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Procedía Materials Science 9 (2015) 496 - 503
International Congress of Science and Technology of Metallurgy and Materials, SAM -
CONAMET 2014
Influence of the unburned residues in fly ash additives on the mechanical properties of cement mortars
Sandra P. Pedrazaa*, Yaneth Pinedaa, Oscar Gutiérreza
aInstituto para la Investigación e Innovación en Ciencia y Tecnología de Materiales INCITEMA, Universidad Pedagógica y Tecnológica de
Colombia, Sede Central, Tunja, Boyacá, Colombia.
Abstract
The cement companies have found in natural and artificial pozzolans a very good option for partial replacement of cement in concrete. Fly Ash is a type of artificial pozzolan, which improves the microstructure of hydrated cement and its durability. Because it is a byproduct of coal combustion, it is economical and abundant for use as an addition to mortars and concrete, making this byproduct a recoverable residue, favoring the environment.
The main objective of this study was to evaluate the changes of the microstructural and mechanical strength occurring in portland cement mortars when different proportions of fly ash are added, comparing the effects of adding fly ash in its original state of industrial waste with those of adding fly ash with a reduced content of unburned residues. For the preparation and strength determination of the mortars, cubes of 50x50x50 mm were cast, using as references standards ASTM C305 and ASTM C109, respectively. Compressive strength was evaluated in curing times of 1, 3, 7, 28, 56, and 118 days. The characteristics of fly ash and Portland cement were evaluated using X-ray Fluorescence (XRF) and X-ray Diffraction (XRD) techniques, and Scanning Electron Microscopy (SEM).
Given that unburned residues in fly ash are defined as the sum of the percentages of volatile matter and fixed carbon contained by it, proximate analyses were carried out - as it is for coal (ASTM D 3172) - to determine the percentage of unburned carbon that is eliminated during the process. As a result, better compressive strengths were obtained when ash containing fewer amounts of unburned carbon was added to mixtures. It is also evident that the optimal amount of fly ash that may be added to mixture of mortar without greatly affecting the mechanical strength is 20 percent.
© 2015TheAuthors. Publishedby 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 Scientific Committee of SAM-CONAMET 2014 Keywords: Fly Ash; Cement; Unburned Coal;
* Corresponding author. Tel.: +57-3138993732; E-mail address: sandra.pedraza@uptc.edu.co
2211-8128 © 2015 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 Scientific Committee of SAM-CONAMET 2014 doi: 10.1016/j.mspro.2015.05.022
1. Introduction
The construction industry has evolved over time, and with it, the research on the raw materials it demands. In the latest years, studies have been performed on materials that may be added to cements and concretes, in order to benefit their production process without lowering their quality. Pozzolans have become a good alternative of cement additive products. Fly Ash is an artificial pozzolan, a byproduct from thermal power plants. Because the production of Portland Cement (PC) entails high energy costs, and that for each ton of cement produced one ton of CO2 is released to the atmosphere (Valdez et al., 2007), adding fly ash to the mix results very beneficial, as it gives utility to the industrial waste of thermal power plants, it reduces CO2 emissions to the atmosphere, and it reduces production costs.
One of the disadvantages of using fly ash as a supplementary material in cements and concretes, is the amount of unburned carbon it contains (Valderrama et al., 2011; Velazquez et al., 2007; Burgos et al., 2012), which is made up of non-reactive, porous and rough-textured particles that increase the amounts of water required by the mixes, an also affect their mechanical properties and their durability (Burgos et al., 2012).
Considering that different authors (Valderrama et al., 2011; Burgos et al., 2012; Molina et al., 2008; Shi et al., 2008) agree the properties of fly ash are very variable, depending on the type of carbon that generates them and the combustion process to which they are submitted, we intend to physically, chemically and mineralogically characterize the fly ash originating from the thermal power plant located in Boyaca (Colombia), as well as the Portland cement used.
The objective of this investigation is to evaluate the effect of unburned residues on the mechanical properties of cement mortars containing fly ash as an additive. To do this, part of the unburned residues in the fly ash will be removed by means of sieving separation techniques. For both samples of fly ash, normal and sieved, the percentage of unburned residues will be determined by means of proximate analyses as described in ASTM D3172 (2013). Thus, the sum of volatile matter and fixed carbon is equal to the percentage of unburned residues in each sample (Velasquez et al., 2007).
To determine which mixture with fly ash additive presents the better compressive strength properties, mortar cement cubes with proportions of 20, 40, 60 and 80% of normal fly ash - just as it arrives from the thermal power plant - and of sieved fly ash will be cast, and they will be compared with one made up of 100% cement.
2. Materials and Methods
2.1. Materials used
The following materials were used during this investigation:
• Cement: Blended Portland cement, classified as Type I. Produced by Cementos Argos S.A., using the commercial denomination "General Purpose Gray Cement", complying with NTC 121 (2014) and NTC 321 (1982) standards.
• Fly Ash: Byproduct of carbon combustion at the thermal power plant TERMOPAIPA, located in the department of Boyaca, Colombia.
• Sand: Sand with similar characteristics to that of standardized Ottawa sand.
• Distilled Water.
2.2. Elimination of unburned carbon
To eliminate a percentage of the unburned residues from normal fly ash (NFA) - in its original state as industrial waste from the Thermal Power Plant of Boyaca - , a granulometric analysis was performed to find the sieving mesh diameter that would retain the unburned residue particles, using as a reference the methodology applied by Velasquez et al. (2007). A set of sieves was used, from #40 - corresponding to an opening of 0.425mm - to # 325 - corresponding to an opening of 0.045mm - , as described in Table 1. This analysis showed that approximately 96% of the initial weight goes through sieve #100, retaining in the first sieves the largest unburned carbon particles (Fig. 1a and 1b). The larger unburned carbon particles were then separated from the NFA by passing it through the #100 sieve, thus
obtaining a sample of unburned carbon-free fly ash (UFFA).
Table 1. Fly Ash Granulometry.
Sieve # Diameter Weigth Ret. Retained Ret. Accum. Passing
(mm) (gr) (%) (%) (%)
40 0.425 0.05 0.017% 0.017% 99.983%
50 0.3 0.22 0.076% 0.093% 99.907%
60 0.25 0.69 0.237% 0.330% 99.670%
80 0.18 1.27 0.436% 0.765% 99.235%
100 0.15 7.22 2.478% 3.244% 96.756%
200 0.075 59.81 20.529% 23.772% 76.228%
325 0.045 195.18 66.992% 90.764% 9.236%
Bottom 26.91 9.236% 100.000% 0.000%
TOTAL 291,35
Proximate analysis of coal determines the percentages of: Moisture (M) - ASTM D 3173 (1996) - , Ash (A) -ASTM D 3174 (1996) - , Volatile Matter (VM) - ASTM D 3175 (1996) - , and Fixed Carbon (FC), calculating this last one using the formula: %FC = 100 - (%M + %Ash + %VM). Proximate analyses were performed on both samples of Fly Ash (NFA and UFFA), in order to find the percentage of unburned carbon (VM+FC) each contained; obtaining the amounts presented in Table 2.
Table 2. Proximate analysis of fly ash.
Ash Type Residual Ash Volatile Fixed Carbon Unburned
Moisture (%) Matter (0% Residue
(%) ° (%) ° (VM+FC) (%)
UFFA 0.38 92.93 3.49 3.2 6.69
NFA 0.34 92.3 4.6 2.76 7.36
2.3. Characterization of materials
The physical, chemical and mineralogical characteristics of fly ash and portland cement were evaluated separately using X-ray Fluorescence (XRF), X-ray Diffraction (XRD), and Scanning Electron Microscopy (SEM). The chemical composition of both types of Fly Ash (NFA and UFFA) resulted in a content of SiO2 + Al2O3 + Fe2O3 larger than 70%, a content of SO3 lower than 5%, and moisture content not exceeding 3%. Density analysis was performed in accordance with ASTM C 188 (1995), obtaining data that shows a uniform distribution, which does not exceed the 5% average variation limit established in standard ASTM C 618 (2000). Thus, the fly ash samples can be classified as Type F, according to standard ASTM C 618 (2000). To determine pore superficial area, volume and size, BET nitrogen adsorption analysis was performed. Loss on ignition was tested following the practice recommended in standard ASTM C 114 (2000). All of these results are reported in Table 3.
Table 3. Physical and chemical characteristics of cement and fly ash.
Cement
Chemical Analysis (%)
Al2Ü3
L.O.I.
Moisture Content Physical Properties
Superficial Area (m2/g) Pore Volume (cm3/g) Pore Size (nm) Density (g/cm3)
22.9 11 3.7 56.8
59.7 29.1 6.64 0.79 0.78 7.66 0.34
59.9 29.1 6.52 0.83 0.59 7.07 0.38
I.........I.........I.........I.........I.........I.........I.........I
10 20 30 40 50 60 70 80
Position [°2Theta] (Copper (Cu))
Fig. 2. Diffractogram of Fly ash.
Fig. 3. Diffractogram of Cement.
The chemical composition of the Fly Ash is similar to that of the Clay Minerals Group Calleja (1982). Two phases can be observed in its diffractogram: an amorphous phase (reactive material), and a larger percentage of a crystalline phase, comprised of quartz and mullite (silicates); indicative of a Fly Ash low in Calcium from anthracites and subbituminous carbon (Fig. 2). In turn, the cement displayed a usual chemical composition, mainly consisting of Alite (C3S), Belite (C2S), Aluminate (C3A) and other characteristic components of added cements (Fig. 3).
The irregular hexagon shape of cement particles can be observed in Figure 4a, which shows a SEM Micrograph. The cenospheres present in fly ash can be observed in Figure 4b, formed by the surface tension generated by the heat of combustion. Porous particles of irregular shape are also recognized, which are what is called "unburned residue".
2.4. Production of mortar cubes.
To determine the optimum percentage of fly ash that may be added to mixtures of Portland cement mortars without affecting compression strength, 50x50x50 mm cubes were cast according to standards ASTM C305 (2000) and ASTM C109 (1999), replacing Portland cement for fly ash, NFA and UFFA, in percentages of 20, 40, 60, and 80%. Mortars of 100% cement were also cast under the same curing conditions, in order to have a target of comparison. This amounts to 162 cubes (Table 4) in total. Fluidizing liquid was also added to the mixtures that included fly ash to make them easier to handle.
Table 4. Material proportion for the elaboration of mortars.
Number Cement NFA UFFA Water Sand Fluidizing
Cubes (g) (g) (g) (ml) (g) (g)
3 250 0 0 121 687.5 0
3 200 50 0 121 687.5 2.5
3 150 100 0 121 687.5 2.5
3 100 150 0 121 687.5 2.5
3 50 200 0 121 687.5 2.5
3 200 0 50 121 687.5 2.5
3 150 0 100 121 687.5 2.5
3 100 0 150 121 687.5 2.5
3 50 0 200 121 687.5 2.5
3. Results
3.1. Compressive strength
All 162 cubes were submitted to compressive strength tests, following standard ASTM C 109 (1999), after curing times of 1, 3, 7, 28, 56 and 118 days. A universal hydraulic machine, reference UH-500 kN1, was used to perform these tests.
The results showed that it is possible to replace 20% of cement for fly ash without affecting quality, given that the mixtures containing a higher percentage of fly ash did not match the strength of the 100%-cement mortar (Fig. 5).
Fig. 5. Compressive strengths for different mortar mixture.
Even though fly ash is branded industrial waste, it has begun to be considered useful because of it pozzolanic properties, which need to be activated to develop products with strength and durability. Hence, mixing fly ash rich in silica (S) with the calcium hydroxide (CH) part of hydrated cement, results in the formation of the well-known Calcium-Silicate-Hydrate (C-S-H) gel, responsible for increased strength in hardened cement.
Fig. 6. Compressive strength comparison of NFA and UFFA mixtures.
The compressive strengths of the mixture with a 20% replacement of cement for fly ash are presented in Figure 6 and Table 5. It is clear that by eliminating the unburned residue particles the compressive strength reached is more advantageous, achieving the strength of the pure cement mortar at 118 days of curing time.
Table 5. Compressive strength of mixture at different curing times. Curing time (days) 100% PC PC/NFA (80/20) PC/UFFA (80/20)
1 2.10 MPa 1.31 MPa 1.485 MPa
3 5.48 MPa 3.76 MPa 3.85 MPa
7 9.57 MPa 6.53 MPa 6.42 MPa
28 18.88MPa 13.97 MPa 14.42 MPa
56 20.59 MPa 17.14 MPa 17.39 MPa
118 21.06 MPa 19.79 MPa 20.82 MPa
4. Conclusions
Eliminating part of the unburned carbon in fly ash is necessary so that mixtures containing a 20% addition of fly ash can achieve similar resistance to that of pure cement mortars. Using sieving techniques to remove the largest unburned particles improves the properties of fly ash, allowing the reuse of the unburned carbon as fuel. The mortar mixtures with added fly ash acquire better resistance with a longer curing time. Adding up to 20% of fly ash to cement mixtures generates economic and environmental benefits, given that Clinker production is expensive and that fly ash storage has a negative environmental impact on locations nearby the thermal power plants that produce them.
Acknowledgements
The authors wish to express their gratitude to: INCITEMA, the Institute for Research and Innovation in Science and Technology of Materials (Instituto para la Investigación e Innovación en Ciencia y Tecnología de Materiales); to the UPTC, the Pedagogical and Technological University of Colombia (Universidad Pedagógica y Tecnológica de Colombia); to the thermal power plant TERMOPAIPA - GENSA; and to the cement company ARGOS (Cementos ARGOS S.A.).
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