulic lime [1,2,3] MH

CaO SiO2  Fe2O3  Al2O3

(1)

There is an increasing emphasis on utilization of secondary raw materials in all of industry sectors recent years. Some investigators have studied the processing and utilization of various secondary raw materials [4,5,6,7,8,9]. Fluidized combustion fly ash (FCFA) also belongs among these substances. FCFA is from the mineralogical point of view comprised of quartz, amorphous aluminum-silica phase, unsoluble anhydrite and free lime. Exactly the anhydrite, the real product of desulphurization process, limits the range of possible mortars only to those with the burning temperature below the point of reverse decomposition of this product. Considering all conditions, manufacturing procedure of mortar hydraulic lime-alike was designed. The mortar brings benefit due to requirement of lower burning temperature, thus its production is economically convenient. Further advantage can be seen in applications where binder both with sufficient plasticity and simultaneously with certain strength and volume stability is needed, in particular in the industry producing both mortar and plaster mixes. Concerning the true development, it was necessary to aim to two crucial issues: whether is possible to produce sufficiently reactive raw meal forming the hydraulic minerals and if is practicable to burn this raw meal at adequate temperature without risk of redecomposition forming calcium oxide and sulfur oxide. 2. Materials and methods Two raw meals were prepared throughout this study. The raw meal contained two components, FCFA and highcalcium limestone. The composition was calculated in order to achieve both values of preferred hydraulic module of 1.7 (corresponding to strongly hydraulic lime) and alumina module of 1.5. FCFFA and FCBFA from the power plant Hodonín were examined, whose chemical composition is shown in Table 1. Table 1. Chemical composition of fluidized fly ashes. Oxide

FCBFA (%)

FCFFA (%)

SiO2

42.71

36.63

Al2O3

19.21

18.67

Fe2O3

4.63

7.27

CaO

17.74

20.86

SO3

9.22

8.58

Other oxides

6.49

7.99

Design was performed in two steps: dosage proportions of the two components were initially computed, the ratios were rounded onto technically acceptable values which were used for calculation of hydraulic and alumina modules reversely. Table 2 presents sample weight proportions and recalculated hydraulic and alumina modules. Table 2. Weight proportions and recalculated modules. Weight proportions and recalculated modules

Fly ash portion (%)

Two-components mixture FCFFA

FCBFA

33.30

28.60

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Dominik Gazdič et al. / Procedia Engineering 172 (2017) 319 – 324 Limestone portion (%)

71.40

66.70

Portion of Fe-correction (%)

-

-

Hydraulic module (-)

1.97

Alumina module (-)

2.55

2.16 3.95

In order to enable to assess the effect of firing temperature on degree of decomposition of anhydrite from FCFA, chemical analysis of prepared raw meals was carried out, Table 3 shows the its results including calculated modules. From the tables above can be concluded that the differences of values of both modules between composition designed and obtained by chemical analysis did not largely exceed 5 %. The greater deviation was observed by hydraulic module of FCFFA, namely 15 %. Table 3. Chemical analysis of raw meal. Two components mixture

Chemical composition

FCFFA

FCBFA

SiO2

14.11

12.29

Fe2O3

3.01

1.41

Al2O3

7.16

5.76

CaO

40.02

44.30

SO3

3.59 3.23

2.59

Other oxides LOI (loss on ignition)

28.88

31.91

MH

1.65

2.28

MA

2.40

4.10

1.74

The raw components were subsequently homogenized and resulting raw meal was fired in a laboratory high temperature furnace under conditions at temperature of 1200 °C and 1250 °C, soak time 5 hours, heating rate 15 °C/min and cooling rate 103 °C/min. After burning were the formed clinkers crushed and ground to a maximum grain size of 0.063 mm. The samples were analysed using XRD analysis and optical microscopy. Microbeams of dimensions of 10 x 10 x 30 mm were subsequently moulded from the clinker which were stored in setting of saturated water vapour where further hydratation occurred. The samples were taken in order to examine their compressive strength at ages of 1, 3, 7 of hydration. The remainders after strength testing were characterised using XRD analysis and thermal analysis (differential thermal analysis). Due to entirely untypical dimensions of the beams were the values of the strength compared to them of reference standards. Hydraulic lime HL 5 and blast-furnace slag Portland cement Hranice CEM II/B-S 32.5 were employed as reference standards. 3. Results and discussion Mineralogical composition of fired clinkers was characterised using XRD and chemical analysis and experimentally by Scanning electron microscope analysis, for the results see Table 4. Table 4. Chemical composition of clinkers. Chemical composition

Clinker from FCFFA (%)

Clinker from FCBFA (%)

1200°C

1250°C

1200°C

1250°C

SiO2

16.40

16.90

17.60

17.80

Fe2O3

3.71

3.81

2.23

2.25

Al2O3

9.36

9.63

8.92

9.08

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Dominik Gazdič et al. / Procedia Engineering 172 (2017) 319 – 324

CaO

58.00

58.02

59.10

60.31

SO3

4.17

4.12

3.89

3.86

Other oxides

3.43

4.06

2.35

2.93

LOI (loss on ignition)

4.93

3.46

5.91

3.77

MH

1.97

1.91

2.06

2.07

MA

2.52

2.53

4.00

4.04

Based on XRD patterns can be claimed that calcite was initially decomposed during the burning, which reacted with anhydrite and aluminosilicate phase from the FCFA subsequently. Belite, E-C2S, was the principal phase in all clinkers. Clein’s salt, C4A3S̄, was by reaction of lime, amorphous aluminum phase and anhydrite released. Free lime, CaO, and clinker mineral brownmillerit, C4AF, were also found in all samples. Creation of Clein’s salt infers that anhydrite favoured entering this mineral. This was apparently the reason why it was retained as undecomposed. Presence of belite was confirmed both by XRD and by optical microscopy, the latter was performed on cut-sections altered by icy vapour of acetic acid. At the pictures taken, brown fine rounded grains of belite with blue grains of alite sporadically enclosed were observed. This beyond, big yellowish grains of free lime were readily to see. Chemical analysis, modules calculated from it respectively, indicates great coincidence between values calculated by the raw meal design and the values which were determined in real terms. The analysis performed helped also to estimate whether the decomposition of calcium sulphate during the clinker firing occur or does not. As each of samples differed in loss on ignition, simple comparison of the values is impracticable. In order to prove the agreement, two approaches were proposed. Approach 1 - Amount of sulphates was referred to the total of oxides CaO, Al2O3, Fe2O3, SiO2 and this ratio was compared to the state before and after firing. Approach 2 - Amount of sulphates was in the two states modified by deduction LOI. The resulting ratio was compared in the same way as in the case 1. Table 5. Parametres of CaSO4 decomposition.

State of sample

Approach 1.

Approach 2.

SO3/(CaO+Al2O3+Fe2O3) (-)

SO3 after deducting LOI (%)

1200 °C

1250 °C

1200 °C

1250 °C

Before firing

0.0600

0.0410

5.05

3.80

After firing at 1200 °C

0.0478

0.0445

4.39

4.13

After firing at 1250 °C

0.0468

0.0433

4.27

4.01

Results presented in Table 5 denote that calcium sulphate from the clinker manufactured from FCFFA was significantly decomposed and, conversely, amount of sulfates from the clinker produced from FCBFA was eiter stable or slightly increased. As this effect had no logical explanation and as the value of 5.05 % by the clinker from FCFFA markedly exceeded the rest of them, a revision of this data was carried out. The revision was performed using chemical analysis of raw materials, i.e. fluidized fly ash and limestone. The amount on sulfates was subsequently calculated according to real dosage of components after deduction loss on ignition. In order to keep the objectivity, amount of sulfates from the sample with FCBFA was provided in the same way, see corrected values in Table 6. Table 6. Parameters of CaSO4 decomposition – modified values. State of sample

SO3 after deducting LOI (%) FCFFA

FCBFA

Before firing

4.10

3.89

After firing at 1200 °C

4.39

4.13

After firing at 1250 °C

4.27

4.01

Dominik Gazdič et al. / Procedia Engineering 172 (2017) 319 – 324

From the comparison of the values presented in Table 5 and 6 can be deduced that values of amount of SO3 by clinker from FCBFA before and after firing, assuming the method mentioned above (Table 6), were no distinct from these obtained by chemical analysis (Table 5). As the amount of SO3 by the clinker from FCFFA before firing deviated from the average, it was more likely to be considered as incorrect. Conversely, there was no significant difference between the value of amount of SO 3 calculated from the chemical analysis of raw materials and their mean, assuming the correctness proved by sample from FCBFA, therefore could be assumed to be more presumable. From these reasons, corrected values of CaSO4 from Table 6 seemed to be more objective. From these values arised that CaSO4 was not almost decomposed during the firing, since the differences among them were likely to be only statistical. If the decomposition would really occur, it can be expected at temperature of 1250 °C and even then will negligibly take place, estimated below 3 %. Taking into considerations very low yield ratio of clinkers manufactured in a laboratory, their technological properties were only indicative examined. The clinkers were ground to a maximum size of grain of 0.063 mm. Cement pastes were manufactured with estimated amount of water. Microbeams were moulded for testing of compressive strength, which were stored in setting of saturated water vapour, see Figure 1.

F Fig. 1. Properties of hydrated samples.

As can be seen from the given results presented in Figure 1, water/clinker ratios of fired hydraulic limes reached substantially lower values than commercially produced lime and only mildly greater values than referential blastfurnace slag Portland cement. The values of compressive strength achieved approximately one third to one half and up to quintuple of values referred to blast-furnace slag Portland cement and to referential hydraulic lime, respectively. It was obvious that the clinkers from FCFFA accomplished better compressive strength using somewhat lower water/clinker ratio than clinkers from FCBFA did. Sharper burning was also beneficial to it. Conclusion To sum up, there is a need to preferentially focus on, aiming to possible utilization of fluidized combustion fly ash as one of raw materials for hydraulic binders, the issue of firing of raw meals rich in calcium sulphate without its decomposition and release of CO2 into the atmosphere. It was proved that virtually all anhydrite present in fluidized fly ash was still bond at temperature of 1250 °C with CaO and aluminum-silica phase into Clein’s salt, C4A3S̄, thus was not released into the air. The reaction of calcium component with silica constituent formed belite, E-C2S, which

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ensured definite hydraulicity of the system. This discovery seems to be of great contribution. As the experimental work performed up to now can be considered to be only preliminary study of the issue discussed, there is a need to prove the promising partial results by detailed research. Acknowledgements This work was financially supported by project number: 14-32942S “Effect of fluidized bed ash on the thermodynamic stability of hydraulic binders” This paper was also elaborated with the financial support of the European Union's "Operational Programme Research and Development for Innovations", No. CZ.1.05/2.1.00/03.0097, as an activity of the regional Centre AdMaS "Advanced Materials, Structures and Technologies". References [1] R. Hanley, S. Pavia, A study of the workability of natural hydraulic lime mortars and its influence on strength, Materials and Structures 41(2) (2008) 373-381 [2] J. Hlaváč, Základy technologie silikátů. Praha: SNTL, 1988. s. 516. [3] EN 459-1, ed.3, Building lime - Part 1: Definitions, specifications and conformity criteria, 2015. [4] J. Papayianni, An investigation of the pouuolanicity and hydraulic reactivity of a high-lime fly-ash, Magazine of Concrete Reaserch 39(138) (1987) 19-28. [5] J.G. García-Díaz, G. Palomo, F. Puertas, Belite cements obtained from ceramic wastes and the mineral pair, CaF2/CaSO4, J. Cem. Concr. Comp. (2011) 33-10. [6] M. Singh, S.N. Upadhayay, P.M. Prasad, Preparation of special cements from red mud, Waste Manag. 16 (1996) 665–70 [7] F. Raupp-Pereira, R. James Ball, J. Rocha, J.A. Labrincha, G. Allen, New based clinkers: belite and lime formulations, J. Cem. Concr. Res. 38 (2008) 511 [8] P. Stroeven, D.D. Bui, E. Sabuni, Ash of vegetable waste used for economic production of low to high strength hydraulic binders, International Ash Utilization Symposium, LEXINGTON, KY, 78(2) (1997.) 153-159. [9] A. Bras, F.M.A. Henriques, M.T. Cidade, Effect of environmental temperature and fly ash addition in hydraulic lime grout behaviour, Construction and Building Materials 24(8) (2010) 1511-1517.