Effect of glass microfibre addition on the mechanical performances of fly ash-based geopolymer composites

Effect of glass microfibre addition on the mechanical performances of fly ash-based geopolymer composites

G Model ARTICLE IN PRESS JASCER 294 1–8 Journal of Asian Ceramic Societies xxx (2017) xxx–xxx Contents lists available at ScienceDirect Journal o...

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ARTICLE IN PRESS

JASCER 294 1–8

Journal of Asian Ceramic Societies xxx (2017) xxx–xxx

Contents lists available at ScienceDirect

Journal of Asian Ceramic Societies journal homepage: www.elsevier.com/locate/jascer

Full Length Article

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Effect of glass microfibre addition on the mechanical performances of fly ash-based geopolymer composites

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Thamer Alomayri Physics Department, Faculty of Applied Science, Umm Al-Qura University, P.O. Box 21955, Makkah, Saudi Arabia

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a r t i c l e

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a b s t r a c t

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Article history: Received 13 May 2017 Received in revised form 15 June 2017 Accepted 21 June 2017 Available online xxx

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Keywords: Geopolymers Composites Mechanical properties

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1. Introduction

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In the present study, various amounts of glass microfibres were introduced into a geopolymer for reinforcement purposes. The influence of these microfibres on the performance of the geopolymer composites was investigated. Results show that the appropriate addition of glass microfibres can improve the mechanical properties of geopolymer composites. In particular, the flexural strength, flexural modulus and impact strength increase at an optimum fibre content of 2 wt%. Further, adding glass microfibres to a plain geopolymer matrix has a significant effect on the pre-cracking behaviour. It substantially enhances the post-cracking response. © 2017 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Ordinary Portland cement (OPC) is a binder used in concrete and cement-based materials. Whilst OPC has served an important role in construction, its production is associated with environmental consequences including significant greenhouse gas emission. The production of one ton of OPC has been found to emit a ton of gaseous CO2 and the cement industry is believed to cause approximately 6% of global emissions of CO2 [1]. Moreover, CO2 emissions are believed to be responsible for anthropogenic effect on climate change. In such a context, there is a need for more sustainable construction materials and production processes. Supplementary cementitious materials and alternatives to OPC have attracted attention. One such example are inorganic cementitious binders known as “geopolymeric cements”. Geopolymers are formed by reacting an alkaline solution such as sodium silicate or sodium hydroxide with an aluminosilicate source such as fly ash, metakaolin, or slag. Fly ash-based geopolymer concrete, in particular, with its excellent engineering properties, is reported to be a sustainable alternative construction material [2]. Geopolymer technology offers an economically workable alternative to inorganic cements in a range of applications including refractory and fire-proof adhesives [3,4]. Geopolymers exhibit high chemical and thermal stability, and have excellent adhesive behaviour, mechanical strength, and long-term durability. The production of geopolymers involves a fraction of CO2 emission making it an environmentally superior process [5]. Such attributes ensure

that geopolymers continue to attract attention as a material of promise concerning the fabrication and application of new materials. Geopolymers suffer brittle failure and sensitivity to cracking as do other ceramics [6–9]. On loading, short disturbed microcracks form. Such microcracks combine to create macrocracks where the composite fails to withstand additional load. The brittle failure and inherent sensitivity to cracking of geopolymers imposes constraints on structural design and undermines durability [10]. Reinforcement of cementitious composites with microfibres has been applied as a useful technique for overcoming material property drawbacks. Microfibres directly impede fracture evolution through arresting and delaying the growth and propagation of microcracks, and indirectly by impeding the coalescing of such to form macrocracks. Microfibres addition also serves to improve the post-peak tension-softening behaviour of brittle materials under tensile load [11]. Microfibres obstruct crack pathways transmitting stress to the interfacial bond between the matrix and the microfibres [12]. Polyvinyl alcohol microfibres have been found to improve material performance in cementitious composites through limiting crack width and quantity in concrete [12]. Recent research has focused of microscopic reinforcement of brittle materials as a means to improve mechanical properties, reduce cracking tendency, and ultimately enhance toughness and ductility. Banthia and Dubeau studied the effect of the carbon and steel micro-fibre reinforcement on cement composite properties [13]. The pair found that it was the carbon micofibres, as opposed to the steel microfibres, that provided strengthening and toughening. Carbon microfibre-reinforced composites exhibited peak loads at greater displacements compared to such of steel

E-mail address: [email protected] http://dx.doi.org/10.1016/j.jascer.2017.06.007 2187-0764/© 2017 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: T. Alomayri, Effect of glass microfibre addition on the mechanical performances of fly ash-based geopolymer composites, J. Asian Ceram. Soc. (2017), http://dx.doi.org/10.1016/j.jascer.2017.06.007

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2 Table 1 Properties of glass fibre.

Glass fibre

Tensile strength (MPa)

Modulus of elasticity (GPa)

Water absorption

Alkali resistance

Corrosion resistance

Color

1700

72

<1%

High

High

White

Table 2 Chemical composition of fly-ash. SiO2

Al2 O3

Fe2 O3

CaO

MgO

SO3

Na2 O

K2 O

LOI

63.13%

24.88%

3.07%

2.58%

0.61%

0.18%

0.71%

2.01%

1.45%

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microfibre-reinforced composites. Solimanm and Nehdi [14] investigated the addition of 0, 4, 8 and 12% wollastonite microfibres to cement. The resultant composite was found to exhibit a compressive strength greater than micro-fibre free control. The pair also found that shrinkage strains has been reduced and cracking resistance increased in the wollastonite microfibres added composites. Low and Beaudoin investigated the effect of reinforcing cement-water and cement-silicafume-water systems with inorganic wollastonite microfibres [15]. The team reported that the addition of wollastonite microfibres into cement and cement-silica fume matrices led to improvements in pre-peak and post-peak load-deflection response causing increased flexural strength and ductility. Ransinchung and Kumar conducted investigations on the influence of the addition of amounts of wollastonite up to a maximum of 15% and microsilica up to a maximum of cement composite properties [16,17]. Ransinchung and Kumar reported that optimal compressive strength was obtained from a mortar with 82.5% cement, 10% wollastonite, and 7.5% microsilica [16,17]. It was also found that a 77.5% cement, 15% wollastonite, and 7.5% microsilica morter exhibited a higher compressive strength than OPC mortar suggesting an alternative for more economical concrete work. While microfibre addition to cement matrices has been investigated, there has been limited research conducted into the effect of the reinforcement of geopolymers with microfibres. The current work aims to meet such a need by studying the effect of addition of glass microfibres to geopolymer composites. The study seeks to incorporate glass microfibres into a geopolymer matrix at different concentrations and characterize the fabricated composite’s flexural strength, flexural modulus and impact strength. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to examine the microstructures of the fly ash and geopolymer.

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2. Experimental procedure

to sodium hydroxide solution was fixed at 2.5. The concentration of sodium hydroxide solution was 10 M, and was prepared and combined with the sodium silicate solution two days before mixing. The glass microfibre was added first to the fly ash at the dosages of 0%,1%, 2% and 3% by weight. The fly ash and glass microfibres were dry mixed for 5 min in a covered mixer at a low speed until homogeneity was achieved. The alkaline solution was then added slowly to the fly ash/glass microfibres in the mixer at a low speed until the mix became homogeneous, then further mixed for another 5 min on high speed. The resultant mixture was then poured into well-greased wooden moulds. All samples were covered with plastic film and cured at 80C for 24 h in an oven before demoulding. They were then dried under ambient conditions for 28 days.

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2.1. Materials

2.3. X-ray diffraction

Glass microfibre was used as reinforcements for the geopolymer-matrix composites. SEM image of glass microfibre is shown in Fig. 1. Fibre properties are summarised in Table 1. Low calcium fly-ash (ASTM class F), collected from the Eraring power station in New South Wales (NSW), was used as the source material to prepare the geopolymer composites. The chemical composition of fly ash is shown in Table 2. The alkaline activator for geopolymerisation was a combination of sodium hydroxide solution and sodium silicate grade D solution. Sodium hydroxide flakes with 98% purity were used to prepare the solution. The chemical composition of sodium silicate used was Na2 O 14.7%, SiO2 29.4% and water 55.9% by mass.

An x-ray diffraction pattern was collected on a D8 Advance diffractometer (Bruker AXS, Germany) using a Cu K␣ source. The data was accumulated using a nominal 2␪ step size of 0.01◦ , a count time of 0.5 s per step and a 2␪ range of 10◦ –90◦ .

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2.2. Preparation of geopolymer composites To prepare the geopolymer matrix, an alkaline solution to fly ash ratio of 0.45 was used and the ratio of sodium silicate solution

Fig. 1. SEM image of glasss microfibre.

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Rectangular bars with a length of 40 mm were cut from the fully cured samples and subjected to three-point bend tests to evaluate their flexural strength. A LLOYD Material Testing Machine (50 kN capacity) with a displacement rate of 1.0 mm/min was employed to perform the tests. In total, five specimens of each composition were tested. The flexural strength (␴F ) was determined using the following equation [8]: 3 Pm S 2 WD2

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2.4. Flexural strength and modulus

F =

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Please cite this article in press as: T. Alomayri, Effect of glass microfibre addition on the mechanical performances of fly ash-based geopolymer composites, J. Asian Ceram. Soc. (2017), http://dx.doi.org/10.1016/j.jascer.2017.06.007

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where Pm is the maximum load at crack extension, S is the span of the sample, W is the specimen width and D is the specimen thickness. The flexural modulus EF was computed using the initial slope of the load–displacement curve, P/X, using the following formula [8]: EF =

S3 4WD3

 P  X

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2.5. Impact strength

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A Zwick Charpy impact tester with a 1.0 J pendulum hammer was employed to determine the impact strength. In all, six bars of 40 mm long were used. The impact strength ( i ) was calculated using the following equation [6]:

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i = E ⁄A

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where E is the impact energy required to break a sample with a ligament of area A. 2.6. Microstructure examination The microstructures of geopolymer composites were examined using a Zeiss Evo 40XVP scanning electron microscope. The specimens were mounted on aluminium stubs using carbon tape and then coated with a thin layer of platinum to prevent charging before the observation.

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3. Experimental results and discussion

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3.1. Chemical analysis of fly-ash

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(3)

X-ray fluorescence (XRF) data provided preliminary information with respect to fly ash suitability for the fabrication of geopolymers. XRF was used to evaluate the chemical composition of the source fly ash. Research by Chen-Tan et al. [19] showed that only the amorphous aluminosilicates in the fly ash are reactive in the geopolymerisation reaction that forms a geopolymer. Table 2 shows Calcium oxide constituting 2.58% of the original fly ash. Such a concentration, (containing <20% of CaO), leads to a class F fly ash classification. The fly ash, as anticipated, contained primarily aluminosilicate iron oxides with traces of minor oxides (Table 2). Silicon to aluminium ratio, iron oxide and calcium content are significant compositional variables in fly ash. The silicon to aluminium ratio in fly ash typically sets the silicon to aluminium ratio for the resulting geopolymer where an alkali-only activating solution is used. Alternatively, the silicon to aluminium ratio in fly ash may affect the composition of the activating sodium aluminate or sodium silicate where there is a desire to obtain a certain silicon to aluminium ratio geopolymer. Researchers have also found that iron oxide presence is considered detrimental as it adversely affects aluminosilicate dissolution in geopolymers [19]. Calcium oxide contents are relevant as it presence cementitious properties impact on the setting time of geopolymers [20]. The mass of carbon remaining in the fly ash from unburned coal is reflected in the loss on ignition (LOI) value. Such remnant carbon is a critical characteristic of fly ash intended for geopolymer concrete production. Air entrainment problems which impact on material performance of geopolymer concretes can arise where higher carbon contents are used [21]. The water required for workability of mortars and concretes depends on the carbon content of fly ashes: the higher the carbon content of a fly ash, the more water is needed to produce a paste of normal consistency.

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3.2. X-ray diffraction (XRD) The x-ray diffraction spectra obtained for the raw materials and the resultant geopolymer are presented in Fig. 2. The XRD pattern of fly ash powder is shown in Fig. 2a. The fly ash are predominantly amorphous, with crystalline inclusions. The crystalline phases were indexed using Powder Diffraction Files (PDFs) from the Inorganic Crystal Structure Database (ISCD). The crystalline phases identified in the fly ash are Quartz (PDF 04-012-0490), Mullite (PDF 01-074-4145) and Magnetite (PDF 04-012-7038). Greater intensity can be identified at 27 for Quartz. While crystalline material in fly ash is typically inert during alkali activation, its presence can affect the properties of geopolymers. Such crystalline phases tend to cause micro cracking reducing strength. The problem is exacerbated crystalline phases have larger particle sizes. The widths of the peaks present the size of crystalline in a particular phase. The crystalline size reduces as the width of the peaks increase. Therefore, the crystalline with a larger size has a higher sharp peak. The degree of disorder can be interpreted by the way it diffracts X-rays to form a pattern. In a noon-crystalline state, X-ray diffraction resulted in a broad diffuse halo rather than sharp diffraction peaks. The broad and amorphous aluminosilicate hump that created at approximately 23◦ 2␪ is characteristic of an amorphous phase. These phases may be alumina and amorphous silica generated by coal combustion. It is the reactivity of such phases that contributes to their participation in geopolymerization reactions. By comparing XRD spectra of the original fly ash with that of the hardened geopolymer paste, major crystalline phases are observed (Fig. 1b). Such major crystalline phases were previously identified in the unreacted fly ash, and are again observable in the Na-based activator prepared geopolymer. The intensity and position of the diffraction peaks for the geopolymer spectrum coincide with those of the fly ash, indicating that the crystalline components play little or no part in the reaction. According to the X-ray data, when the fly ash was activated with alkaline solution, there is no clear evidence of new crystalline phases. The same crystalline phases were identified in the fly ash and geopolymer .There is, however, a broad “amorphous hump” from a centre at 23◦ 2␪ to 35◦ 2␪. It has been noted that this hump is indicative of geopolymeric reaction [22]. Chen-Tan et al. [19] reported that amorphous aluminosilicates alone in the fly ash are reactive in the geopolymer forming reaction, known as geopolymerisation. Watt and Thorne [23] used XRD to investigate various types of fly ashes and reported that the majority of fly ashes contained quartz, mullite, magnetite, and hematite. Other crystalline phases were not found in significant amounts. Fly ash reactivity is reported to be linked with non-crystalline phase glass [19,22]. The non-crystalline components, referred to as the “glass”, are an aluminosilicate glassy material .The diffraction pattern reflects such glass content as a broad “hump”. 3.3. Morphology of fly ash and geopolymer The morphology of fly ash affects bulk characteristics of resultant fabricated geopolymers [24]. Spherical morphology is optimal for geopolymer production as it permits workability at low liquids to solids mix ratio [24]. In the current study, SEM analysis was used to evaluate the surface morphologies of the fly ash and the synthesized geopolymer, focusing on particle shape. The typical fly ash particle morphology is presented in Fig. 3. The majority of the particles appear glassy such is recognized by a smooth surface texture and absence of angular crystalline shapes. The fly ash particles in cross section reveal a typical spherical morphology. Such has been previously observed [25] and arises as a result of local temperature and composition variations affecting coal combustion and capture process of fly ash. These particles act as lubricant to reduce the friction between aggregate in concrete. Thus, a fly ash with a higher

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Fig. 2. XRD pattern of (a) fly ash and (b) geopolymer [Legend: M = mullite, 2 = quartz, F = magnetite].

Previous studies have also report micropore presence in geopolymers attributed to the quartz disturbance in geopolymer matrices. Quartz, as mentioned, hinders the geopolymer reaction through causing interfacial separation, and interfacial microcracks formation after curing [29]. The behaviour of quartz is illustrative of the detrimental influence crystalline phases have on the mechanical properties of the geopolymer.

3.4. Flexural stress–strain curve

Fig. 3. SEM image of fly ash.

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amount of round-shaped particles is a more ideal fly ash for concrete in terms of workability [26]. The fly ash was also found to contain porous and non-spherical particles. Such morphology has a propensity to impede the performance of geopolymer slurry made from the fly ash [25]. SEM analysis of the polished surface of the fabricated geopolymers showed non smooth surfaces with micocracking, indicating incomplete gepolymerization. The presence of randomly distributed micropores and undissolved solid particles of fly ash (see Fig. 3) supporting that part of the particles avoided the chemical reaction. The implication is that crystalline phases refrain from reaction in geopolymerization, and present merely as inactive fillers in geopolymer network. The consequence is that increased dispersed small sized pores present in the geopolymer matrix contributing to the overall porosity. Duxson et al. [27] reported that trace quantity of unreacted and/or insoluble phases of hematite, quartz, and gypsum tend to hinder dissolved alumina and silica’s homogeneous distribution and transport causing an increase in micropores quantity in the geopolymer matrix.

The influences of fibres on the stress–strain behaviour of polymer composites are well known. Previous studies reported that polymer specimens in high stress tend to deform at the precracking stage [30]. At the post-cracking stage, that is, once the first crack appeared, the control specimen of polymer failed on reaching maximum stress. The control was unable to sustain extra stresses. In contrast, fibre reinforced polymer specimens were found to be able to sustain stress throughout the post peak stage. The fibres tended to hold and bridge cracks on the polymer as the polymer specimen initially cracked and deformed. It was also found that pullout behaviour continuing well in the post-cracking stage resulted in loading being transferred to fibres and the typical brittle failure of polymers was prevented [30]. The fibres/polymer composites were found to be capable to absorb a substantial amount of energy to propagate the crack through the fibres which lie at the tip of the crack. The energy required was found to be higher than that to initiate a new crack in the composites. As a result, a new crack was formed some distance away from the first one instead cracking all the way through and breaking. As cracks increased further, the stress-strain curves were found to gradually flatten under bending stress before it fractures (Fig. 4). Q3 Fig. 5 shows typical flexural stress-strain curves for the unreinforced geopolymer matrix and the glass microfibregeopolymer composites with different microfibre contents. From the stress–strain curves in Fig. 5, it is interesting to note that geopolymer composites displayed some non-linearity during fracture whereas a linear behaviour was observed for neat geopolymer. This shift from linearity to nonlinearity is indicative of improved mechanical properties in the composite samples. Adding glass microfibres to a plain geopolymer matrix has a significant effect on the pre-cracking behaviour. It substantially enhances the post-

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Fig. 6. Flexural strength of geopolymer composites as a function of microfibre content. Fig. 4. SEM image of geopolymer.

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cracking response. From the stress–strain relationship in Fig. 5, it is evident that specimens reinforced with glass microfibre exhibited the ability to carry higher strain level. The test results show that the microfibre reinforced geopolymer composites were able to accommodate the highest strain value compared to the unreinforced geopolymer. The high strain capacity is attributed to the contribution of the reinforcing glass microfibres to crack arresting and bridging. Such crack arresting, bridging mechanisms and crack deflection results in non-linear stress-strain curves for the geopolymer composites. This implies the feasibility of using glass micro fibres to mitigate the brittle failure in geopolymers. These results suggest that the glass microfibres are able to introduce additional mechanisms of failure and energy consumption composites. Glass microfibres may act as stoppers to crack growth by bridging the cracks.

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3.5. Mechanical properties of geopolymer composites

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3.5.1. Flexural strength Flexural strength is the ability of a material under load to resist deformation. Flexural strength as a value should indicate the maximum stress experienced within a material at point of rupture [31].

It is known that fibres commonly increase the flexural strength of the composites, but on the other hand, they may have a detrimental effect on the flexural strength with rising fibre content beyond the optimal fibre weight content. The decrease in the flexural strength above the optimal concentration can be attributed to a weak interfacial bond between the fibre and the matrix, probably due to the agglomeration of the fibres [7]. The interfacial bond plays a crucial role in the materials capability of transferring stresses and elastic deformations from the matrix to the fibres. If the interfacial bond is weak the fibres are unable to carry part of the external load applied to the composite. On the other hand, a strong interfacial bond allows a good stress transfer from the matrix to the fibres and increases the yield strength of the composites [7]. The effects of fibre content on the flexural strength of glass microfibre-reinforced geopolymer composites are shown in Fig. 6. Geopolymer composites reinforced with 2 wt% glass microfibres are found to have greater flexural strength than the non-reinforced controls samples (Fig. 6). Good dispersion of glass microfibres throughout the geopolymer matrix increasing adhesion at the matrix/microfibre interface is believed to underpin enhancements in flexural strength. Such dispersion and adhesion is believed to allow optimum stress-transfer operation from matrix to microfibres improving of material strength properties. Fibre-matrix adhesion is a determinant of composite quality in fibre reinforced

Fig. 5. Typical stress–strain curves of geopolymer composites with various glass microfibre contents (a) 0 wt.%, (b) 1 wt.%, (c) 2 wt.% and (d) 3 wt.%.

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Fig. 7. Flexural modulus of geopolymer composites as a function of microfibre content.

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composites where the matrix acts to transfer load to fibres through shear stresses at interfaces. The process is reliant on good bonding between the fibres and the polymeric matrix [32–34]. The 3 wt% addition of glass microfibres, however, reduced flexural strength. Processing events are believed to have led to the failure of 3 wt% glass microfibre reinforcement to further improve strength properties. Viscosity rises during the mixing of glass microfibres and paste rendering typical degassing insufficient before curing in the case of higher glass content. A complete degassing process is required for composites to limit void formation. For composites containing glass microfibres at 3 wt%, degassing is mandatory as void formation causes specimen failure. Moreover, the viscous mixture that results on 3 wt% glass microfibres addition to geopolymer further compromises microfibre–matrix adhesion. High viscosity causes wettability reduction, and matrix/microfibre interfacial adhesion is more likely also reducing resultant material strength [35,36]. The results of similar studies with geopolymer composites prepared with short fibres have been published. The patterns emerges that increasing fibre loading in geopolymer composites increases flexural strength to an point at which on further addition of fibre, flexural strength decreases. Poor interaction and dispersion of fibres in matrices after a particular concentration underpins flexural strength reduction. Chen et al. [37] studied the flexural behaviour of sweet sorghum fibre reinforced fly ash-based geopolymer, and reported an increasing flexural strength up to 2 wt% fibre content at which on further addition of fibre, flexural strength decrease. The team concluded that sweet sorghum fibre at 2 wt% is optimum, permitting greater load to be transferred throughout the composite delaying micro-crack growth and enhancing flexural strength overall. Poor workability and agglomeration of fibres leading air bubble entrapment in composites are common issues when optimum fibre content is exceeded. Lin et al. [38] studied the flexural behaviour of metakaolin-based geopolymers composites by reinforcing the composites with short carbon fibre. Up to 4.5 wt.% of carbon fibre flexural strength was reported to increase as fibre content was increased. Beyond 4.5 wt.% of carbon fibre, at 6 and 7.5 wt.%, the flexural strength decreased due to fibre damage and fibre/matrix interface high shear stress formation. 3.5.2. Flexural modulus Flexural modulus is an indicator of a material’s stiffness in static bending condition [39]. Fig. 7 depicts the variation in flexural modulus of the composite as a function of wt% loading of glass microfibres. The use of glass microfibres in low strength geopolymer mixtures increases their flexural modulus significantly

Fig. 8. Impact strength of geopolymer composites as a function of microfibre content.

compared to unreinforced matrices (control). The magnitude of the improvement is dependent on the microfibre contents. This increase is more for the composites with 2 wt% glass microfibres in comparison with geopolymer composites with 1 wt% glass microfibres. This enhancement in the flexural modulus after micorfibres additions is likely to be attributable to higher modulus of glass microfibres which serve as backbones for the composites. The presence of glass microfibres delays the formation of the first crack, enable the composites to accommodate large strains before failure. Unlike plain geopolymer the presence of microfibres imparts considerable energy absorption capacity to stretch and debond the mcirofibres before the complete fracture of the material occurs. Flexural modulus improvements in geopolymer composites reinforced with natural fibres have also been reported [7]. Notwithstanding such, glass microfibre reinforcement beyond optimum concentrations causes a rapid decrease in geopolymer composite’s flexural modulus. Such decreases can be attributable poor dispersion of microfibres within geopolymer resins at higher concentrations. When glass microfibres are poorly dispersed inside matrices they tend to form microfibre agglomerations acting as stress concentrators and cause flexural modulus reduction. Moreover, geopolymer contains high pre-existing content levels of other constituents such as hematite and quartz which are inactive fillers. Particularly undesirable, is quartz presence in source materials when designing geopolymers as such hinders geopolymer binding causing micro-cracking, which reduces material strength. As quartz particle size increases, the issue becomes more significant. The results in the current study are consistent with findings of previous studies in which the flexural modulus of geopolymer composites was found to decrease on cotton fibre addition exceeding 0.5 wt.% [7]. 3.5.3. Impact strength Impact strength refers to a material’s ability to withstand an applied load without failure. Impact strength can also refer to the quantity of energy required to propagate a crack. Matrix-fibre interfacial bonding, and the properties of constituent matrix and fibres will determine the impact strength of fibre reinforced polymers. On sudden force, fibre pull out, fibre fracture and matrix deformation act to dissipate impact energy [40,41]. Typically impact strength increases as fibre content increases in fibre reinforced polymer composites due to increases in fibre pull out and fibre breakage [41]. Microfibre addition impacts on the Charpy impact strength for glass microfibres reinforced geopolymer composites is shown in Fig. 8. The content of microfibre content has a significant impact on

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Charpy impact strength value. The composite’s impact strength was found to increase as glass microfibre was increased up to a maximum of 2 wt%, and then it decreases thereafter (Fig. 8). A number of mechanisms underpin beneficial effects when glass microfibres are incorporated into geopolymer composites. Firstly, microfibres serve as a filler filling voids within the hydrated geopolymer paste. Such filling leads to denser microstructures. Secondly, microfibres may micro-bridge cracks in geopolymers consequently improving composite ability to absorb fracture energy on impact loading. Finally, the utilization of microfibre could increase embedded fibres quantity improving interfacial bonding strength of the fibre-matrix interface. The impact strength, as mentioned, is primarily dependent on the fibre, polymer, and fibre-matrix interfacial bonding quality. Composites require good interfacial bonding to show enhanced impact strength. Good microfibre and matrix bonding ensures that great loads are required for debonding or microfibre pullouts to occur. Microfibres interface with cracks contributing to crack advancement resistance in the sample during the impact test. Microfibres, on contact with propagating cracks, nucleate the formation of tiny craze cracks producing a large free surface capable of absorbing mechanical energy as potential surface energy. The result is that as composite microfibre content increases, composite impact strength also increases. Composites at 2 wt% microfibres have greater impact strength than pure control samples (Fig. 8). However, composite impact strength decreases beyond 2 wt% microfibres. Composite materials typically have a certain composition at which optimum strength will be optimal. The required composition will typically be when the paste used is sufficient to enable perfect bonding surrounding the fibres. An increase of microfibres concentration reduces the concentration of paste respectively causing a reduced wetting of microfibres by the geopolymer paste beyond 2 wt% microfibres. The implication is poor interfacial adhesion and reductions in impact strength. To achieve workable fibre embedment and greater mechanical properties, a lower binder viscosity should be obtained. Adding microfibre beyond 2 wt% caused a recognizable rise in matrix viscosity, which permitted the introduction of residual air bubbles through being trapped in the geopolymer during mould pouring or through mixing. Such conditions can result in degradation of impact energy. When microfibres are loading at lower concentrations there is more potential for uniform dispersion, and less potential for microvoid formation, both of which conditions contribute to strength improvement. Results of earlier studies also support that short fibre addition can enhance the mechanical properties of geopolymer composites reinforced with such fibres. Alomayri and Low [6] observed a reduction in impact strength due to fibre agglomerations degrading interfacial adhesion between fibre and matrix where short fibres had been added to geopolymers at 0.7 and 1.0 wt%. The team found that while pure geopolymer paste had an impact strength of 1.9 kJ/m2 , the addition of 0.5 wt% short natural fibres resulted in impact strength higher at 4.5 kJ/m2 . However, beyond 0.5 wt% adding more fibres, such as at 0.7 and 1.0 wt %, caused a reduction in composite impact strength.

4. Conclusions Geopolymer-glass microfibre composites have been synthesized and investigated in terms of XRD, SEM, stress-strain curves, flexural strength, flexural modulus and impact strength. The XRD analysis indicated that the geopolymers contained an amorphous phase as well as the crystalline phases that were initially present in the fly ash precursor. Glass microfibre was found to substantially enhance the post-cracking response of geopolymer composites

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with 1, 2 and 3 w% loading giving more favourable results compared to pure geopolymer matrix. Flexural strength and modulus properties were also found to be enhanced due to the reinforcing effect of glass microfibre. In particular, glass microfibre addition improved fibre–matrix adhesion giving greater strength property results for the geopolymer composites. Due to the toughness mechanism provided by glass microfibres, the presence of glass microfibers increased the ability of samples to absorb fracture energy on impact loading. However, glass microfibre addition beyond 2 wt% led to a reduction in mechanical properties due to the glass microfibre’s effect on the fibre–matrix adhesion limiting the mechanisms of fibre pull-out and fibre debonding. Therefore, the mechanical properties of the geopolymer composites composites with 3 wt% glass microfibres were lower than those of the geopolymer composites with 2 wt% glass microfibres. Uncited references

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