Effect of Slag Addition on Mechanical Properties of Fly ... - Science Direct

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ScienceDirect Procedia Engineering 151 (2016) 191 – 197

International Conference on Ecology and new Building materials and products, ICEBMP 2016

Effect of slag addition on mechanical properties of fly ash based geopolymers Michal Marcin*, Martin Sisol, Ivan Brezani Technical University of Košice, Faculty of Mining, Ecology, Process Control and Geotechnologies, Institute of earth resources, Department of mineral processing, Park Komenského 19, 043 84, Košice, Slovak Republic

Abstract This work describes improvement of mechanical properties of alkali activated binders – geopolymers made of fly ash from coal combustion in fusion boiler. The effect of addition of amorphous phase on physico-mechanical properties of geopolymers was examined. The studied properties were: flexural strength, compressive strength and water absorption. Effect of preparation process on the final properties of samples was also discussed. Amorphous phase was added in form of slag from the same coal combustion boiler. Samples were tested after 7, 28 and 90 days. With increasing amount of slag, the ascending trend of compressive strength was observed at the early stages of maturing. Compressive strength values measured after 90 days of maturing were found almost the same for all slag concentrations, and outperformed the compressive strength value of reference sample. On the contrary, the flexural strength was not positively affected by addition of the slag. Regarding the water absorption, the higher the slag addition, the lower water absorption was measured. © 2016 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-reviewunder underresponsibility responsibility of the organizing committee of ICEBMP Peer-review of the organizing committee of ICEBMP 2016 2016. Keywords: Fly ash; slag; alkali activation; geopolymers; mechanical properties

1. Introduction At present, most of the worldwide production of energy is made in heating plants by using combustion of fossil fuels, most of it coal. This process produces substantial part of solid and gaseous wastes. These products include fly ash, slag, clinker and gypsum. Combustion is the oldest and most common way of using coal. The combustion

* Corresponding author. Tel.: +421-556-022-992; E-mail address: [email protected]

1877-7058 © 2016 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 ICEBMP 2016

doi:10.1016/j.proeng.2016.07.380

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Michal Marcin et al. / Procedia Engineering 151 (2016) 191 – 197

process is based on the exothermic reaction between carbon in fuel and oxygen from the atmosphere. The product of reaction is not only heat but also water vapor and carbon dioxide [1]. The combustion of coal in power plants and heating plants occurs most often in powder form and in three types of boilers, whether it is a fusion boiler, granulation boiler or fluid boiler. The granulation boilers use less valuable fuels and operation is well below the melting point of ash. In the core of the flame, temperature depends on the type of fuel being in the range of 1100 – 1300 °C and the formation of molten slag is undesirable [2]. In fusion boilers, temperatures higher than the melting point of ash are used, about 1400 – 1600 °C. Ashes are melting and are removed in the liquid form from the boiler. This creates slag and only a small amount is collected from electrostatic precipitators as fly ash [1,3]. Fly ashes can be used as alkali activated materials and utilized in synthesis of geopolymers. The term geopolymer was first used by Joseph Davidovits. He defined the material that is formed in inorganic polycondensation called geopolymerization. In geopolymerization reaction, three-dimensional structures of AlO4 and SiO4 tetrahedra are created. According to him, only material with peak at about 55 ppm in 27Al NMR spectrum may be called geopolymer. Only materials produced by alkaline activation of metakaolin comply with this condition. Later the term geopolymer was used for all alkali activated aluminosilicate. Nowadays, research in field of geopolymers is focused mostly on using secondary raw materials like fly ash [4,5,6]. Geopolymers now represent a new group of organic substances, because they have significant environmental and energy potential. They belong in a group of the inorganic polymer covalently bound macromolecules with the chain consisting of -Si-O-Al-O-. Geopolymers are obtained from the chemical reaction of alumino-silicate oxides with sodium silicate solutions in a highly alkaline environment. As an alkali activating solution, strongly alkaline aqueous solution of sodium or potassium hydroxide is most commonly used [5,7,8]. Geopolymer structure is created by sialate network which is composed of SiO4 and AlO4 tetrahedron, which are connected to each other through its own oxygen atoms. The empirical formula of geopolymers, also known as poly(sialates) is Mn{-(SiO2)z-AlO2}n · wH2O, where M is a cation such as K+, Na+, or Ca2+; n is the degree of polycondensation and z is 1, 2, or 3 [4,9]. Geopolymerization is a complex multi-stage process. The reaction rate and chemical composition of the final reaction products is dependent on a numbers of factors which can be properties of the feed material, such as chemical and phase composition, grain size, and material composition of the activating solution, such as, the water content and the presence of soluble silicates [10]. Geopolymers are materials with many excellent properties such as high mechanical strength, resistance to low and high temperatures, resistance to aggressive environments or flame resistance [11]. This paper describes the effect of addition of amorphous phase – slag from coal combustion in fusion boiler on the physico-mechanical properties of geopolymers based on fly ash. Two different procedures were used to prepare the samples – the first one was stirred immediately after addition of the activating solution, the second one after onehour exposure to activation solution. Effect of preparation process on the final properties of samples is also discussed. 2. Materials and methods Material used for alkali activation was fly ash (FA) and slag (S) from the same combustion process from District Heating Plant in Košice (Slovakia). Combustion temperature is 1400–1550 °C. Slag was used as a filler to a fly ash based binding material. Material was homogenized before alkali activation. No other treatment was applied to material. Partial chemical analyses of the fly ash and slag are indicated in Table 1. The fly ash and slag are characterised by a SiO2/Al2O3 ratio of 2.59 for FA and 2.63 for S. Table 1. Partial chemical composition of used materials. (Wt. %)

SiO2

Al2O3

CaO

Fe2O3

MgO

LOI

Fly ash

45.26

17.46

5.61

9.10

3.22

15.48

Slag

58.22

22.17

2.82

7.95

7.29

0


To determine the leachability of Al and Si from fly ash and other solid alumosilicates [12], their concentrated suspensions were prepared in the 10 M NaOH alkali activator. The suspensions, containing 24 g of solid dispersed in 50 ml, were mixed for 24 hours, centrifuged, filtered and diluted with 5% cont. HCl before the analysis of the elemental concentrations by atomic absorption spectroscopy (AAS) (Table 2). Table 2. Content of SiO2 and Al2O3 in the 10 M NaOH leaching solution after 24 hours. Material

Fly ash -1

Slag

SiO2 (mg·l )

780

290

Al2O3 (mg·l-1)

675

235

The activation solution was prepared by mixing solid NaOH pellets with Na-water glass and water. Sodium water glass from the Kittfort Praha Co. with the density of 1.328 – 1.378 g/cm3 was used. It contains 36 – 38 % Na2SiO3 and the molar ratio of SiO2/Na2O is 3.2 – 3.5. Solid NaOH with the density of 2.13 g/cm3 was obtained from Kittfort Praha Co. containing at least 97 – 99.5 % of NaOH. From the fly ash – slag mixture (FAS) samples with different amount of slag were prepared. Samples with 15%, 30%, 50% and 65% by weight and also a reference sample which was created only from fly ash (FA) were prepared. FA was first mixed with S and after their homogenization, activation solution was added. The value of the Ms modulus in the activation solution was adjusted to 1.25. Overall concentration of the alkaline activation agent was adjusted to 9% Na2O in the binder mass. The water-to-fly ash ratio was adjusted to 0.30. FAS mixture was stirred with activation solution for 10 minutes, until creation of homogenous mixture. Mixture was then filled into prismatic molds with the dimensions 40×40×160mm and compacted on the vibration table VSB-40. The pastes were cured in a hot air drying chamber at 80 °C for 6 hours. Thereafter, the samples were removed from the forms, marked, and stored in laboratory conditions till the moment of the strength test. The values of compressive strength were determined after 7, 28, and 90 days according to the Slovak Standard STN EN 12390-3. A part of the samples was kept for 28 days at laboratory temperature, then a water absorption test according to the Slovak Standard STN 73 1316 was performed. To examine the method of preparations of samples, one was synthetized by stirring sample immediately after addition of the activating solution (FAS) and the second sample after one hour exposure of activation solution (FASE). The goal was to find out if the prolonged exposure to activating solution before stirring has any significant effect on properties of prepared geopolymeric materials. Sample with 50% addition of slag was chosen for this experiment. Mechanical properties were examined only on 90th day of ageing. Water absorption test was performed on 28 days old samples. The compressive strength of the hardened samples was determined after 7, 28, and 90 days using the hydraulic machine Form+Test MEGA 100-200-10D. 3. Results and discussion Properties and composition of feed material plays an important role in alkali activation. Samples were synthetized from fly ash with addition of slag by several proportions. The effect of slag addition on final compressive strength and flexural strength as a function of time was examined. The resulting strengths are shown in Figures 1 and 2. Flexural strength results shows that maximum strength of all samples is achieved on 90th day, but slag addition in samples proves that these strengths were lower than the flexural strength of reference sample. Flexural strength of all samples was rising over the time. On the contrary, results of compressive strength tests showed that addition of slag increases compressive strengths of the mixtures compared to the reference sample. The samples with 65% addition of the slag achieved the highest compressive strength values at 7 and 28 days, at 90 days, the small decrease was observed. Results also showed that on 90th day, all the compressive strength values were similar, which means that the added portion of slag have no significant effect on compressive strength of hardened material, but when compared to reference sample, the compressive strength of all the samples is higher.

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Fig. 1. Flexural strengths of fly ash – slag geopolymers after 7, 28 and 90 days

Fig. 2. Compressive strengths of fly ash – slag geopolymers after 7, 28 and 90 days.

Water absorption was examined over 24 hours and was measured after 10, 40, 90, 360 and 1440 minutes. Measured water absorption of prepared geopolymeric materials is shown in Figure 3. Results show that increase in the portion of slag in sample decreases the water absorption. The highest water absorption was measured for reference sample, 17.81%, the lowest was measured for sample with 65% addition of slag, 10%. Slag has a positive effect to reduce the water absorption in investigated geopolymers.


Fig. 3. Water absorption of fly ash – slag geopolymers over 24 hours.

Comparison of compressive and flexural strengths of FASE and FAS samples with 50% addition of slag are shown in Figures 4 and 5. Figures 4 and 5 show that prolonged exposure to activation solution for 1 hour have a negative effect on both compressive and flexural strengths. Both of the samples showed a decrease of strengths. Compressive strength was lower by 25.76% and flexural strength was lower by 26.78%. FAS and FASE water absorption is shown on Figure 6. It’s clear that FAS have lower water absorption 10.16% against 12.55% for FASE. Samples have the same amount of fly ash and slag, but they showed different mechanical properties, which was caused by different stirring method. Exposure of material to activation solution before stirring has a negative effect on resulting properties of prepared geopolymers.

Fig. 4. Flexural strengths of FAS and FASE geopolymers after 90 days.

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Fig. 5. Compressive strengths of FAS and FASE geopolymers after 90 days.

Fig. 6. Water absorption of FAS and FASE geopolymers after 24 hours.

4. Conclusion Alkali activated materials – geopolymers, are a new generation of inorganic binders. Any aluminosilicate materials can be used to prepare geopolymers, including fly ash and slag. This paper describes possibilities of preparation of geopolymers from fly ash with addition of different amount of slag. The results show, that slag has a positive effect on compressive strengths of the produced material, but the amount of it has no significant role on the final strengths. Increase in the amount of slag in mixture had positive effect on water absorption. The higher the slag addition, the lower water absorption was measured. Exposure of the mixture to the activation solution before stirring had a negative effect on all monitored characteristics.

Acknowledgements This work was supported by the research grant project VEGA, no. 1/0843/15, and APVV 0423-11.


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