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Alkali-activated metakaolin mortars using glass waste as fine aggregate: Mechanical and photocatalytic properties
Construction and Building Materials 235 (2020) 117510
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Alkali-activated metakaolin mortars using glass waste as fine aggregate: Mechanical and photocatalytic properties Ruby Mejía de Gutiérrez ⇑, Mónica A. Villaquirán-Caicedo, Luis A. Guzmán-Aponte Composite Materials Group (CENM), Materials Engineering School, Universidad del Valle, Calle 13 # 100-00, Cali, Colombia
h i g h l i g h t s Alkali-activated mortars based on metakaolin and TiO2 particles were evaluated. Glass bottle waste was used as an alternative fine aggregate. The compressive strength was 31.5 MPa at 28 days of curing. The thermal conductivity of the mortars 100G10Ti was 0.87 W/m.K. The self-cleaning photocatalytic capacity of these mortars was increased 72.4%
a r t i c l e
i n f o
Article history: Received 5 November 2018 Received in revised form 18 September 2019 Accepted 5 November 2019
Keywords: Alkali-activated mortars Metakaolin Titanium dioxide Glass bottle waste Photocatalysis Self-cleaning properties
a b s t r a c t In the present study, properties such as density, absorption, porosity, compressive strength, thermal conductivity and photocatalytic activity of alkali-activated mortars based on metakaolin added with TiO2 particles were evaluated. TiO2 particles were incorporated in proportion of 10 wt%. Ground glass (GB) obtained from glass bottle waste was used as an alternative fine aggregate in proportion up to 100% as replacement of natural sand (S). The alkaline activator was a mix of potassium silicate and hydroxide. The photocatalytic activity was measured by the removal of Rhodamine B and the microstructure of the developed material was studied by scanning electron microscopy. The results obtained in this study reveal a reduction in the percentage of absorption of 16.3%, and porosity of 7% for mortars when S is replaced by GB. The mortar prepared with 100% GB reached a compressive strength of 31.5 MPa compared to 28.0 MPa for reference mortar (100% S). The thermal conductivity was reduced by approximately 25%. Finally, the self-cleaning photocatalytic capacity using GB as fine aggregate increased 72.4%. This type of cementitious materials contributes to mitigate environmental pollution and reduces the maintenance costs of structures. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction In recent decades the interest of using photocatalysis in building materials to obtain materials with self-cleaning and antibacterial properties has grown rapidly to help mitigate environmental pollution and reduce the maintenance costs of exposed outdoor structures [1–5]. For the implementation of these new technologies in traditional cementing materials based on Ordinary Portland Cement (OPC), the addition of particles of semiconductor materials, such as TiO2, has been used [1–8]. The main advantages of TiO2 are its relatively low cost, its high chemical stability and nontoxicity, ⇑ Corresponding author at: Composites Materials Group, School of Materials Engineering, Universidad del Valle, Ciudad Universitaria Meléndez, Calle 13 # 10000, Building E47, Second Floor, Cali, Colombia. E-mail address: [email protected] (R. Mejía de Gutiérrez). https://doi.org/10.1016/j.conbuildmat.2019.117510 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
good optical transparency and its highly oxidizing power, which makes it an excellent candidate for many photocatalytical applications with construction materials [6,9]. This oxide presents three crystal structures: anatase, rutile and brookite; of these the anatase phase exhibits the highest photocatalytic activity [8,9]. These novel properties are generated from reactive oxygen species, hydroxyl radicals and superoxide radical ions (OH and O 2 ), after exposure to sunlight, particularly to the UV-a fraction that coincides with the banned TiO2 band [6]. The first radical is formed for the oxidation of water and the second for the reduction of O2 molecule. The two radicals are highly reactive species and they can degrade organic and inorganic species in further reactions. This TiO2 technology has been applied in some buildings constructed with cement-based materials, between these the Church ‘‘dives in Misericordia” (Rome, Italy) and the public building ‘‘Cité de la Musique et des Beaux-Arts” (Chambery, France) are two of the
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most referenced. However, there are also concrete constructions and finished pavements in Belgium, France, Italy, Monaco, Morocco, Japan and China [10,11]. A state-of-the-art report titled ‘‘Application of Titanium Dioxide Photocatalysis to Construction Materials” was published by the RILEM Technical committee 194-TDP in 2011, which includes applications for self-cleaning, antibacterial and air purification [8–11]. In recent years, other authors have suggested that the incorporation of TiO2 can also contribute to the durability of the cementing material. Fiore et al. [12] studied the effect of TiO2 as a protective barrier against the deterioration of reinforced concrete because of carbonation process. The authors proved that photocatalytic concrete products applied on the surface of concrete improved the carbonation resistance and reduced the corrosion propagation rate of reinforcement bars with respect to cement coatings without any photocatalysts [12]. In addition, the use of different wastes such as blasted copper slag [13] and ground glass as a partial replacement of cement and fine aggregate in cementitious materials has been evaluated obtaining promising results in various applications such as roads, trails, concrete or structural and non-structural masonry [14–21]. The review written by Paul et al. [16] analyzes the engineering properties (mechanical and durability properties) of cement- based materials where glass powder has been used as both binder and fine aggregate. The authors consider that there is potential for its use due to the large volumes generated from glass waste, especially those that go to landfills being this an alternative material to the raw materials used in the concrete. They also identify the fields that require research, among which they point to the importance of a life cycle analysis to quantify the impact of the use of glass in concrete. Specifically, finely ground glass presents pozzolanic properties and this makes it attractive as a supplementary cementitious material [16,18]. Some studies have shown that this type of aggregate can be a used as a substitute for natural sand without affecting the mechanical properties of the mortar, on the contrary with a positive effect on its thermal properties [19,21]. Likewise, one of the alternatives proposed by Chen and Poon to increase the performance of the photocatalytic properties of conventional mortars based on OPC in air purification applications was the partial replacement of the fine aggregate or sand by ground glass of various tonalities, obtaining improvements in the photocatalytic activity due to the properties of the glass to transmit light, especially for those of clear or transparent colour [20]. In recent years, the glass waste has also been incorporated in alternative binders such as alkali activated materials (AAMs). The AAMs are produced by the reaction of silica and alumina-rich precursors (Fly ash, metakaolin, blast furnace slag and others industrial by-products and wastes) with an alkaline substance generally at low temperature [22]. AAMs depending of the type of raw materials used and their corresponding design can present properties such as high early compressive strength, high resistance to aggressive chemicals and excellent fire resistance [23,24]. In this type of materials, the glass has been used as a precursor, source for produce alkali activators and replacement of the fine aggregate [25–35]. This has been possible due to its chemical instability in alkaline conditions and higher content of silica rich glassy phase [30]. The studies mainly have been realized with fly ash or slag as precursor obtaining highly satisfactory results. Specifically, the incorporation of waste glass as fine aggregate can enhance some properties of AAMs mortars such as workability, thermal conductivity, resistance to fire, and resistance to sulfate attack [25,33]. Rivera et al. [26] used bottle glass as precursor and window glass as aggregate to fabricate tiles with acceptable mechanical properties. Torres-Carrasco et al. [27,28] used sodium silicate solutions formed from glass wastes as alkaline activator in the preparation of AAMs based on fly ash and slags. Hajimohammadi et al. [29]
evaluated the properties of a geopolymer based on fly ash and glass aggregates, the authors concluded that this type of aggregate is a suitable alternative to fine sand for concrete applications. Arulrajah et al. [31] synthesized recycled glass-fly ash geopolymers, the glass used had the maximum particle size of 4.75 mm similar to a fine aggregate, validating the viability of the use of these materials as low carbon footprint masonry elements, with a mechanical performance of up to 17.42 MPa after 7 days of curing. Luhar et al. [30], in a recent review, present the different applications of GB in geopolymer composites; the authors revised the previous studies on rheological properties and compressive and flexure strength. In general, the different studies show that the use of glass can enhance some properties of AAMs in order to produce an innovative and promising eco-supplementary cementitious material. This is consistent with what other authors have reported [25,29,32–35]. On the other hand, the synthesis and properties of metakaolin (MK)-based geopolymers have been widely studied over the past decades. The activators traditionally used are hydroxides (Na+, K+, and Ca2+), silicates (Na + and K + ), carbonates or sulfates. Rocha et al. [36] compared potassium and sodium – based activators concluding that the mechanical properties are similar although better thermal performance was obtained using potassium silicate and potassium hydroxide. Villaquiran et al. [37,38] studied the mechanical and thermal properties of MK geopolymer using different types of activators, potassium silicate, rice husk ash (RHA), silica fume (SF) and potassium hydroxide. The authors conclude that the exposure of these geopolymers to high temperatures achieves compressive strengths of up to 137.6 MPa (RHA100) and 148.5 MPa (SF100); this result is mainly attributed to the higher densification and formation of crystalline products such as leucite and mullite. Additionally, they mentioned that the alkaline activators based on commercial potassium silicate provide a higher carbon footprint whereas activators based on RHA are more ecofriendly. Recently, the physicomechanical properties of geopolymers based on MK including TiO2 have been studied [39,40]. Guzmán-Aponte et al. [39] indicate that up to 10% TiO2 does not affect the mechanical properties of the geopolymer, although it does reduce the fluidity and setting times of the mixture. Asim et al. [40] in a recent review paper mentioned the geopolymer types with photocatalytic activity; MK basedgeopolymers incorporating TiO2 were included as materials with good efficiency. This article aims to evaluate the mechanical, physical and photocatalytic properties of a geopolymeric mortar based on metakaolin added with particles of TiO2 (10 wt%), which uses ground glass residue as a replacement of sand in proportions of 0, 50 and 100 wt% MK was selected as precursor in this study because its white colour, amorphous character and availability on the country; the final objective is developing an inorganic coating for use on architecture applications. 2. Experimental methodology 2.1. Materials Commercial high purity metakaolin (Metamax, MK) was used as a precursor to produce geopolymeric material. The chemical composition, determined by X-ray fluorescence (XRF), using a Phillips MagiX-Pro PW 2440 spectrometer equipped with a Rhodium tube and a maximum power of 4 KW, is presented in Table 1. The molar ratio SiO2/Al2O3 of MK was 1.97. As an addition, titanium dioxide nanoparticles were used (TiO2 - Aeroxide - P25), whose composition is included in Table 1. Particle size and distribution analysis were performed with a Mastersizer-2000 laser Granulometer from Malvern Instruments with a Hydro2000MU dispersion unit; distilled water was used as a dispersant medium. The mean particle sizes D [4.3] of the MK and titanium oxide were 7.76 and 1.59 lm, respectively (Fig. 1). As a fine aggregate for the manufacture of geopolymeric mortars, glass residues (GB) obtained from the recycling of bottles were used, which, after a thorough cleaning and washing process with water to remove all their contaminants, were
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Table 1 Chemical composition and physical properties of raw materials. Compound (% by Weight)
SiO2 Al2O3 TiO2 Fe2O3 Na2O MgO K2O P2O5 CaO Others Physical properties Density (Kg/m3) Mean Particle Size, D (4,3) (mm)
Precursor
Addition
Fine Aggregate
MK
AEROXIDE TiO2 – P25
Glass bottle GB
52.02 44.95 1.730 0.47 0.30 0.19 0.16 0.06 0.02 – 2500
0.20 0.30 99.50 0.10 – – – – – – 3505
72.27 1.49 0.08 0.62 13.37 0.26 0.51 – 11.15 1.60 2411
7.76
1.59
–
dried and after conditioned mechanically in a jaw mill. The chemical composition of GB is also presented in Table 1, the XRF results show that the glass waste is composed of three main oxides: SiO2 (72.27%), Na2O (13.37%), and CaO (11.15%). This composition suggests that the glass is soda-lime type glass. It is worth noting that this type of glass is the most widely used glass to produce containers and windows. As a reference aggregate, river sand (S) was used. Fig. 2 shows the particle size distribution of the two types of fine aggregates used. This was determined by sieve analysis in accordance with the ASTM C 136 standard [41]. The fineness module of the sand was 2.06 and the fineness module of the ground glass was 3.27. The densities of S and GB were 2650 and 2411 kg/m3 respectively. Fig. 3 shows the X-ray diffraction patterns for MK and TiO2. For the X-ray analysis, a Bruker diffractometer equipped with a wide-angle goniometer RINT2000 was used, using the Ka1 signal of Cu at 45 kV and 40 mA. A 0.02° pitch was used within a range of 5–70° at a rate of 5°/min. Information processing was performed using the X’pert HighScore Plus software package, version 2.2.5. It can be observed in Fig. 3 that MK has a high level of amorphicity due to the halo located between 20° and 30° 2h angle and presents small traces of a crystalline product identified as anatase (ICSD 9852), which corresponds to the observed peaks approximately at 25.3° (100%), 37.8° (18.6%), 48.1° (24%), 53.9° (15%) and 55.1° (15%) 2h. This
Fig. 2. Particle size distribution for sand and ground glass. matches with the percentage of TiO2 reported in the chemical composition of MK (Table 1). There is also evidence that the TiO2 (Aeroxide – P25) used as addition is mainly in the anatase phase (A) which is an important phase because of its photocatalytic potential [6]; however, it has a reduced amount of rutile phase, R (ICSD 33838), less than 1%, identified in the XRD by the peak located at 27.46° 2h which corresponds to the main peak of this phase (100% intensity). This result contrasts with the one presented by Sangchay et al. [42], who report that P25 is composed of 80% anatase and 20% rutile. The X-ray diffraction spectrum of GB (Fig. 3) exhibits a broad amorphous shoulder attributed to the glassy or amorphous phase. The position of the amorphous shoulder is between the angles of 15° and 35° 2h.
2.2. Preparation of geopolymers The geopolymeric pastes were prepared using the molar ratios SiO2/Al2O3 = 2.5 and K2O/SiO2 = 0.28, based on results from previous research [37]. A potassium hydroxide (KOH) analytical-grade reagent and commercial potassium silicate
Fig. 1. Particle Size distribution for TiO2 particles and MK.
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R. Mejía de Gutiérrez et al. / Construction and Building Materials 235 (2020) 117510 Scanning electron Microscopy with Energy-dispersive spectroscopy (SEM/EDS) was performed using a JEOL JSM-6490LV and 20 kV of accelerating voltage. The samples were coated with Au and observed in a low vacuum mode. The photocatalytic properties of the mortars were studied using the UNI 11259– 2016 method [47]. This method is based on the degradation of the pigment Rhodamine B (RhB) once applied on the surface of the sample and exposed to UV-a irradiation. The solution of RhB with concentration of 5 104 g/ml was spread manually with a brush on the specimen, then dried in an electric oven at 40 °C for 20 min, a total of two layers of the pigment were applied to each material evaluated. UV-A irradiation was supplied by two mercury lamps (Electrolux T8 20w/BLB) located inside a black acrylic dome. These lamps emit light with an intensity of 10.3 W M2, which was measured with a Delta Ohm HD 2102.2 Photoradiometer using the filter for UV-a light range (k = 360 nm) at 5 mm. Changes in colour on the surface of the material before and after irradiation were registered with a portable spectrophotometer X-Rite, Ci60 (Photometric Solutions International, Victoria, Australia). The equipment readings were expressed in colorimetric coordinates L*, A* and b* in the CIE LAB system. The coordinate L* represents units in the white range to black, the coordinate a* represents values between red and green and the coordinate b* represents values between yellow and blue. The Rhodamine B removal analysis was performed by comparing the colour parameter a* prior to the irradiation of the sample in the UV source (A* (0 h)), with the colour obtained after UV irradiation during 4 h (A * (4 h)) and 26 h (A * (26 h)). Degradation efficiencies (R4 and R26) were calculated using Eqs. (1) and (2). A material is considered photocatalytic if R4 > 20% and R26 > 50% [47,48].
Fig. 3. X-ray diffraction patterns of raw materials. (K2SiO3) distributed by Pan American Chemicals (SiO2 = 26.38%, K2O = 13.06%, H2O = 60.56%) were dosed to obtain the activator solution module required in the geopolymer mixture (Ms: SiO2/K2O = 0.76). Geopolymeric mortars were manufactured with a geopolymer:fine aggregate ratio of 1:2; where the natural sand (S) was replaced in quantities of 50% and 100% by ground glass (GB). Mortars with and without addition of TiO2 were prepared for comparative analysis and the percentage of addition of TiO2 according to the binder was 10% (wt). The liquid/solid ratio (L/S) remained fixed at 0.40 for all manufactured materials. These parameters were selected from previous studies [39]. The compositions and mixing codes used are presented in Table 2. The mixing process of the solid (MK and titanium dioxide) and liquids (activator) components was performed in a HOBART Vulcan 1249 mixer. The solids were homogenized for 15 s, and the activating solution (KOH + K2SiO3) was added and mixed for 3 min at low speed, followed by 2 min at medium speed and 1 min at low speed until a homogeneous paste was obtained. Finally, the fine aggregate was incorporated, and the mixture was continued at slow speed for another 2 min; the total mixing time was 8.25 min. The geopolymeric pastes and mortars were then casted into cubic silicone moulds of dimensions 20 mm 20 mm 20 mm. The obtained samples were cured at room temperature (25 °C) for 24 h and then demoulded and taken to a humidity chamber (relative humidity > 90% and 25 °C) until the test age was reached (7 and 28 days). 2.3. Instrumental and analytical characterization techniques The fluidity and setting times were evaluated for geopolymeric pastes, MK and MK + 10% TiO2, using the procedures described in ASTM C230-14 [43] and ASTM C191-18 [44]. The compressive strength was evaluated at ages of 7 and 28 days using an INSTRON 3369 Universal test machine with a capacity of 50 kN at a deformation rate of 1 mm/min. In each case, a minimum of three specimens were tested. The test was made according to the ASTM C109/C109M-10 standard using 20 mm cubes on the side [45]. The density, pore volume, and water absorption capacity of each material were also determined according the procedures detailed in ASTM C642-13 [46]. It should be noted that the drying of the sample was performed at 60 °C for 48 h. The thermal conductivity was obtained by using a thermal constants Analyzer TP 500 S Hot Disk with a heating power of 956.69 mW for 40 s, using cubic samples of 20 mm 20 mm 20 mm.
R4 ð%Þ ¼ ½
a ð0hÞ a ð4hÞ 100 a ð0hÞ
R26 ð%Þ ¼ ½
ð1Þ
a ð0hÞ a ð26hÞ 100 a ð0hÞ
ð2Þ
3. Results and discussion 3.1. Workability and setting time The addition of TiO2 particles affected the workability and the setting time of MK geopolymer. The fluidity of reference geopolymer (0% TiO2) was 100.6 mm; the initial and final setting time was 58.8 and 73 min respectively. The addition of 10 wt% TiO2 particles decreased the fluidity 20.4% and the initial and final setting time decreased by 25.9% and 30.1%, respectively. It should be noted that the liquid/solid ratio (L/S) remains fixed at 0.40, so the reduction of the workability of MK geopolymer blended with 10% TiO2 is related to the greater demand for water due to its fine particle size. This was also observed in previous studies of the authors by including different percentages of TiO2 and evaluating different L/S ratios [39]. This is consistent with that reported by Duan et al. [49] using geopolymers based on fly ash. The reduction of the setting time of the geopolymers with the addition of TiO2 at a constant L/S ratio has been also reported in previous researchers; this effect can be attributed to a nucleation and filling effect that contributes to accelerate the degree of reaction of the geopolymer [39,49]. 3.2. Compressive strength Fig. 4 shows the results of the compressive strength at ages 7 and 28 days of curing. The compressive strength results of MK geopolymer blended mortars with 10% TiO2 were between 27.4
Table 2 Composition of the mixtures. Mixture
100S 100S10Ti 50S 50S10Ti 100G 100G10Ti
Precursor and Addition, wt%
Aggregate
L/S Ratio
MK
TiO2
Sand
GB
100 90 100 90 100 90
0 10 0 10 0 10
100
0
50
50
0
100
0.40
Molar ratio SiO2/Al2O3
K2O/SiO2
2.5
0.28
R. Mejía de Gutiérrez et al. / Construction and Building Materials 235 (2020) 117510
Fig. 4. Compressive strength at 7 and 28 days.
and 33.1 MPa for all the materials developed; in addition, it was found that the material at the age of 28 days improved its compressive strength up to 20%. Analysing the effect of the addition of TiO2 in the mortars with different types of aggregate, it was found that the TiO2 particles increased the strength by 17.4% for the samples with 100% sand and 5.8% for the samples with 100% glass at age of 7 days; these increases were 12.9% and 3.2% respectively at 28 days of curing. The positive effect in the compressive strength of the addition of TiO2 is due to the greater densification of the matrix that contributes to the decrease in the proportion of pores present (Fig. 5). Additionally, the incorporation of TiO2 nanoparticles can act as nucleation points for the formation of aluminium-hydrated potassium silicate geopolymeric gel (K-A-SH) due to its fine particle size, which also contributes to the gaining of greater mechanical properties [1]. Previous studies at the level of MK pastes added with 10% TiO2 showed a compressive strength at 28 days of up to 43.25 MPa, value 10.3% higher than that obtained without the addition [39]. About the use of waste glass as a fine aggregate in cementitious materials there are controversies regarding the positive or negative effects on mechanical strength, which is attributed mainly to the difference in size and distribution of the particles of glass used in the different published studies [15,17,19,21,50,51]. In the present investigation, it was found that the partial or total replacement of the sand by GB did not affect negatively the mechanical performance to compression of the developed materials. Thus, for
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the mortars produced with GB (100G), an increase in the compressive strength at 28 days of 12.33% was observed compared to the mortars where sand was used (100S). The increase observed in the present study can be attributed to several factors, first the particle size used, since about 5.8% of the glass used as a fine aggregate had a size less than 0.3 mm. In addition, its composition (Table 1) and amorphous character (Fig. 3) contributed to generate a better interface between the gel (matrix) and the aggregate due to the contribution of soluble silica to the geopolymer system; in other words, the glass particles are involved in the reaction with the alkaline activator and allow the development of a more compact geopolymer matrix [25,30]. This is consistent with the results obtained by Hajimohammadi et al. [29]. The total substitution of the fine aggregate by GB in the samples with TiO2 (100G10Ti) generated an increase of just 2.7%. This indicated that the compressive strength of MK geopolymer was more positively affected by the addition of TiO2 compared with the change of aggregate. 3.3. Density, absorption and porosity The density, absorption and porosity properties evaluated in the mortars are shown in Fig. 5. In general, the density of the samples
Fig. 6. Thermal conductivity of mortars with TiO2.
Fig. 5. Mortar properties: (a) Bulk Density, (b) Water absorption and porosity.
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linked to that the sand suffers from the phenomenon of swelling due to high hydrolysis capacity [52]. As a result, during the drying and hardening process of the geopolymer, the water in excess of the geopolymeric gel (K-A-S-H) remained in the pores of the material and in the subsequent drying was released leaving a higher percentage of cavities or pores. A lower porosity was obtained in the compound samples partially or totally made with glass as fine aggregate, being this behaviour consistent with that reported in previous studies [17]. This is attributed to the lower absorption capacity of the recycled glass aggregate and a possible pore refinement by gel formation due to the presence of amorphous or reactive silica in the aggregate [18]. The reduction in the percentage of absorption and porosity in the mortars when S was totally replaced by GB was 16.3% and 7% respectively. The highest densification, lower porosity and water absorption capacity of the 100G mortars is consistent with the highest mechanical strength observed for this type of mortars, 31.5 MPa compared to 28.0 MPa of the 100S mortars. Fig. 7. Efficiency of removal of rhodamine B on the various samples evaluated using mercury lamp. (I = 10.3 W m2, k = 360 nm).
with sand and GB as a fine aggregate (Fig. 5a) fluctuated between 2191 and 2484 kg/m3, where the highest density corresponded to the samples with 100% GB. In turn, the samples with higher sand content (100S) showed greater porosity (Fig. 5b), this is possibly
3.4. Thermal conductivity The results of thermal conductivity obtained for the materials added with nanoparticles of TiO2 and the porosity are reported in Fig. 6. Additionally, the thermal conductivity is influenced by type and degree of crystallization of aggregate [53]. It was found that the samples manufactured with 100% Sand (100S10Ti) presented the highest conductivity. The complete replacement of the sand
Fig. 8. Appearance of samples 100G10Ti (a) before application of RhB pigment; (b) Once applied; After UV exposure: (c) R4 and (d) R26.
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by glass (100G10Ti) decreased thermal conductivity by 24.60%. This behaviour was previously found by other studies [19,21], which attributed this decrease to the greater thermal conductivity of sand compared to that of glass. Sikora et al. [19] reported reductions up to 58% when the sand is replaced totally by GB in Portland cement mortars. The conductivity of the dry sand is approximately 1.76 w/m.K at room temperature, while the conductivity of glass is approximately 1.1 w/m.K at the same temperature [54,55]. The thermal conductivity obtained for mortars 100S10Ti was lower than traditional construction materials such as Portland concrete with light aggregates (1.35 w/m.K) and lower than traditional Portland cement mortars (1.40 w/m.K) [56]. The thermal conductivity for mortars 100G10Ti was similar to the conductivity of limestone mortar (0.87 w/m.K) and gypsum mortar (0.80 w/m.K) [42]. Hajimohammadi et al. [57] used glass as a substitute of sand aggregate in order to produce geopolymer foams based on fly ash and slag; the authors reported a thermal conductivity of 0.15 W/m.K with 600 kg/m3 density and concluded that this is the most favourable insulating material. In the present study the thermal conductivity obtained in 100G10Ti was 0.87 W/m.K with 2398 kg/m3 density and 22% of porosity.
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(100S10Ti, 50S10Ti, 100G10Ti) meet the criteria (R4 > 20% and R26 > 50%) required to be considered as materials with photocatalytic properties [7,39,47,48]. The degradation efficiencies R4 of the geopolymeric mortars added with 10% TiO2 was between 22.18% and 34.24%, while R26 was in the range of 51.26%–88.37%. The use of 100% glass as fine aggregate (100G10Ti) increased R26 by 72.4% compared to when using sand (100S10Ti). The increase in the efficiency of removal of RhB is attributable to the capacity of the glass to reflect the light [20], and in this sense a greater exposure of UV-a radiation is guaranteed on the TiO2 particles embedded in the material, thus allowing a greater photonic efficiency. Fig. 8 shows the colour appearance of the 100G10Ti specimens before and after of the efficiency test. In this figure, a significant change in colour can be observed when comparing the specimen after the application of Rhodamine B (Fig. 8b) and the specimen subjected to UV irradiation during 4 h and 26 h (Fig. 8c and Fig. 8d, respectively). The value obtained from R4 and R26 for 100G10Ti was 22.18% and 88.37% respectively. These values exceed the requirement of the standard in a 2.18% and 38.4% respectively [47,48]. 3.6. Scanning electron microscopy (SEM) analysis
3.5. Photocatalytic properties Fig. 7 shows the results of the measurement of the photocatalytic properties in the mortar samples coated with the pigment Rhodamine B (RhB). It is observed that the geopolymeric materials without addition of TiO2 (100S, 50S, 100G) have potential to degrade the pigment, which has also been reported in studies conducted by Falah and MacKenzie [2], who attributed this behavior to the ability to attract and retain organic cationic species by the hydroxyl groups present on the surface of the geopolymer. The results reported in the present study showed that, regardless of the type of aggregate used, the samples added with TiO2
Fig. 9 shows the superficial morphology of the geopolymer mortars studied at 28 days of curing by using SEM. The samples present an irregular surface, with the presence of pores, and large particles of sand and glass. For samples with addition of glass (Fig. 9b and c), it is possible to distinguish glass particles by their smooth and angular surface immersed inside the geopolymer gel. Additionally, it is observed a better interface between the glass aggregate and the geopolymer matrix. This is attributable to the surface reaction of the glass particles with the alkaline activator. Furthermore, presence of small pores and microcracks were identified. In recently published review papers by Liu et al. [25] and
Fig. 9. SEM/EDS micrographs of the samples (a) 100S10Ti (b) 50S10Ti and (c) 100G10Ti.
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Table 3 Energy dispersive X-ray spectroscopy (EDS) data of 100S10Ti and 100G10Ti samples. Sample
Point
100S10Ti
1 2 1 2
100G10Ti
Element, % O
Al
Si
K
Ca
Ti
35.15 47.68 – 46.20
– 12.48 – 13.97
64.85 21.20 69.88 21.20
– 14.08 – 14.07
– – 30.12 –
– 4.56 – 4.56
Luhar et al. [30] reported that the binding between geopolymer matrix and aggregate surface is usually better when is used waste glass to replace sand in AAMs. The authors attribute the good quality of the interface to the formation of silica-rich geopolymer gel around glass particles. Table 3 shows the elemental composition present in the points assessed using SEM/EDS technique for the 100S10Ti (Fig. 9a) and 100G10Ti samples (Fig. 9c). For 100S10Ti, the composition of the Spectrum 1 is Si and O, this is attributed to a particle of sand. In spectrum 2 the geopolymeric gel is observed, consisting of the elements Si, Al, K, O and Ti, indicating that titanium is distributed in the gel. The results of the EDS analysis for the 100G10Ti (Spectrum 1) show smooth particles that correspond to glass particles composed mainly of Si and Ca. The spectrum 2 corresponds to the geopolymeric gel with elements Si, Al, K, O and Ti in proportions similar to those in spectrum 2 of the 100S10Ti sample. It is worth highlighting that the TiO2 particles coexist in their previous form in the structure of the gel that compose the material and these results corroborate those found in previous studies [2,39]. 4. Conclusions According to the results obtained in this study, it is concluded that: - The mechanical performance of the geopolymeric mortars based on MK using sand as aggregate (100%S) was improved by 16% when ground glass was used as a replacement for sand and 10% of TiO2 particles were added (100G10Ti). - The glass particles used as a fine aggregate generate a better interface with the gel (matrix) due to surface reaction of the particle with the alkali present in the mix. This allowed the development of a more compact geopolymer matrix with smaller porosity and greater resistance. - In geopolymeric mortars, replacing sand by ground glass slightly reduces properties such as density and water absorption; however, the thermal conductivity is reduced by approximately 25%, making it possible to obtain mortars with thermal conductivities similar to gypsum or limestone mortars. - Finally, the self-cleaning photocatalytic capacity (R26) of the mortars added with 10 wt% TiO2 was increased 72.4% when 100% of the fine aggregate (S) is replaced by ground glass (GB).
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors, who are members of the Composite Materials Group (GMC), from the Excellence Centre for Novel Materials (CENM), gratefully acknowledge financial support from Universidad del Valle under the ‘‘Bolsa Concursable’’ 139-2017 (Cali, Colombia).
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Alkali-activated metakaolin mortars using glass waste as fine aggregate: Mechanical and photocatalytic properties Ruby Mejía de Gutiérrez ⇑, Mónica A. Villaquirán-Caicedo, Luis A. Guzmán-Aponte Composite Materials Group (CENM), Materials Engineering School, Universidad del Valle, Calle 13 # 100-00, Cali, Colombia
h i g h l i g h t s Alkali-activated mortars based on metakaolin and TiO2 particles were evaluated. Glass bottle waste was used as an alternative fine aggregate. The compressive strength was 31.5 MPa at 28 days of curing. The thermal conductivity of the mortars 100G10Ti was 0.87 W/m.K. The self-cleaning photocatalytic capacity of these mortars was increased 72.4%
a r t i c l e
i n f o
Article history: Received 5 November 2018 Received in revised form 18 September 2019 Accepted 5 November 2019
Keywords: Alkali-activated mortars Metakaolin Titanium dioxide Glass bottle waste Photocatalysis Self-cleaning properties
a b s t r a c t In the present study, properties such as density, absorption, porosity, compressive strength, thermal conductivity and photocatalytic activity of alkali-activated mortars based on metakaolin added with TiO2 particles were evaluated. TiO2 particles were incorporated in proportion of 10 wt%. Ground glass (GB) obtained from glass bottle waste was used as an alternative fine aggregate in proportion up to 100% as replacement of natural sand (S). The alkaline activator was a mix of potassium silicate and hydroxide. The photocatalytic activity was measured by the removal of Rhodamine B and the microstructure of the developed material was studied by scanning electron microscopy. The results obtained in this study reveal a reduction in the percentage of absorption of 16.3%, and porosity of 7% for mortars when S is replaced by GB. The mortar prepared with 100% GB reached a compressive strength of 31.5 MPa compared to 28.0 MPa for reference mortar (100% S). The thermal conductivity was reduced by approximately 25%. Finally, the self-cleaning photocatalytic capacity using GB as fine aggregate increased 72.4%. This type of cementitious materials contributes to mitigate environmental pollution and reduces the maintenance costs of structures. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction In recent decades the interest of using photocatalysis in building materials to obtain materials with self-cleaning and antibacterial properties has grown rapidly to help mitigate environmental pollution and reduce the maintenance costs of exposed outdoor structures [1–5]. For the implementation of these new technologies in traditional cementing materials based on Ordinary Portland Cement (OPC), the addition of particles of semiconductor materials, such as TiO2, has been used [1–8]. The main advantages of TiO2 are its relatively low cost, its high chemical stability and nontoxicity, ⇑ Corresponding author at: Composites Materials Group, School of Materials Engineering, Universidad del Valle, Ciudad Universitaria Meléndez, Calle 13 # 10000, Building E47, Second Floor, Cali, Colombia. E-mail address: [email protected] (R. Mejía de Gutiérrez). https://doi.org/10.1016/j.conbuildmat.2019.117510 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
good optical transparency and its highly oxidizing power, which makes it an excellent candidate for many photocatalytical applications with construction materials [6,9]. This oxide presents three crystal structures: anatase, rutile and brookite; of these the anatase phase exhibits the highest photocatalytic activity [8,9]. These novel properties are generated from reactive oxygen species, hydroxyl radicals and superoxide radical ions (OH and O 2 ), after exposure to sunlight, particularly to the UV-a fraction that coincides with the banned TiO2 band [6]. The first radical is formed for the oxidation of water and the second for the reduction of O2 molecule. The two radicals are highly reactive species and they can degrade organic and inorganic species in further reactions. This TiO2 technology has been applied in some buildings constructed with cement-based materials, between these the Church ‘‘dives in Misericordia” (Rome, Italy) and the public building ‘‘Cité de la Musique et des Beaux-Arts” (Chambery, France) are two of the
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most referenced. However, there are also concrete constructions and finished pavements in Belgium, France, Italy, Monaco, Morocco, Japan and China [10,11]. A state-of-the-art report titled ‘‘Application of Titanium Dioxide Photocatalysis to Construction Materials” was published by the RILEM Technical committee 194-TDP in 2011, which includes applications for self-cleaning, antibacterial and air purification [8–11]. In recent years, other authors have suggested that the incorporation of TiO2 can also contribute to the durability of the cementing material. Fiore et al. [12] studied the effect of TiO2 as a protective barrier against the deterioration of reinforced concrete because of carbonation process. The authors proved that photocatalytic concrete products applied on the surface of concrete improved the carbonation resistance and reduced the corrosion propagation rate of reinforcement bars with respect to cement coatings without any photocatalysts [12]. In addition, the use of different wastes such as blasted copper slag [13] and ground glass as a partial replacement of cement and fine aggregate in cementitious materials has been evaluated obtaining promising results in various applications such as roads, trails, concrete or structural and non-structural masonry [14–21]. The review written by Paul et al. [16] analyzes the engineering properties (mechanical and durability properties) of cement- based materials where glass powder has been used as both binder and fine aggregate. The authors consider that there is potential for its use due to the large volumes generated from glass waste, especially those that go to landfills being this an alternative material to the raw materials used in the concrete. They also identify the fields that require research, among which they point to the importance of a life cycle analysis to quantify the impact of the use of glass in concrete. Specifically, finely ground glass presents pozzolanic properties and this makes it attractive as a supplementary cementitious material [16,18]. Some studies have shown that this type of aggregate can be a used as a substitute for natural sand without affecting the mechanical properties of the mortar, on the contrary with a positive effect on its thermal properties [19,21]. Likewise, one of the alternatives proposed by Chen and Poon to increase the performance of the photocatalytic properties of conventional mortars based on OPC in air purification applications was the partial replacement of the fine aggregate or sand by ground glass of various tonalities, obtaining improvements in the photocatalytic activity due to the properties of the glass to transmit light, especially for those of clear or transparent colour [20]. In recent years, the glass waste has also been incorporated in alternative binders such as alkali activated materials (AAMs). The AAMs are produced by the reaction of silica and alumina-rich precursors (Fly ash, metakaolin, blast furnace slag and others industrial by-products and wastes) with an alkaline substance generally at low temperature [22]. AAMs depending of the type of raw materials used and their corresponding design can present properties such as high early compressive strength, high resistance to aggressive chemicals and excellent fire resistance [23,24]. In this type of materials, the glass has been used as a precursor, source for produce alkali activators and replacement of the fine aggregate [25–35]. This has been possible due to its chemical instability in alkaline conditions and higher content of silica rich glassy phase [30]. The studies mainly have been realized with fly ash or slag as precursor obtaining highly satisfactory results. Specifically, the incorporation of waste glass as fine aggregate can enhance some properties of AAMs mortars such as workability, thermal conductivity, resistance to fire, and resistance to sulfate attack [25,33]. Rivera et al. [26] used bottle glass as precursor and window glass as aggregate to fabricate tiles with acceptable mechanical properties. Torres-Carrasco et al. [27,28] used sodium silicate solutions formed from glass wastes as alkaline activator in the preparation of AAMs based on fly ash and slags. Hajimohammadi et al. [29]
evaluated the properties of a geopolymer based on fly ash and glass aggregates, the authors concluded that this type of aggregate is a suitable alternative to fine sand for concrete applications. Arulrajah et al. [31] synthesized recycled glass-fly ash geopolymers, the glass used had the maximum particle size of 4.75 mm similar to a fine aggregate, validating the viability of the use of these materials as low carbon footprint masonry elements, with a mechanical performance of up to 17.42 MPa after 7 days of curing. Luhar et al. [30], in a recent review, present the different applications of GB in geopolymer composites; the authors revised the previous studies on rheological properties and compressive and flexure strength. In general, the different studies show that the use of glass can enhance some properties of AAMs in order to produce an innovative and promising eco-supplementary cementitious material. This is consistent with what other authors have reported [25,29,32–35]. On the other hand, the synthesis and properties of metakaolin (MK)-based geopolymers have been widely studied over the past decades. The activators traditionally used are hydroxides (Na+, K+, and Ca2+), silicates (Na + and K + ), carbonates or sulfates. Rocha et al. [36] compared potassium and sodium – based activators concluding that the mechanical properties are similar although better thermal performance was obtained using potassium silicate and potassium hydroxide. Villaquiran et al. [37,38] studied the mechanical and thermal properties of MK geopolymer using different types of activators, potassium silicate, rice husk ash (RHA), silica fume (SF) and potassium hydroxide. The authors conclude that the exposure of these geopolymers to high temperatures achieves compressive strengths of up to 137.6 MPa (RHA100) and 148.5 MPa (SF100); this result is mainly attributed to the higher densification and formation of crystalline products such as leucite and mullite. Additionally, they mentioned that the alkaline activators based on commercial potassium silicate provide a higher carbon footprint whereas activators based on RHA are more ecofriendly. Recently, the physicomechanical properties of geopolymers based on MK including TiO2 have been studied [39,40]. Guzmán-Aponte et al. [39] indicate that up to 10% TiO2 does not affect the mechanical properties of the geopolymer, although it does reduce the fluidity and setting times of the mixture. Asim et al. [40] in a recent review paper mentioned the geopolymer types with photocatalytic activity; MK basedgeopolymers incorporating TiO2 were included as materials with good efficiency. This article aims to evaluate the mechanical, physical and photocatalytic properties of a geopolymeric mortar based on metakaolin added with particles of TiO2 (10 wt%), which uses ground glass residue as a replacement of sand in proportions of 0, 50 and 100 wt% MK was selected as precursor in this study because its white colour, amorphous character and availability on the country; the final objective is developing an inorganic coating for use on architecture applications. 2. Experimental methodology 2.1. Materials Commercial high purity metakaolin (Metamax, MK) was used as a precursor to produce geopolymeric material. The chemical composition, determined by X-ray fluorescence (XRF), using a Phillips MagiX-Pro PW 2440 spectrometer equipped with a Rhodium tube and a maximum power of 4 KW, is presented in Table 1. The molar ratio SiO2/Al2O3 of MK was 1.97. As an addition, titanium dioxide nanoparticles were used (TiO2 - Aeroxide - P25), whose composition is included in Table 1. Particle size and distribution analysis were performed with a Mastersizer-2000 laser Granulometer from Malvern Instruments with a Hydro2000MU dispersion unit; distilled water was used as a dispersant medium. The mean particle sizes D [4.3] of the MK and titanium oxide were 7.76 and 1.59 lm, respectively (Fig. 1). As a fine aggregate for the manufacture of geopolymeric mortars, glass residues (GB) obtained from the recycling of bottles were used, which, after a thorough cleaning and washing process with water to remove all their contaminants, were
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3
Table 1 Chemical composition and physical properties of raw materials. Compound (% by Weight)
SiO2 Al2O3 TiO2 Fe2O3 Na2O MgO K2O P2O5 CaO Others Physical properties Density (Kg/m3) Mean Particle Size, D (4,3) (mm)
Precursor
Addition
Fine Aggregate
MK
AEROXIDE TiO2 – P25
Glass bottle GB
52.02 44.95 1.730 0.47 0.30 0.19 0.16 0.06 0.02 – 2500
0.20 0.30 99.50 0.10 – – – – – – 3505
72.27 1.49 0.08 0.62 13.37 0.26 0.51 – 11.15 1.60 2411
7.76
1.59
–
dried and after conditioned mechanically in a jaw mill. The chemical composition of GB is also presented in Table 1, the XRF results show that the glass waste is composed of three main oxides: SiO2 (72.27%), Na2O (13.37%), and CaO (11.15%). This composition suggests that the glass is soda-lime type glass. It is worth noting that this type of glass is the most widely used glass to produce containers and windows. As a reference aggregate, river sand (S) was used. Fig. 2 shows the particle size distribution of the two types of fine aggregates used. This was determined by sieve analysis in accordance with the ASTM C 136 standard [41]. The fineness module of the sand was 2.06 and the fineness module of the ground glass was 3.27. The densities of S and GB were 2650 and 2411 kg/m3 respectively. Fig. 3 shows the X-ray diffraction patterns for MK and TiO2. For the X-ray analysis, a Bruker diffractometer equipped with a wide-angle goniometer RINT2000 was used, using the Ka1 signal of Cu at 45 kV and 40 mA. A 0.02° pitch was used within a range of 5–70° at a rate of 5°/min. Information processing was performed using the X’pert HighScore Plus software package, version 2.2.5. It can be observed in Fig. 3 that MK has a high level of amorphicity due to the halo located between 20° and 30° 2h angle and presents small traces of a crystalline product identified as anatase (ICSD 9852), which corresponds to the observed peaks approximately at 25.3° (100%), 37.8° (18.6%), 48.1° (24%), 53.9° (15%) and 55.1° (15%) 2h. This
Fig. 2. Particle size distribution for sand and ground glass. matches with the percentage of TiO2 reported in the chemical composition of MK (Table 1). There is also evidence that the TiO2 (Aeroxide – P25) used as addition is mainly in the anatase phase (A) which is an important phase because of its photocatalytic potential [6]; however, it has a reduced amount of rutile phase, R (ICSD 33838), less than 1%, identified in the XRD by the peak located at 27.46° 2h which corresponds to the main peak of this phase (100% intensity). This result contrasts with the one presented by Sangchay et al. [42], who report that P25 is composed of 80% anatase and 20% rutile. The X-ray diffraction spectrum of GB (Fig. 3) exhibits a broad amorphous shoulder attributed to the glassy or amorphous phase. The position of the amorphous shoulder is between the angles of 15° and 35° 2h.
2.2. Preparation of geopolymers The geopolymeric pastes were prepared using the molar ratios SiO2/Al2O3 = 2.5 and K2O/SiO2 = 0.28, based on results from previous research [37]. A potassium hydroxide (KOH) analytical-grade reagent and commercial potassium silicate
Fig. 1. Particle Size distribution for TiO2 particles and MK.
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R. Mejía de Gutiérrez et al. / Construction and Building Materials 235 (2020) 117510 Scanning electron Microscopy with Energy-dispersive spectroscopy (SEM/EDS) was performed using a JEOL JSM-6490LV and 20 kV of accelerating voltage. The samples were coated with Au and observed in a low vacuum mode. The photocatalytic properties of the mortars were studied using the UNI 11259– 2016 method [47]. This method is based on the degradation of the pigment Rhodamine B (RhB) once applied on the surface of the sample and exposed to UV-a irradiation. The solution of RhB with concentration of 5 104 g/ml was spread manually with a brush on the specimen, then dried in an electric oven at 40 °C for 20 min, a total of two layers of the pigment were applied to each material evaluated. UV-A irradiation was supplied by two mercury lamps (Electrolux T8 20w/BLB) located inside a black acrylic dome. These lamps emit light with an intensity of 10.3 W M2, which was measured with a Delta Ohm HD 2102.2 Photoradiometer using the filter for UV-a light range (k = 360 nm) at 5 mm. Changes in colour on the surface of the material before and after irradiation were registered with a portable spectrophotometer X-Rite, Ci60 (Photometric Solutions International, Victoria, Australia). The equipment readings were expressed in colorimetric coordinates L*, A* and b* in the CIE LAB system. The coordinate L* represents units in the white range to black, the coordinate a* represents values between red and green and the coordinate b* represents values between yellow and blue. The Rhodamine B removal analysis was performed by comparing the colour parameter a* prior to the irradiation of the sample in the UV source (A* (0 h)), with the colour obtained after UV irradiation during 4 h (A * (4 h)) and 26 h (A * (26 h)). Degradation efficiencies (R4 and R26) were calculated using Eqs. (1) and (2). A material is considered photocatalytic if R4 > 20% and R26 > 50% [47,48].
Fig. 3. X-ray diffraction patterns of raw materials. (K2SiO3) distributed by Pan American Chemicals (SiO2 = 26.38%, K2O = 13.06%, H2O = 60.56%) were dosed to obtain the activator solution module required in the geopolymer mixture (Ms: SiO2/K2O = 0.76). Geopolymeric mortars were manufactured with a geopolymer:fine aggregate ratio of 1:2; where the natural sand (S) was replaced in quantities of 50% and 100% by ground glass (GB). Mortars with and without addition of TiO2 were prepared for comparative analysis and the percentage of addition of TiO2 according to the binder was 10% (wt). The liquid/solid ratio (L/S) remained fixed at 0.40 for all manufactured materials. These parameters were selected from previous studies [39]. The compositions and mixing codes used are presented in Table 2. The mixing process of the solid (MK and titanium dioxide) and liquids (activator) components was performed in a HOBART Vulcan 1249 mixer. The solids were homogenized for 15 s, and the activating solution (KOH + K2SiO3) was added and mixed for 3 min at low speed, followed by 2 min at medium speed and 1 min at low speed until a homogeneous paste was obtained. Finally, the fine aggregate was incorporated, and the mixture was continued at slow speed for another 2 min; the total mixing time was 8.25 min. The geopolymeric pastes and mortars were then casted into cubic silicone moulds of dimensions 20 mm 20 mm 20 mm. The obtained samples were cured at room temperature (25 °C) for 24 h and then demoulded and taken to a humidity chamber (relative humidity > 90% and 25 °C) until the test age was reached (7 and 28 days). 2.3. Instrumental and analytical characterization techniques The fluidity and setting times were evaluated for geopolymeric pastes, MK and MK + 10% TiO2, using the procedures described in ASTM C230-14 [43] and ASTM C191-18 [44]. The compressive strength was evaluated at ages of 7 and 28 days using an INSTRON 3369 Universal test machine with a capacity of 50 kN at a deformation rate of 1 mm/min. In each case, a minimum of three specimens were tested. The test was made according to the ASTM C109/C109M-10 standard using 20 mm cubes on the side [45]. The density, pore volume, and water absorption capacity of each material were also determined according the procedures detailed in ASTM C642-13 [46]. It should be noted that the drying of the sample was performed at 60 °C for 48 h. The thermal conductivity was obtained by using a thermal constants Analyzer TP 500 S Hot Disk with a heating power of 956.69 mW for 40 s, using cubic samples of 20 mm 20 mm 20 mm.
R4 ð%Þ ¼ ½
a ð0hÞ a ð4hÞ 100 a ð0hÞ
R26 ð%Þ ¼ ½
ð1Þ
a ð0hÞ a ð26hÞ 100 a ð0hÞ
ð2Þ
3. Results and discussion 3.1. Workability and setting time The addition of TiO2 particles affected the workability and the setting time of MK geopolymer. The fluidity of reference geopolymer (0% TiO2) was 100.6 mm; the initial and final setting time was 58.8 and 73 min respectively. The addition of 10 wt% TiO2 particles decreased the fluidity 20.4% and the initial and final setting time decreased by 25.9% and 30.1%, respectively. It should be noted that the liquid/solid ratio (L/S) remains fixed at 0.40, so the reduction of the workability of MK geopolymer blended with 10% TiO2 is related to the greater demand for water due to its fine particle size. This was also observed in previous studies of the authors by including different percentages of TiO2 and evaluating different L/S ratios [39]. This is consistent with that reported by Duan et al. [49] using geopolymers based on fly ash. The reduction of the setting time of the geopolymers with the addition of TiO2 at a constant L/S ratio has been also reported in previous researchers; this effect can be attributed to a nucleation and filling effect that contributes to accelerate the degree of reaction of the geopolymer [39,49]. 3.2. Compressive strength Fig. 4 shows the results of the compressive strength at ages 7 and 28 days of curing. The compressive strength results of MK geopolymer blended mortars with 10% TiO2 were between 27.4
Table 2 Composition of the mixtures. Mixture
100S 100S10Ti 50S 50S10Ti 100G 100G10Ti
Precursor and Addition, wt%
Aggregate
L/S Ratio
MK
TiO2
Sand
GB
100 90 100 90 100 90
0 10 0 10 0 10
100
0
50
50
0
100
0.40
Molar ratio SiO2/Al2O3
K2O/SiO2
2.5
0.28
R. Mejía de Gutiérrez et al. / Construction and Building Materials 235 (2020) 117510
Fig. 4. Compressive strength at 7 and 28 days.
and 33.1 MPa for all the materials developed; in addition, it was found that the material at the age of 28 days improved its compressive strength up to 20%. Analysing the effect of the addition of TiO2 in the mortars with different types of aggregate, it was found that the TiO2 particles increased the strength by 17.4% for the samples with 100% sand and 5.8% for the samples with 100% glass at age of 7 days; these increases were 12.9% and 3.2% respectively at 28 days of curing. The positive effect in the compressive strength of the addition of TiO2 is due to the greater densification of the matrix that contributes to the decrease in the proportion of pores present (Fig. 5). Additionally, the incorporation of TiO2 nanoparticles can act as nucleation points for the formation of aluminium-hydrated potassium silicate geopolymeric gel (K-A-SH) due to its fine particle size, which also contributes to the gaining of greater mechanical properties [1]. Previous studies at the level of MK pastes added with 10% TiO2 showed a compressive strength at 28 days of up to 43.25 MPa, value 10.3% higher than that obtained without the addition [39]. About the use of waste glass as a fine aggregate in cementitious materials there are controversies regarding the positive or negative effects on mechanical strength, which is attributed mainly to the difference in size and distribution of the particles of glass used in the different published studies [15,17,19,21,50,51]. In the present investigation, it was found that the partial or total replacement of the sand by GB did not affect negatively the mechanical performance to compression of the developed materials. Thus, for
5
the mortars produced with GB (100G), an increase in the compressive strength at 28 days of 12.33% was observed compared to the mortars where sand was used (100S). The increase observed in the present study can be attributed to several factors, first the particle size used, since about 5.8% of the glass used as a fine aggregate had a size less than 0.3 mm. In addition, its composition (Table 1) and amorphous character (Fig. 3) contributed to generate a better interface between the gel (matrix) and the aggregate due to the contribution of soluble silica to the geopolymer system; in other words, the glass particles are involved in the reaction with the alkaline activator and allow the development of a more compact geopolymer matrix [25,30]. This is consistent with the results obtained by Hajimohammadi et al. [29]. The total substitution of the fine aggregate by GB in the samples with TiO2 (100G10Ti) generated an increase of just 2.7%. This indicated that the compressive strength of MK geopolymer was more positively affected by the addition of TiO2 compared with the change of aggregate. 3.3. Density, absorption and porosity The density, absorption and porosity properties evaluated in the mortars are shown in Fig. 5. In general, the density of the samples
Fig. 6. Thermal conductivity of mortars with TiO2.
Fig. 5. Mortar properties: (a) Bulk Density, (b) Water absorption and porosity.
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R. Mejía de Gutiérrez et al. / Construction and Building Materials 235 (2020) 117510
linked to that the sand suffers from the phenomenon of swelling due to high hydrolysis capacity [52]. As a result, during the drying and hardening process of the geopolymer, the water in excess of the geopolymeric gel (K-A-S-H) remained in the pores of the material and in the subsequent drying was released leaving a higher percentage of cavities or pores. A lower porosity was obtained in the compound samples partially or totally made with glass as fine aggregate, being this behaviour consistent with that reported in previous studies [17]. This is attributed to the lower absorption capacity of the recycled glass aggregate and a possible pore refinement by gel formation due to the presence of amorphous or reactive silica in the aggregate [18]. The reduction in the percentage of absorption and porosity in the mortars when S was totally replaced by GB was 16.3% and 7% respectively. The highest densification, lower porosity and water absorption capacity of the 100G mortars is consistent with the highest mechanical strength observed for this type of mortars, 31.5 MPa compared to 28.0 MPa of the 100S mortars. Fig. 7. Efficiency of removal of rhodamine B on the various samples evaluated using mercury lamp. (I = 10.3 W m2, k = 360 nm).
with sand and GB as a fine aggregate (Fig. 5a) fluctuated between 2191 and 2484 kg/m3, where the highest density corresponded to the samples with 100% GB. In turn, the samples with higher sand content (100S) showed greater porosity (Fig. 5b), this is possibly
3.4. Thermal conductivity The results of thermal conductivity obtained for the materials added with nanoparticles of TiO2 and the porosity are reported in Fig. 6. Additionally, the thermal conductivity is influenced by type and degree of crystallization of aggregate [53]. It was found that the samples manufactured with 100% Sand (100S10Ti) presented the highest conductivity. The complete replacement of the sand
Fig. 8. Appearance of samples 100G10Ti (a) before application of RhB pigment; (b) Once applied; After UV exposure: (c) R4 and (d) R26.
R. Mejía de Gutiérrez et al. / Construction and Building Materials 235 (2020) 117510
by glass (100G10Ti) decreased thermal conductivity by 24.60%. This behaviour was previously found by other studies [19,21], which attributed this decrease to the greater thermal conductivity of sand compared to that of glass. Sikora et al. [19] reported reductions up to 58% when the sand is replaced totally by GB in Portland cement mortars. The conductivity of the dry sand is approximately 1.76 w/m.K at room temperature, while the conductivity of glass is approximately 1.1 w/m.K at the same temperature [54,55]. The thermal conductivity obtained for mortars 100S10Ti was lower than traditional construction materials such as Portland concrete with light aggregates (1.35 w/m.K) and lower than traditional Portland cement mortars (1.40 w/m.K) [56]. The thermal conductivity for mortars 100G10Ti was similar to the conductivity of limestone mortar (0.87 w/m.K) and gypsum mortar (0.80 w/m.K) [42]. Hajimohammadi et al. [57] used glass as a substitute of sand aggregate in order to produce geopolymer foams based on fly ash and slag; the authors reported a thermal conductivity of 0.15 W/m.K with 600 kg/m3 density and concluded that this is the most favourable insulating material. In the present study the thermal conductivity obtained in 100G10Ti was 0.87 W/m.K with 2398 kg/m3 density and 22% of porosity.
7
(100S10Ti, 50S10Ti, 100G10Ti) meet the criteria (R4 > 20% and R26 > 50%) required to be considered as materials with photocatalytic properties [7,39,47,48]. The degradation efficiencies R4 of the geopolymeric mortars added with 10% TiO2 was between 22.18% and 34.24%, while R26 was in the range of 51.26%–88.37%. The use of 100% glass as fine aggregate (100G10Ti) increased R26 by 72.4% compared to when using sand (100S10Ti). The increase in the efficiency of removal of RhB is attributable to the capacity of the glass to reflect the light [20], and in this sense a greater exposure of UV-a radiation is guaranteed on the TiO2 particles embedded in the material, thus allowing a greater photonic efficiency. Fig. 8 shows the colour appearance of the 100G10Ti specimens before and after of the efficiency test. In this figure, a significant change in colour can be observed when comparing the specimen after the application of Rhodamine B (Fig. 8b) and the specimen subjected to UV irradiation during 4 h and 26 h (Fig. 8c and Fig. 8d, respectively). The value obtained from R4 and R26 for 100G10Ti was 22.18% and 88.37% respectively. These values exceed the requirement of the standard in a 2.18% and 38.4% respectively [47,48]. 3.6. Scanning electron microscopy (SEM) analysis
3.5. Photocatalytic properties Fig. 7 shows the results of the measurement of the photocatalytic properties in the mortar samples coated with the pigment Rhodamine B (RhB). It is observed that the geopolymeric materials without addition of TiO2 (100S, 50S, 100G) have potential to degrade the pigment, which has also been reported in studies conducted by Falah and MacKenzie [2], who attributed this behavior to the ability to attract and retain organic cationic species by the hydroxyl groups present on the surface of the geopolymer. The results reported in the present study showed that, regardless of the type of aggregate used, the samples added with TiO2
Fig. 9 shows the superficial morphology of the geopolymer mortars studied at 28 days of curing by using SEM. The samples present an irregular surface, with the presence of pores, and large particles of sand and glass. For samples with addition of glass (Fig. 9b and c), it is possible to distinguish glass particles by their smooth and angular surface immersed inside the geopolymer gel. Additionally, it is observed a better interface between the glass aggregate and the geopolymer matrix. This is attributable to the surface reaction of the glass particles with the alkaline activator. Furthermore, presence of small pores and microcracks were identified. In recently published review papers by Liu et al. [25] and
Fig. 9. SEM/EDS micrographs of the samples (a) 100S10Ti (b) 50S10Ti and (c) 100G10Ti.
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Table 3 Energy dispersive X-ray spectroscopy (EDS) data of 100S10Ti and 100G10Ti samples. Sample
Point
100S10Ti
1 2 1 2
100G10Ti
Element, % O
Al
Si
K
Ca
Ti
35.15 47.68 – 46.20
– 12.48 – 13.97
64.85 21.20 69.88 21.20
– 14.08 – 14.07
– – 30.12 –
– 4.56 – 4.56
Luhar et al. [30] reported that the binding between geopolymer matrix and aggregate surface is usually better when is used waste glass to replace sand in AAMs. The authors attribute the good quality of the interface to the formation of silica-rich geopolymer gel around glass particles. Table 3 shows the elemental composition present in the points assessed using SEM/EDS technique for the 100S10Ti (Fig. 9a) and 100G10Ti samples (Fig. 9c). For 100S10Ti, the composition of the Spectrum 1 is Si and O, this is attributed to a particle of sand. In spectrum 2 the geopolymeric gel is observed, consisting of the elements Si, Al, K, O and Ti, indicating that titanium is distributed in the gel. The results of the EDS analysis for the 100G10Ti (Spectrum 1) show smooth particles that correspond to glass particles composed mainly of Si and Ca. The spectrum 2 corresponds to the geopolymeric gel with elements Si, Al, K, O and Ti in proportions similar to those in spectrum 2 of the 100S10Ti sample. It is worth highlighting that the TiO2 particles coexist in their previous form in the structure of the gel that compose the material and these results corroborate those found in previous studies [2,39]. 4. Conclusions According to the results obtained in this study, it is concluded that: - The mechanical performance of the geopolymeric mortars based on MK using sand as aggregate (100%S) was improved by 16% when ground glass was used as a replacement for sand and 10% of TiO2 particles were added (100G10Ti). - The glass particles used as a fine aggregate generate a better interface with the gel (matrix) due to surface reaction of the particle with the alkali present in the mix. This allowed the development of a more compact geopolymer matrix with smaller porosity and greater resistance. - In geopolymeric mortars, replacing sand by ground glass slightly reduces properties such as density and water absorption; however, the thermal conductivity is reduced by approximately 25%, making it possible to obtain mortars with thermal conductivities similar to gypsum or limestone mortars. - Finally, the self-cleaning photocatalytic capacity (R26) of the mortars added with 10 wt% TiO2 was increased 72.4% when 100% of the fine aggregate (S) is replaced by ground glass (GB).
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors, who are members of the Composite Materials Group (GMC), from the Excellence Centre for Novel Materials (CENM), gratefully acknowledge financial support from Universidad del Valle under the ‘‘Bolsa Concursable’’ 139-2017 (Cali, Colombia).
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