Effect of waste glass incorporation on the properties of geopolymers formulated with low purity metakaolin

Journal Pre-proof Effect of waste glass incorporation on the properties of geopolymers formulated with low purity metakaolin O. Burciaga-Díaz, M. Durón-Sifuentes, J.A. Díaz-Guillen, J.I. Escalante-Garcia PII:

S0958-9465(19)31335-6

DOI:

https://doi.org/10.1016/j.cemconcomp.2019.103492

Reference:

CECO 103492

To appear in:

Cement and Concrete Composites

Received Date: 13 March 2019 Revised Date:

11 November 2019

Accepted Date: 19 December 2019

Please cite this article as: O. Burciaga-Díaz, M. Durón-Sifuentes, J.A Díaz-Guillen, J.I. EscalanteGarcia, Effect of waste glass incorporation on the properties of geopolymers formulated with low purity metakaolin, Cement and Concrete Composites, https://doi.org/10.1016/j.cemconcomp.2019.103492. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

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Effect of waste glass incorporation on the properties of geopolymers formulated with low purity

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metakaolin

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O. Burciaga-Díaza,*, M. Durón-Sifuentesa , J.A Díaz-Guillena , J.I. Escalante-Garciab

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a

Tecnológico Nacional de México, Instituto Tecnológico de Saltillo, Blvd. Venustiano Carranza # 2400, Col. Tecnológico, C.P. 25280 Saltillo, Coahuila, México.

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b

Cinvestav Saltillo Av. Industria Metalúrgica No. 1062, Parque Industrial, Ramos Arizpe, Coahuila, MX

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C.P. 25900, Saltillo, Coahuila, México

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*Corresponding author: [email protected]

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Abstract

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Given the worldwide abundant resources of low purity kaolin and urban waste glass, their use to produce

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alkali activated cements seem to be a promising area in the search for alternative sustainable cements with

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environmental advantages over Portland cement. Under this perspective, the properties of silicate activated

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low purity metakaolin (MK) and flat soda lime silicate waste glass (WG) at MK/WG mass ratios of 100/0,

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85/15 and 70/30 were investigated. The chemical composition of the pastes was varied in terms of

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modulus (Ms=0.8-1.2) SiO2/Na2O of the activating solution with additions of 12 and 16 wt.% Na2O. Cubic

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samples dry cured at 20°C were used to follow the compressive strength from 1 to 90 days, which reached

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from 17 to 56 MPa; the highest values were attained by samples activated with Ms= 0.8- 1 and 12%Na2O.

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The products of the reactions were evaluated by X- ray diffraction, infrared spectroscopy, scanning

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electronic microscopy and nuclear magnetic resonance. The microstructures included unreacted MK, WG

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and N-A-S-H, while the incorporation of WG favored the formation of a more complex reaction product

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similar to a (N,C)-A-S-H type gel.

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Keywords: geopolymers, waste glass, low-purity metakaolin, compressive strength, microstructural

29

characterization.

30 31

1. Introduction

32

Geopolymers are alkali-activated materials conceived as alternative binders to Portland cement (PC),

33

which are produced by the reaction of synthetic or natural amorphous aluminosilicate precursors with

34

concentrated alkaline solutions to form N-A-S-H type reaction products characterized by high compressive

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strength and stability in different service conditions [1,2,3]. The development of these non-clinker based

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cements is of interest to reduce the environmental impact related to the manufacture of PC, which is

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reported to be about 8% of the anthropogenic CO2 emissions [4,5].

38

By their wide availability fly ash and calcined clays of metakaolin are among the most studied raw

39

materials to formulate geopolymers [6]. However, diverse studies have recently reported the possibility of

40

using alkali-activated materials as a medium to incorporate other urban wastes with cementitious potential

41

such as the soda lime silicate glass in form of alkaline activator or as partial replacement to produce pastes,

42

mortars and concretes with improved environmental advantages [7,8,9,10].

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Soda lime silicate glass is an amorphous material with a chemical composition consisting essentially of

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SiO2 (65-75%), CaO (6-12%), Na2O (12-15%), Al2O3 (0.5-5%) and Fe2O3 (0.1-3%); it is widely used to

45

produce bottles, flasks and flat glass for use in the building industry. Once the glass fulfills its

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functionality, it is discarded in quantities exceeding the 65 million tons per year worldwide [11]. From this

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amount more than 46 million tons are yearly land filled due to logistic difficulties for its reuse in the

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manufacture of other glass articles, which is unsustainable as this does not decompose in the environment

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[9,12]. As an attempt to increase the recyclability, powdered waste glass from different sources has been

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used in PC materials and more recently as addition in alkali-activated cements, due to the minor

51

requirements of infrastructure and preconditioning operations [13,14,15,16]. In this regard, Carrasco et al.,

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[17] outlined the feasibility of using soda-lime glass dissolved in NaOH solutions as a source to substitute

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the sodium silicate in type F fly ash based geopolymers; they reported strength values greater than the 37

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MPa observed with commercial waterglass, due to the formation of dense reaction products conformed of

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N-A-S-H gel and zeolite-like crystalline phases.

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In other research, Tho-in et al., [18] observed 7 day compressive strengths of 34-48 MPa in silicate-

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activated geopolymer pastes of fly ash with 10-20% of powder waste glass replacements. Bobirică et al.,

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[19] studied the addition of 10, 20 and 30% of glass replacement in mixtures of fly ash activated with

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NaOH, fly ash-slag with NaOH and fly ash with sodium silicate. The general trend showed decreasing

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compressive strength with increasing amount of waste glass in the formulations. Balaguer-Pascual et

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al.,[20] reported the effect of NaOH concentration and metakaolin (MK) content as replacement of glass

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powder on the mechanical properties of mortars; they concluded that a solution of 5M NaOH and 8%MK

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was enough to reach 30-35 MPa at 91 days, they also noted that higher concentration delayed the

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formation of geopolymeric gel. Other authors have also reported the feasibility of adding waste glass from

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different sources as fluorescent lamps [21 ], liquid crystal displays [22,23,24], color bottles [25] and glass

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fiber [26] in geopolymeric binders; nonetheless, limited information was found on the effect of ground

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waste glass on the properties of geopolymers pastes formulated with low purity dehydroxilated kaolin

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clays. The latter are worldwide abundant and their investigation in the production of new binders becomes

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essential [27, 28].

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Therefore, the purpose of this study is to examine the effect the soda lime silicate waste glass addition on

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the compressive strength and microstructural evolution of geopolymer pastes formulated with a low purity

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clay with ~49% kaolinite, through the use X-ray diffraction (XRD), infrared spectroscopy (FT-IR),

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scanning electron microscopy (SEM) and magic-angle spinning nuclear magnetic resonance (NMR).

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2. Experimental procedure

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2.1 Materials

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Composite pastes of geopolymer were fabricated using metakaolin partially substituted with clear flat soda

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lime silicate waste glass (WG). The metakaolin (MK) was produced in a laboratory furnace (FELISA, FE-

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363) by calcination of low purity kaolin, at 750°C for 6h. The mean particle size of the MK (d50) was of

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28.8 µm and 10% finer than 2.9 µm, while the WG recollected from an urban zone, cleaned, dried and then

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ground in a ball mill, resulted in a material with mean particle size (d50) of 45.3 µm. The X-ray diffraction

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analysis in Fig. 1 showed that the WG was amorphous with a halo at 15-40° 2θ without showing any

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crystalline phase. Previous to the thermal treatment, the kaolinite showed characteristic reflections of

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kaolinite (Al2Si2O5(OH)4), alunite (KAl3(SO4)2(OH)6), quartz and cristobalite but after the calcination at

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750°C only quartz and cristobalite remained.

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Fig. 1. X-ray diffraction patterns of the used raw materials (waste glass, kaolinite and metakaolin obtained

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at 750°C for 6h).

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The chemical composition of both materials, determined by X-ray fluorescence spectrometry is shown in

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Table 1. Following the procedure described elsewhere [29], assuming that all the Al2O3 was present as MK

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(based on the XRD analysis), the mineral was estimated to contain ~49% kaolinite and the remaining 51%

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was crystalline SiO2 in form of quartz and cristobalite. The MK in combination with the WG was

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chemically activated with solutions of sodium silicate modulus (Ms) SiO2/Na2O=2.0 and sodium hydroxide

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flakes blended in proportions intended to cover a relatively wide range of variation.

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Table 1. Chemical composition of the kaolin mineral, soda-lime waste glass and sodium silicate by X-ray

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fluorescence

Oxide (wt.%) SiO2 Al2O3 CaO MgO TiO2 K2O Fe2O3 MnO Na2O P2O5 ZrO2 SO3 CaCO3 H2O LOI Physical properties Blaine (m2/Kg)

100 101

2.2 Sample synthesis and test procedures

Metakaolin

Waste glass

Sodium silicate

74.44 22.47 ----0.50 0.58 0.32 ----0.49 0.08 0.75 -------

69.65 0.85 14.45 2.89 0.10 0.39 0.50 --9.69 --0.02 0.55 ----0.8

29.5 --------------14.7 --------55.8 ---

1112

574

---

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Geopolymer pastes of precursors based on MK/WG in weight proportions of 100/0, 85/15 and 70/30 were

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elaborated by mean of alkaline activation using sodium silicate solutions with Ms = 0.8, 1.0 and 1.2 adding

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12 and 16 % of Na2O relative to the mass binder (MK+WG). Additionally, six mortars were elaborated

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using limestone sand with particle size <4 mm with aggregate:binder weight ratios of 3:1 and 2:1 using

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formulations of pastes activated with Ms = 1 and 12%Na2O; the details are given in Table 2. Before the

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manufacture of the samples, the alkaline solutions were prepared by dissolving NaOH flakes in water and

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then mixing with sodium silicate; the solutions were left to cool down to 20°C. The water to binder ratio

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was kept constant at 0.36, which allowed for a good fluidity of the pastes.

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Table 2. Compositions of pastes prepared to study the effect of the chemical composition on the strength

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development on geopolymer pastes Formulation number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Mortar 1 Mortar 2 Mortar 3 Mortar 4 Mortar 5 Mortar 6

113

a

MK* (%) 100 85 70 100 85 70 100 85 70 100 85 70 100 85 70 100 85 70 100 100 85 85 70 70

WG+ (%) 0 15 30 0 15 30 0 15 30 0 15 30 0 15 30 0 15 30 0 0 15 15 30 30

Msa

Na2O (%)

water/binder ratio

12 0.8 16

12 0.36

1.0 16

12 1.2 16

1.0

12

Aggregate:binder ratio --------------------------------------2:1 3:1 2:1 3:1 2:1 3:1

Ms = Modulus SiO2/Na2O of the alkaline solutions for the chemical activation. All % are in mass basis.

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MK* = Metakaolin ; WG+ = Waste glass

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The alkaline solutions and the MK+WG powders were blended for 3 min, in a bench-top mixer with

117

planetary motion to get pastes. The pastes were then cast in polypropylene cubic molds with sides of 2.5

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cm; the molds were vibrated on a shaking table (Controls Model 55-C0159/LZ) for 30 s to eliminate

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entrapped air bubbles. The samples were left covered with plastic film and moist cloth to avoid water

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losses and further transferred to isothermal chambers to be cured at 20°C ± 2°C with a relative humidity of

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80% for up to 90 days.

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Four samples for each formulation were tested for compressive strength (CS) at the curing ages of 1, 3, 7,

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14, 28 and 90 days using an automatic hydraulic machine (Controls model 50-C7024) at a constant loading

124

rate of 350 N/s; the average was reported. After the CS tests, fragments of hardened pastes were immersed

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in acetone for 2 days and then dried in a vacuum oven (VWR 1430M, Sheldon Manufacturing, inc) for at

126

least 3 days at 38°C to evaporate the water and interrupt the reaction processes. Chosen samples were then

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ground in a planetary mill (Restch PM 400/2), using agate media, to pass the #140 sieve (105 µm). The

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ground powders were characterized by XRD using a powder diffractometer Empyrean, PANalytical in a

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range of 7°- 60°(2θ) using a step size of 0.03° and 2 s per step, using CuKa (1.542 Ǻ) radiation. The

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ground powders were also mixed with KBr for characterization by Fourier transform infrared spectroscopy

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(FTIR) using a Nicolet AVANTAR 320 FT-IR device.

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For SEM analysis, samples were mounted in resin and polished using silicon carbide sand paper of

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numbers from 60 to 1200 and diamond pastes down to 1/4 µm. Gold coating was applied to make the

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samples conductive under the microscope (JEOL JSM-66110LV) accessorized with a detector Oxford

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instruments X-Max of 20 mm2. Representative backscattered electron images of microstructures were

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taken at 500 magnifications in high vacuum mode using an accelerating voltage of 20 kV, spot size of 4–5,

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and a working distance of 10 mm.

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using a Bruker Avance II 300 NMR spectrometer (Billerica, MA) with a range of frequency of 31P to 15N

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and field strength of 7.05 T. Powdered paste samples with particle size <145 µm were used for

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characterization. The 29Si MASNMR spectra were acquired at a frequency of 59.59 MHz applying 5 kHz

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spinning rate on a 4 mm CP-MAS probe using zirconia rotors. Single pulse experiments were carried out

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by applying 90° pulses of 4.0 µs with CW 1H decoupling field strength of 35.0 kHz, and recycle delays of

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10 s. The

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[Si(CH3)4] at 0.0 ppm. The observed

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where a Si tetrahedron is connected to n Si tetrahedra with n varying from 0 to 4 and m is the number of

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neighboring AlO4 tetrahedra.

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The 27Al MAS-NMR spectra were acquired at 78.15 MHz, with the mentioned NMR spectrometer. Single-

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pulse experiments were carried out at 10 kHz spinning rate by applying (π/2) excitation pulses of 2.0 µs

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and 0.6 s repetition delays. The 27Al chemical shifts were referenced relative to a 1.0M AlCl3 solution with

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pH = 2. The spectra were deconvoluted using the DMFIT free software by means of the Gauss–Lorentz

152

curve fitting [30].

Si and

27

Al MAS-NMR (Magic-Angle Spinning Nuclear Magnetic Resonance) spectra were acquired

29

Si chemical shift of the acquired spectra was referenced externally to tetramethylsilane 29

Si resonances were analyzed using the Qn(mAl) nomenclature,

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3. Results and discussion

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3.1 Compressive strength (CS)

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Fig. 2 gathers the compressive strength vs time results for geopolymeric composite pastes with 0, 15 and

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30% of waste glass. The graph shows a strong influence of the activation variables on the development of

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mechanical strength. After 1 day samples with 12%Na2O and Ms= 0.8 presented a rapid strength gain

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greater than 17 MPa, which increased over time and stabilized in a range of 45-52 MPa after 90 days; there

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was a trend of decreasing CS with increasing amount of WG, similar to previous reports for fly ash based

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geopolymers [19]. A similar behavior was observed for binders with Ms= 1.0; however, the strength levels

162

were slightly lower at early (4-12MPa) and later ages (45-52 MPa) than the formulations with Ms= 0.8. On

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the other hand, it seems that an increased content of Si-O-Si species in the activating solution, represented

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by a higher modulus of Ms =1.2, was unfavorable for the setting and strength gain of binders with

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12%Na2O at 1 day, which implies a deceleration of the dissolution processes and condensation of reaction

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products, under such conditions and unlike to the observed with the use of Ms =0.8 and 1.0, the composite

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with 15%WG developed the highest strength (~39 MPa) at 90 days, while samples with 0 % and 30%WG

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reached 35 and 32 MPa, respectively. The reason of the superior mechanical behavior of the sample 85/15

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with Ms=1.2 and 12%Na2O is unclear and more research is necessary.

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171 172

Fig. 2. Effect of the activation variables on the compressive strength of geopolymer pastes with 0, 15 and

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30% of waste glass (WG).

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The amount of Na2O had an important effect on the activation of the composites; with 16%Na2O, the trend

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of lower CS with higher Ms was more evident. An explanation for that is that although a higher

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concentration of Na2O could have fostered the dissolution process of the raw materials, the

178

geopolymerization reactions were delayed by the low mobility and stability of ionic species in the liquid

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media inducing the formation of non-uniform reaction products of lower strength in agreement with

180

previous reports [20, 29]. This can also explain why a higher %Na2O was less effective for binders

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100%MK but favorable for composites with 15 and 30%WG, that showed the highest CS values at later

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ages; the results suggest that although the geopolymerization of the MK was delayed to some extent, the

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dissolution of WG particles was probably favored by a higher alkalinity (16%Na2O) [31]. In this regard the

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solubility of amorphous materials as WG in highly alkaline environments of pH above 10.7 [12,32, 33]

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provided soluble silicates, that polymerized when combined with silicoaluminate amorphous precursors

186

forming reaction products with good strength [29] which correlates with the findings discussed.

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It is noteworthy that for 16%Na2O, the formulations with Ms=1 and Ms= 1.2 did not develop strength

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during the first 7 days of curing, but after 14 days it surpassed the 10 MPa and at 90 days, all formulations

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exceeded 20 MPa; the composite with 30%WG had the highest CS above 35 MPa which is acceptable for

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structural and non-structural applications in the construction industry. Finally, the results show that the

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incorporation of WG at replace levels of 30% by MK, is feasible to get composite geopolymers with

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acceptable mechanical properties in the conditions used for this study.

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Based on the results, pastes 100/0, 85/15 and 70/30 with Ms=1.0 and 12%Na2O were used to prepare

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mortars of aggregate: binder ratios of 2:1 and 3:1 these were cured at 20°C for up to 28 days. Fig. 3 shows

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that higher amounts of sand resulted in lower strengths in agreement with other reports [34]; also, similarly

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to the pastes, higher WG contents lowered the strength. After 1 day all samples surpassed the 2 MPa with a

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substantial increase of strength after 14 days [35]. At 28 days, the mortars with 100/0 developed the highest

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CS of 31 and 25 MPa for aggregate:binder ratios of 2:1 and 3:1, respectively, while the mortars 85/15 and

199

70/30 showed among 22-27 MPa and 17-21 MPa, respectively. The strength shown by the pastes and

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mortars could be suitable for the manufacture of prefabricated building elements as masonry blocks,

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concretes or even as coatings and adhesives.

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203 204 205

Fig. 3. Compressive strength results of mortars with aggregate binder ratios 2:1 and 3:1 activated with Ms =1.0 and 12%Na2O.

206 207

3.2 Fourier transform infrared spectroscopy (FT-IR)

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Fig. 4 shows infrared spectra for composite geopolymers chemically activated using sodium silicate

210

solutions of Ms=1.0 and 12%Na2O after 28 days of curing; the assignment of the signals was based on

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references [36,37,38]. Previous to the alkaline activation the WG spectrum showed a broad band with a

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maximum at 1041cm-1 representative of Si-O-Si asymmetric stretching mode in the SiO4 tetrahedral groups

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existent in the amorphous structure of the glass. Another band of lower intensity was noted at 779 cm–1,

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corresponding to O-Si-O stretching vibrations bonds in the SiO4 groups, likewise in the silicates. The MK

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spectrum shows a broad peak with a maximum at 1095 cm-1 indicating the presence of asymmetric

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stretching vibrations of Si-O-Si and Al-O-Si within the TO4 (T=Si or Al) tetrahedral structure typical of an

217

amorphous aluminosilicate. The shoulder appearing at 802 cm-1 is directly related to the existence of Al-O

218

vibrations in the tetrahedral coordinated AlO4 molecule.

219

220 221

Fig. 4. FT-IR spectra corresponding to the raw materials (MK and WG) and geopolymer pastes with 0, 15

222

and 30% WG cured at 28 days activated with Ms=1.2 and 12%Na2O .

223

224

After alkaline activation, the spectra showed significant differences relative to the unreacted WG and

225

MK, evidencing their reactions and the formation of new products. The main distinctive feature was

226

the disappearance of the bands at 802 cm-1 and 779 cm-1 and the shifting of the vibrational band in the

227

range of 1041-1095 cm-1 towards lower frequencies for samples 100/0, 85/15 and 70/30. Some

228

reports have indicated that the shifting of this band is related to the presence of non-bridging oxygen

229

in the network and to the gradual incorporation of Si and Al into geopolymeric gel as well as on the

230

state of hydration of the reaction products [39].

231

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This indicates that the original silicate and/or aluminosilicate structures in the MK and WG were

233

significantly depolymerized under the alkaline attack to form new aluminosilicate hydrated reaction

234

products; possibly (N,C)-A-S-H-like compounds with different SiO2/Al2O3 ratio compared with the

235

starting raw materials [37,40]. A slightly noticeable band at 1423 cm-1 suggests the presence of O-C-

236

O bonds of CO32- groups, assigned to carbonated phases [41];a decreased intensity of the CO32- band

237

with higher %WG is noteworthy as it implies that its incorporation was beneficial to limit the

238

carbonation. Another feature detected included symmetrical stretching vibrations of Al-O bonds at

239

698 cm-1 corresponding to the formation of new Al-O-Si cyclic structures after the geopolymerization

240

process; such structural units are common in geopolymers and aluminosilicate glasses as pointed by

241

other reports [42].

242

Finally, the stretching bands of H-OH at 3400 cm-1 and the H-O-H bending frequency at ~1646 cm-1 are

243

representative of the absorbed and molecular water incorporated in the porous microstructure and possibly

244

linked into the N-A-S-H-type reaction products, or even in the atomic structure of intermixed silica gel.

245 246

3.3 X-ray diffraction (XRD)

247

Fig. 5 shows diffraction patterns of selected compositions with MK/WG ratio of 100/0, 85/15 and 70/30

248

activated with Ms= 1.0 and 12%Na2O; the patterns of the MK and WG are included for reference. After the

249

alkaline activation all the patterns showed similar characteristics, evidencing similarities among the

250

hydration products for all %WG, curing age and strength. The shifted hump among 20-35° 2θ denotes the

251

formation of aluminosilicate-type reaction products of amorphous nature which are responsible of the

252

strength developed by the binders. The remanence of quartz and cristobalite peaks evidences that such

253

crystalline phases did not take part in the reaction process; however, such phases could act as filler possibly

254

improving the mechanical properties of the geopolymers [29]. Crystalline carbonate phases were not

255

detected suggesting that the carbonation observed by FT-IR may have occurred during the sample

256

preparation and it was limited only to the surface of the powder samples [41]. It has also been considered

257

that XRD detects phases on contents greater than about 4-5%; probably the carbonation in the binders did

258

not exceed such percentage.

259

260 261

Fig. 5. XRD patterns from pastes with 0, 15 and 30% WG activated with Ms=1.0 and 12% Na2O at 1, 14

262

and 28 days

263 264

3.4 Scanning electron microscopy (SEM) and EDS analysis

265

Fig. 6 shows backscattered electron (BSE) microstructures of polished samples of the formulations 100/0,

266

85/15 and 70/30 with 12%Na2O and Ms=1.0. A brighter grey is correlated to higher atomic number density

267

of irradiated zone, so it is possible to identify the different phases in the microstructures. For binder 100/0

268

in Fig. 6 (a), unreacted MK particles were noted as the brighter areas, some MK particles had reaction rims

269

of gray dark tone evidencing that the conditions of chemical activation were suitable to form new reaction

270

products that condensed as a dense geopolymeric matrix bearing some unreacted quartz (SiO2) particles;

271

the latter were inert to the alkaline attack and probably acted as filler reinforcing the microstructures [43].

272

Although, the 100/0 paste had 52.5 MPa at 28 days, some micro cracks were formed, probably as a

273

consequence of drying during the SEM analysis in the vacuum column of the microscope, without affecting

274

the structural integrity of the binder.

275

Similar features were observed in the microstructure of the paste 85/15 (Fig. 6 (b)), although the matrix of

276

reaction products seemed more dense with some small particles of WG fully reacted, readily distinguished

277

as small gray dark spots. The intense dissolution of the MK particles resulted in the formation of dark inner

278

products marked as (IP-MK) by a mechanism of reaction in solid state as the grains were consumed from

279

outer to inner. This indicates differences in the chemical composition and density among the reaction

280

products formed by the consumption of MK particles, and the main binding phase in the outer products

281

(OP) [44]. According to the EDS spot analysis discussed in next section, the matrix was composed by Si,

282

Al and Na, suggesting the formation of N-A-S-H geopolymeric gel in agreement with the compressive

283

strength reached by such formulation.

284

On the other hand, in the microstructure of the geopolymer 70/30 (Fig. 6 (c)) there was a higher amount of

285

unreacted WG particles with angular morphology. It seems that the dissolution of MK particles was mainly

286

favored under the activation conditions (Ms=1 and 12%Na2O), as noted by the formation of inner products

287

(IP-MK) which were not clearly identified around the WG particles. The interface between the WG

288

particles and the matrix seem to have provided a weak mechanical interlocking, which could be one of the

289

possible causes of the lower CS developed by this formulation.

290

291 292

Fig. 6. Scanning electron microscopy of samples with 0 (a), 15 (b) and 30% WG (c) activated with Ms=1.0

293

and 12% Na2O at 28 days.

294 295

Fig. 7 presents energy dispersive X-ray spectroscopy (EDS) results acquired by means of spot analyses

296

taken throughout the microstructure of samples 100/0, 85/15 and 70/30 at 1, 28 and 90 days. Data are

297

plotted in ternary composition diagrams of Al-Si-Na normalized to 100 at.%. As references, the atomic

298

composition of the unreacted MK and WG were also included. For the geopolymer with 100%MK (Fig. 7

299

a) most of the analysis collected in the outer products (OP) at 1 and 28 days located relatively close to each

300

other with an average compositional ratio Si/Al = 1.85, which reduced to ~ 1.4 over time due to the

301

progressive dissolution of MK particles, in the alkaline medium, providing Al to the OP. The difference in

302

chemical composition between the unreacted MK and the OP demonstrates that the main reaction product

303

was a N-A-S-H type gel [45].

304

In sample 85/15 the OP showed a higher Si/Al ratio of 2.1 signifying that part of the WG was effectively

305

dissolved, releasing silicon that was incorporated in the reaction products; this is in agreement with the

306

observations of SEM were zones of fully reacted WG (small particles) were identified. At 90 days the

307

composition of the OP was richer in Al evidencing the advance in the reactions of dissolution of MK. In

308

contrast, the zones around of WG particles, identified as IP-WG had an average composition of 80%Si-

309

10%Al-10%Na with ratio Si/Al ~ 8.0 indicating that the Si dissolved from the WG and precipitated as

310

silica gel in the nearby zones. On the other hand the inner products around unreacted MK (IP-MK)

311

revealed a composition of 30%Si- 40%Al- 30%Na with a ratio Si/Al =0.75 evidencing a preferential

312

incorporation of Al and Na with respect to the OP and IP-WG zones.

313

For the formulation 70/30, the OP showed a trend skewed towards the silicon-rich apex at longer curing

314

times, due to dissolution of WG particles. At 90 days, the N-A-S-H gel formed presented a ratio Si/Al =

315

3.1, higher than that registered in samples 100/0 and 85/15 [46]. This pattern can be explained because

316

when WG is added to the composition and partially dissolved, more SiO2 is incorporated, which tends to

317

increase the content of Si in the OP. Similar to binder 85/15, the IP-WG had a ratio Si/Al of 9.4 with low

318

compositional variation at the different curing ages, while the IP-MK showed higher incorporation of Al

319

and Na with an average composition of 30%Si- 40%Al-30%Na, demonstrating the formation of N-A-S-H

320

products with different chemical composition and Si/Al ratio.

321

322 323

Fig. 7. Chemical composition of hydration products from binders 100/0 (a); 85/15 (b); and 70/30 (c) at 1,

324

28 and 90 days.

325 326

3.5 Solid state 27Al and 29Si MAS NMR spectroscopy

327

The atomic structure of geopolymers made with fly ash and high purity kaolinite clays has been

328

investigated successfully using NMR spectroscopy by means of the Gaussian peak deconvolution

329

corresponding to the 27Al and 29 Si nuclei [47]. Fig. 8 presents the 27 Al NMR spectra from the unreacted

330

MK and silicate -activated pastes 100/0, 85/15 and 70/30 after 28 days of curing at 20°C. In Fig. 8 (a) the

331

asymmetry of the MK spectrum denotes the presence of Al in different coordination sites as a result of the

332

structural disorder induced in the material during the thermal treatment of the low purity kaolin clay. The

333

maximum resonance shoulder at 2 ppm is due to the presence of octahedrally coordinated aluminum

334

(AlO6), the peak at ~ 21 ppm corresponds to AlO5 and the shoulder at 58 ppm is identified as four-fold

335

coordinated Al which is in agreement with other studies [48,49]. The interaction of the alkaline solution

336

with the aluminum sites of the MK was evident during the synthesis of the geopolymers, as the reaction

337

progresses the coordination of aluminum (IV, V and VI) changed completely to IV to form new products,

338

regardless of the level of WG incorporated in the pastes [50].

339

From the deconvolution results shown in Fig. 8 (b), the main peaks from formulations 100/0, 85/15 and

340

70/30, corresponded to two overlapped signals of four fold-coordinated Al. The curve at 50 ppm is due to

341

AlO4 present in the atomic structure of the unreacted MK particles (noted by SEM), while the signal at 58

342

ppm, is referred to tetrahedral Al incorporated in the N-A-S-H type reaction products formed after the

343

alkaline activation.

344 345

Fig. 8. NMR spectra of the 27Al of unreacted MK and alkali activated pastes with 0, 15 and 30%WG (a);

346

deconvolution (b) and relative fraction of AlO4 in N-A-S-H and unreacted MK (c).

347 348

The quantification of the area under the curves showed in Fig. 8 (c) represents the relative fraction of each

349

nucleus in the formulations. For binder 100/0, the 57.9% of AlO4 was incorporated in the N-A-S-H-type

350

reaction products and the remaining 42.1% was from the unreacted MK. Higher %WG resulted in higher

351

fractions of AlO4 in the N-A-S-H gel, but lower than that corresponding to the unreacted MK particles.

352

This implies that the incorporation of the WG as partial replacement at 15 and 30%, favored a higher

353

reactivity of the MK particles to form N-A-S-H. This could be explained due to a lower reactivity of the

354

WG relative to the MK, so that as the amount of WG increased (85/15 and 70/30), that of MK reduced,

355

hence a higher concentration of alkaline solution was available to react with less MK, resulting in a more

356

intense dissolution of MK particles and condensation of reaction products. The above is verified for

357

samples 85/15 and 70/30 that showed a contribution of 69 and 64% of AlO4 in the N-A-S-H phase,

358

respectively, which was higher than that observed in the formulation 100/0 (~57.9%).

359

The analysis of the

360

wide resonance band among -80 ppm to -120 ppm with a maximum peak at -95 ppm attributed to Q3

361

signals that represent the connection of silicon tetrahedron through 3 of its vertices (oxygen bridges). The

362

asymmetry of the curve suggests that this may be formed by the superposition of Q3 (-93 ppm) and Q4 (-

363

101 ppm) signals also found in the atomic structure of the glasses, conferring its amorphous nature [51].

364

The MK spectrum had a broad band in the range of -95 ppm to -120 ppm with two resonance peaks at -108

365

ppm and at -112 ppm, corresponding to Q4 sites characteristic of quartz and cristobalite, which are

366

crystalline impurities contained in the mineral. It has been reported that phases that have all the

367

crystallographically equivalent atomic sites will exhibit very sharp resonance peaks indicating that the

368

structure of the material is highly crystalline; on the other hand, wide and asymmetric curves are typical of

369

materials with amorphous structure [52].

370

After the alkaline activation, the geopolymers showed a main signal at -93 ppm corresponding to Q4(3Al)

371

nuclei representative of reaction products where tetrahedral units of Si are connected to Al atoms [53]. The

372

remanence of Q4 (0Al) signals is observed at -109 and -112 ppm, which confirms that the crystalline phases

373

as quartz and cristobalite did not participate in the reaction processes, as previously observed by FT-IR and

374

XRD.

375

The asymmetry and width of the 29Si NMR bands from binders 100/0, 85/15 and 70/30 reveal a variety of

376

electron distributions around the Si nuclei in the alkali activated samples, with the existence of different Si

377

coordination environments [54]. The decomposition of the spectrum corresponding to the formulation

378

100/0 in Fig. 9 (b) shows that the reaction products consist of Q4(4Al), Q4 (3Al) and Q4(2Al) sites in which

29

Si RMN spectra is shown in Fig. 9. The spectrum of the WG in Fig. 9 (a) shows a

379

SiO4 structures are bonded by oxygen bridges at 4, 3 and 2 Al atoms, respectively. The union of these

380

species and the variability in the coordination number of Si result in the formation of polymer chains

381

organized in a three-dimensional amorphous network characteristic of the N-A-S-H gel [55]. At -104 ppm,

382

the Q4 (1Al) signal indicated unreacted MK particles, while the signals at -108 and -110 ppm originated

383

from the crystalline phases quartz and cristobalite. At -113 ppm appears a curve of polymerized structures

384

of Q4(0Al) condensed as silica gel originated from a fraction of the sodium silicate used as activator that

385

did not participate in the reactions; it has been proposed that the condensed silica gel intermixed with N-A-

386

S-H gel enhances the CS [29].

387

Similar to the previously discussed, the binders 85/15 and 70/30, also showed the remanence of unreacted

388

crystalline phases, and condensation of Q4(4Al), Q4 (3Al) and Q4(2Al) structures representative of the N-

389

A-S-H gel formation. The Q4(1Al) signal corresponding to unreacted MK was also identified in Fig. 9 (c)

390

and (d). The increased amount of WG contents (15% and 30%) favored a higher intensity of the Q4(0Al)

391

sites at -108 ppm by the incorporation of polymerized silicon structures from unreacted WG and silica gel

392

that precipitated after the alkaline activation.

393

A noteworthy feature was the possible appearing of small signals corresponding to Q1, Q2(1Al) and Q2 at -

394

79 ppm, -81 ppm and -84 ppm, respectively. These nuclei are typical of C-(A)-S-H formation which is a

395

reaction product commonly reported for hydrated cement Portland and alkali activated slags due to the

396

calcium content [56]. As mentioned in Table 1, the WG is a source of calcium (~14 %CaO) hence during

397

the alkaline activation, the WG could be partially dissolved releasing Si and Ca leading to the later

398

formation of C-(A)-S-H; nonetheless, it cannot be concluded if it condensed as a separate phase,

399

intermixed with N-A-S-H or even if a more complex reaction product as the (N,C)-A-S-H was formed.

400

Nonetheless, it is clear that such results demonstrate that the addition of WG in MK geopolymers had a

401

great effect on the structural characteristics of the reaction products, which undoubtedly may affect the

402

mechanical properties and durability of pastes, mortars and concretes [57].

403 404

Fig. 9. NMR spectra of the 29 Si of raw materials (MK and WG) and alkali activated pastes with 0, 15 and

405

30%WG (a) and their deconvolution (b, c, d).

406 407

Fig. 10 presents a simple illustrative structural model, proposed for the (N,C)-A-S-H reaction products of

408

composite binders of MK/WG assuming a suitable structural compatibility between de C-S-H and N-A-S-

409

H gels. It is observed that an amorphous structure was formed through the union of different structural

410

Qn(mAl) units as Q4(4Al), Q4(3Al) y Q4(2Al) with cavities of sufficient size to incorporate Na+ ions that

411

balance the negative electronic charge of the tetrahedral units of Al. The partial dissolution of the WG also

412

favored the condensation of C-(A)-S-H that could be linked to the N-A-S-H gel through aluminum or

413

silicon tetrahedra; however, more research is required to gain understanding regarding the structural

414

compatibility of both gels and the chemical restrictions to know the conditions that can determine their

415

condensation as separated phases or as (N,C)-A-S-H type complex reaction products [45].

416

417 418

Fig. 10. Two-dimensional simple structural model proposed for the (N,C)-A-S-H gel.

419 420

4. Conclusions

421

This paper presents results of geopolymer binders formulated by mixtures of metakaolin (MK) and waste

422

glass (WG). Both materials are abundant in different regions and have potential to be used as building

423

materials. The compressive strength, microstructural and structural characterization of pastes and mortars

424

allow the following conclusions:

425

•

Soda lime silicate waste glass can be effectively incorporated as partial substitute of low purity metakaolin

426

(with ~49% of kaolinite) at levels of up 30% to produce geopolymer pastes with compressive strength of

427

17-55 MPa after 90 days of curing at 20°C.

428

•

the metakaolin and condensation of reaction products, delaying the setting and hardening in pastes.

429 430

Alkaline solutions of increased modulus (Ms) from 0.8 to 1.2 were less effective towards the dissolution of

•

The amount of Na2O has an important effect on the activation of the composite pastes. The lower

431

concentration of 12%Na2O was more effective for strength development than the use of 16%Na2O for

432

pastes. This is interesting as common geopolymers of high purity metakaolin require high %Na2O and the

433

use of an initial 24h curing at temperatures above 60°C.

434

•

Mortars with Ms= 1.0 and 12%Na2O showed a positive trend for compressive strength gains over time. At

435

28 days of curing, mortars of MK/WG with a 100/0 ratio developed 31 and 25 MPa for aggregate:binder

436

ratios of 2:1 and 3:1, respectively; while mortars 85/15 and 70/30 with higher amount of WG showed

437

among 22-27 MPa and 17-21 MPa, respectively.

438

•

Infrared spectroscopy and X-ray diffraction demonstrated the formation of new amorphous aluminosilicate

439

reaction products after the alkaline activation, and lower carbonation was noted for higher waste glass

440

contents. The crystalline phases of quartz and cristobalite from the raw materials did not take part in the

441

geopolymerization reactions acting as fillers.

442

•

Scanning electron microscopy of a binder with 30% waste glass showed a weak mechanical interlocking

443

between unreacted glass particles and the matrix, which was deemed as one of the possible reason of the

444

reduced strength compared with samples with 15% and 0% waste glass.

445

•

Energy dispersive spectroscopy analyses indicated that the matrixes of reaction products were composed

446

by Si, Al and Na, suggesting the formation of N-A-S-H geopolymeric with different chemical composition

447

and intermixed with condensed silica gel as well as with unreacted particles of metakaolin and waste glass.

448

•

Nuclear magnetic resonance demonstrated the condensation of Q4(4Al), Q4 (3Al) and Q4(2Al) structures

449

representative of the N-A-S-H gel formation in binders with 100% metakaolin, but the incorporation of 15

450

and 30% waste glass favored a higher incorporation of polymerized silicon structures and the formation of

451

C-(A)-S-H distinguished by the appearing of Q1, Q2(1Al) and Q2 nuclei intermixed with N-A-S-H resulting

452

possibly in a more complex product as the (N,C)-A-S-H.

453 454

Acknowledgements

455 456

This research was financially supported by the Technological National of Mexico (TecNM) through the

457

project 6167.17-P. The National Council of Science and Technology (Conacyt) is also acknowledged by

458

the scholarship 711334 awarded to Durón-Sifuentes.

459 460

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