Effect of surface-modified fly ash on compressive strength of cement mortar

Materials Today: Proceedings xxx (xxxx) xxx

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Effect of surface-modified fly ash on compressive strength of cement mortar Ivan Mironyuk a, Tetiana Tatarchuk a,b,⇑, Natalia Paliychuk b, Iryna Heviuk c, Alexander Horpynko c, Oleg Yarema c, Ihor Mykytyn a a b c

Department of Chemistry, Vasyl Stefanyk Precarpathian National University, 57, Shevchenko Str., Ivano-Frankivsk 76018, Ukraine Educational and Scientific Center of Materials Science and Nanotechnology, Vasyl Stefanyk Precarpathian National University, 201, Galytska Str., Ivano-Frankivsk 76018, Ukraine PJSC Ivano-Frankivskcement, Ivano-Frankivsk Region 77422, Ukraine

a r t i c l e

i n f o

Article history: Received 19 September 2019 Accepted 8 October 2019 Available online xxxx Keywords: Cement Fly ash Surface modification Compressive strength Pozzolanic reaction

a b s t r a c t This paper presents the experimental results on the impact of modified coal fly ash on mechanical performance of cement materials. Fly ash used in the experiments was received from Burshtyn thermal power plant (Ukraine). The surface modification was carried out by the acidic treatment in order to increase its pozzolanic activity. The impact of raw fly ash and surface-modified fly ash on the cement mortar strength have been compared. It was found in the experiments that the replacement of cement by 5% (wt) of modified fly ash increase the compressive strength value from 61.3 MPa to 63.2 MPa compared to unmodified fly ash at age 28 days. The mineral phase formation, structure and chemical compositions of the modified fly ash/cement composites were analyzed by XRD, SEM and EDS. This research is of great importance and opens a new way for the future investigation and utilization of coal fly ash. Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the XVII International Freik Conference on Physics and Technology of Thin Films and Nanosystems.

1. Introduction Every year the billions of tons of various cement compositions, including Portland cement, are produced in the world causing the atmosphere pollution by greenhouse gas carbon dioxide [1]. In order to make production more eco-friendly, researches are being carried out in the direction of replacing part of cement in concrete by the mineral additives, such as fly ash [2–4]. The fly ash is produced by thermal power plants, formed during the burning of coal and causes the atmosphere pollution [5]. The fly ash is one of the active mineral additives, which is most widely used in almost all countries of the world as a pozzolanic additive to cement [6,7]. Its main component is amorphous silica, which can interacts with calcium hydroxide in cement mortars, leads to formation of calcium hydrated silicates and improves the strength of the cement matrix [8].

⇑ Corresponding author at: Department of Chemistry, Vasyl Stefanyk Precarpathian National University, 57, Shevchenko Str., Ivano-Frankivsk 76018, Ukraine. E-mail address: [email protected] (T. Tatarchuk).

Since Portland cement contains about 65% of the lime and part of it remains free after the concrete hydration, then there is a need to introduce a pozzolanic additive into the concrete in order to neutralize the lime and in general improve the mechanical properties of the concrete [3]. The using of fly ash as a mineral additive can reduce the cement price, increase the density of the structure and the stability of concrete to corrosive aggressive environments [9]. The advantage of fly ash is related to their pozzolanic activity in solidifying cement. In current research, it is proposed to use the coal fly ash obtained from Burshtyn thermal power plant (Ukraine) as a pozzolanic additive in cement [10]. The million tons of fly ash microspheric particles, accumulated in the settling cesspools as result of years of thermal power plant work, cause a serious environmental problem. The fly ash particles have microspherical shape with size of 2–100 mm. The main components of fly ash are quartz, mullite and magnetite in the form of ultra-small crystallites, which are interconnected by the vitreous phase [10]. Nowadays many works have been carried out to investigate the possibility to obtain multifunctional high-strength corrosionresistant concrete by introducing of fly ash filler [11–15]. However, fly ash has a low adhesion and low pozzolanic activity if its surface

https://doi.org/10.1016/j.matpr.2019.10.016 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the scientific committee of the XVII International Freik Conference on Physics and Technology of Thin Films and Nanosystems.

Please cite this article as: I. Mironyuk, T. Tatarchuk, N. Paliychuk et al., Effect of surface-modified fly ash on compressive strength of cement mortar, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.10.016

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is untreated [9,16,17]. Therefore, the surface modification is necessary in order to enhance the fly ash reactivity. Thus, in this paper a new method of chemical modification of fly ash is proposed. It provides the creation of additional hydroxyl groups on the fly ash particles surface increasing their chemical reactivity and pozzolanic activity. The modified fly ash has been obtained by ion-exchange process of alkali and alkaline-earth metals substitutions on the fly ash particles surface by the protons. At the same time, the utilization of the coal fly ash will reduce the price of Portland cement and solve the environmental problems on the territory around the Burshtyn thermal power plant (Ukraine). 2. Experimental 2.1. Materials properties Raw materials in the experiment were coal fly ash (obtained from Burshtyn thermal power plant, Ukraine) and Portland cement (obtained from PJSC Ivano-Frankivsk cement, Ukraine). The chemical composition of the coal fly ash and Portland cement used to obtain the cement mortars were analysed by EDS and described in Table 1. 2.2. Preparation of surface-modified fly ash Raw fly ash (FA), obtained from Burshtyn thermal power plant, was used to obtain the modified fly ash (FAM). Surface modification of glass microspheres was carried out by the acidic treatment under certain temperature and constant stirring. Such surface activation destroys the integrity of the glass shell and leads to the replacement of Na+, K+, Mg2+cations by protons. The protonated surface become more active in the pozzolanic reaction. The more details of this process will be described in the patent.

Table 2 The composition of cement mortars with fly ash. Material

Labeling

A

B

C

Portland cement (Type I) (%) Fly ash (%) Modified fly ash (%)

FA-0 FA-5 FAM-5

100 – –

95 5 –

95 – 5

patterns were recorded in the range of 10-70owith step of 0.02 using X-ray diffractometer STOE STADI P with Cuka anode (k = 0.154 nm). The surface morphology and elemental chemical composition of the samples were done by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) by using REMMA-102-02 Scanning Electron Microscope-Analyzer (JCS SELMI, Ukraine). 3. Results and discussion 3.1. Compressive strength Fig. 2 demonstrate the impact of the fly ash content (5% wt) on the compressive strength of the cement mortar samples at ages 2-days, 7-days and 28-days. It can be seen that after 2 days the compressive strength of cement mortars containing fly ash and modified fly ash decreased. The compressive strength of cement mortars at 7 days has a highest value for sample FA-5 (53.6 MPa) compared to samples FA-0 and FAM-5 (51.9 MPa and 51.2 MPa respectively). However, at age 28 days the composition FAM-5 shown the best result: the compressive strength is 63.2 MPa in comparison with 61.3 MPa for FA-0 and 60.5 MPa for FA-5 samples respectively. It proves that the modified fly ash can accelerate strengthening of late age mortar due to pozzolanic reaction.

2.3. Mixture proportions 3.2. X-ray diffraction analysis (XRD) Two types of coal fly ashes (raw fly ash (FA-5) and modified fly ash (FAM-5)) were used to replace cement at level of 5% by weight of the cement. After dry mixing the cement and fly ashes in a mixer, the required amount of water was added and mixing was continued to achieve an uniform consistency in the mixture. The proportions of admixture for cement mortars varies as shown in Table 2. The mortar specimens were prepared for mechanical testing and characterization. 2.4. Characterization The compressive strength test was carried out according to DSTU EN 196-1: 2015 using the testing machine Walter + Bai AGDB300/20 SUPER at PJSC ‘‘Ivano-Frankivskcement” (Fig. 1). The prisms of sizes 40 mm  40 mm  160 mm were prepared from a cement mortars. The water-cement ratio was maintained at 0.40. The mortar specimens were kept into water environment at temperature 25 °C and tested at the ages of 2, 7 and 28 days. Chemical compositions of cement samples were characterized by X-ray diffraction. The specimens were ground to fine powder and XRD

The X-ray data (Fig. 3) showed that diffraction patterns have diffraction peaks corresponding to the main phases in cement compositions: Portlandite Ca(OH)2, calcite CaCO3, alitheCa3SiO5, calcium silicate hydrate (C-S-H) etc. In all samples there is also a quartz phase (SiO2). It is known that the early age strength after cement hydration is mainly determined by tricalcium silicate (C3S), and the late age strength is determined mainly by tricalcium silicate (C3S) and dicalcium silicate (C2S), where as tricalcium aluminate (C3Al) and tetracalcium aluminate ferrite (C4AF) are negligible impact on early age and late age strength. From the data of the X-ray analysis, it can be concluded that the processes of hydration of Portland cement with fly ash (5%) proceed more intensively than similar processes in comparison of cement without the fly ash addition. And this, in turn, should result in the formation of a significant number of low-calcium hydrosilicates C-S-H, which are characterized by greater resistance in aggressive environments. The analysis of the concrete compositions diffractograms shows a decrease in the intensity of the peaks of portlandite Ca(OH)2 for FAM-5 cement composition which is

Table 1 The chemical composition of the coal fly ash and Portland cement used to obtain the cement composition. Component

SiO2 (wt%)

Al2O3 (wt%)

Fe2O3 (wt%)

CaO (wt%)

MgO (wt%)

SO3 (wt%)

R2 O (wt%)

LOI (wt%)

FA Portland cement

58.33 21.42

17.16 5.05

11.65 2.92

2.13 64.64

0.50 0.92

4.46 2.66

1.79 0.95

0.90 0.75

Please cite this article as: I. Mironyuk, T. Tatarchuk, N. Paliychuk et al., Effect of surface-modified fly ash on compressive strength of cement mortar, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.10.016

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Fig. 1. Testing machine Walter + Bai AGDB300/20 SUPER (PJSC ‘‘Ivano-Frankivskcement”).

3.3. Scanning electron microscopy (SEM) of hydrated cement mortars

Fig. 2. Compressive strength of cement mortars with FA of 0.5% (wt) at ages 2, 7, and 28 days.

Fig. 4 show a microstructure of raw fly ash (Fig. 4a) and cement mortars without and with fly ash at age 28 days (Fig. 4b–d). In Fig. 4b it can be seen deep cracks that, apparently, were formed as a result of the washing of easily soluble substances such as Portlandite. Such a process will be accompanied by a reduction in the corrosion resistance of cement mortars without fly ash. The SEM data showed that the chemically modified fly ash has a significantly higher reactivity and pozzolanic activity (Fig. 4c) compared to unmodified fly ash (Fig. 4d). It is clearly seen that the modified fly ash microsphere are placed in the C-S-H gel and are strongly kept by the cement mortar. Since portland cement contains about 65% of lime and part of this lime becomes free and accessible during the hydration process, the pozzolanic additives (fly ash, for example) introduced into the concrete mixture, react with lime, form concrete, improving its properties. It is seen that the glass shell of the modified fly ash leads to an intensive chemical reaction with portlandite Ca(OH)2 in the presence of water and the formation of strong contact with the binding gel. The absence of pores on the surface indicates the binding of portlandite into insoluble silicate compounds, which inhibits the rinsing of easily soluble compounds and results in increased corrosion resistance of the studied concrete compositions with the addition of modified fly ash. Conversely, modified fly ash composition is quite dense, homogeneous, represented by fine crystalline phases, which, according to their morphology, can be attributed to the formation of hydrated aluminosilicate compounds. 4. Conclusions

Fig. 3. XRD patterns of cement mortars at age of 28 days.

explained by the active interaction of the modified fly ash with calcium hydroxide and the formation of calcium hydrosilicates (C-S-H) (Fig. 3).

Surface-modified coal fly ash was prepared in order to investigate its possibility to replace the some part of cement in concrete. The partial replacement of cement by surface-modified fly ash improves the compressive strength of cement mortars due to higher dispersion between particles and pozzolanic reaction. The SEM data shown that the spherical particles of modified fly ash fill the pores in cement matrix and kept strongly by cement mortar. The replacement of cement by modified fly ash demonstrate a significant advantage for improving the mechanical property of concrete, reduce its price and solves the environmental problem related to the accumulation of greenhouse gas CO2 in the atmosphere. Thus, it is relevant to study the possibility of using such active mineral additive as modified fly ash to obtain multifunctional high-strength corrosion-resistant concrete.

Please cite this article as: I. Mironyuk, T. Tatarchuk, N. Paliychuk et al., Effect of surface-modified fly ash on compressive strength of cement mortar, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.10.016

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Fig. 4. Microstructures of (a) raw fly ash and cement mortars (b) FA-0; (c) FA-5; (d) FAM-5 at age 28 days.

Acknowledgements The authors would like to thank the PJSC IvanoFrankivskcement (Ukraine) for funding this research (Projects ‘‘Composite building materials based on the cement and fly ash, obtained from thermal power plants”, Project Number 0119U100713). References [1] F. Mushtaq, M. Zahid, I.A. Bhatti, S. Nasir, T. Hussain, J. Environ. Manage. 240 (2019) 27–46. [2] Y.K. Cho, S.H. Jung, Y.C. Choi, Constr. Build. Mater. 204 (2019) 255–264. [3] P. Turgut, F. Demir, J. Clean. Prod. 226 (2019) 270–281. [4] E.R. Teixeira, A. Camões, F.G. Branco, J.B. Aguiar, R. Fangueiro, Waste Manag. 94 (2019) 39–48. [5] E.M. van der Merwe, C.L. Mathebula, L.C. Prinsloo, Powder Technol. 266 (2014) 70–78.

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Please cite this article as: I. Mironyuk, T. Tatarchuk, N. Paliychuk et al., Effect of surface-modified fly ash on compressive strength of cement mortar, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.10.016