materials

Article Innovative Geopolymer-Epoxy Composites: Preparation, Microstructure and Mechanical Properties

Giuseppina Roviello 1, Laura Ricciotti 1,*, Oreste Tarallo 2, Claudio Ferone 1, Francesco Colangelo 1, Valentina Roviello 3 and Raffaele Cioffi 1

1 INSTM Research Group Napoli Parthenope, Centro Direzionale Napoli, Dipartimento di Ingegneria, Università di Napoli ‘Parthenope’, Isola C4, Napoli 80143, Italy; [email protected] (G.R.); [email protected] (C.F.); [email protected] (F.C.); raffaele.cioffi@uniparthenope.it (R.C.) 2 Dipartimento di Scienze Chimiche, Università degli Studi di Napoli “Federico II”, Complesso Universitario di Monte S. Angelo, via Cintia, Napoli 80126, Italy; [email protected] 3 Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, p.le Tecchio 80, Napoli 80126, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-081-547-6799

Academic Editors: Arie van Riessen and Prabir K. Sarker Received: 14 March 2016; Accepted: 2 June 2016; Published: 9 June 2016

Abstract: The preparation and characterization of composite materials based on geopolymers obtained from fly ash and epoxy resins are reported for the first time. These materials have been prepared through a synthetic method based on the concurrent reticulation of the organic and inorganic components that allows the formation of hydrogen bonding between the phases, ensuring a very high compatibility between them. These new composites show significantly improved mechanical properties if compared to neat geopolymers with the same composition and comparable performances in respect to analogous geopolymer-based composites obtained starting from more expensive raw material such as . The positive combination of an easy synthetic approach with the use of industrial by-products has allowed producing novel low cost aluminosilicate binders that, thanks to their thixotropicity and good adhesion against materials commonly used in building constructions, could be used within the field of sustainable building.

Keywords: geopolymer; fly ash; composites; epoxy resin; SEM; mechanical properties

1. Introduction Geopolymers are a family of inorganic materials obtained by reaction between an aqueous alkaline solution and an aluminosilicate source [1,2]. This reaction yields an amorphous three-dimensional structure in which SiO4 and AlO4 tetrahedra are linked by corner-shared O atoms. Geopolymers are characterized by interesting mechanical properties, low shrinkage, thermal stability, freeze-thaw, chemical and fire resistance, long term durability and recyclability. For these reasons, they have the potential for utilization as Ordinary Portland (OPC) replacement in a wide range of applications, such as fireproof barriers, materials for high temperatures, matrices for hazardous waste stabilization, toolings and moldings [3,4]. Moreover, with respect to the manufacturing of OPC that consumes a significant amount of natural materials and energy release of a large quantity of carbon dioxide in the atmosphere [5–8], the use of geopolymer-based materials in concrete applications could significantly reduce the CO2 emissions [9] thanks to the “low carbon” footprint of several raw materials with a high concentration

Materials 2016, 9, 461; doi:10.3390/ma9060461 www.mdpi.com/journal/materials Materials 2016, 9, 461 2 of 15 of aluminosilicates from which they can be prepared, i.e., dehydroxylated kaolinite (metakaolin, MK) or industrial waste such as fly ash. Materials 2016, 9, 461 2 of 15 Fly ash (FA) is a fine powder by-product transported by flue gas after the combustion of coal in coal-firedFly ash power (FA) stationsis a fine powder typically by-product made up transpor of smallted glass by flue spheres, gas after consisting the combustion primarily of coal of silicon, in aluminium,coal-fired iron, power and stations calcium typically oxides made [10– 14up]. of The small development glass spheres, of FA-basedconsisting primarily geopolymer of silicon, concretes couldaluminium, contribute iron, to recyclingand calcium waste oxides into [10–14]. construction The development material, thus of FA-based reducing, geopolymer at the same concretes time, CO 2 emissionscould contribute [15–17]. to recycling waste into construction material, thus reducing, at the same time, CO2 emissionsHowever, [15–17]. FA-based geopolymers typically exhibit brittle behavior with low tensile strength, ductility,However, and fracture FA-based toughness, geopolymers thus limitingtypically upexhibi to nowt brittle the behavior actual possibilitywith low tensile to use strength, these very promisingductility, materials and fracture for extensivetoughness, and thus practical limiting up applications to now the in actual constructions. possibility to This use limit these could very be, in principle,promising overcome materials for by extensive developing and geopolymerpractical applic composites,ations in constructions. but, to the This best limit of our could knowledge, be, in principle, overcome by developing geopolymer composites, but, to the best of our knowledge, very very little is reported on this topic. Only recently, Li et al. [18] described the utilization of chitosan little is reported on this topic. Only recently, Li et al. [18] described the utilization of chitosan biopolymers for the implementation of composite systems based on geopolymers obtained from fly biopolymers for the implementation of composite systems based on geopolymers obtained from fly ash,ash, in which in which the the formation formation of of a a three-dimensional three-dimensional cross-linked cross-linked composite composite is achieved is achieved by means by means of a of a networknetwork of of hydrogen hydrogen bonds bonds betweenbetween geopolymer geopolymer and and chitosan chitosan macromolecules (Figure (Figure 1). 1).

Figure 1. Schematic representation of the mechanism of interaction between geopolymer (on the left Figure 1. Schematic representation of the mechanism of interaction between geopolymer (on the left and on the right, in the hypothesis of Na/Al = 1:1) and N-carboxymethyl chitosan macromolecules and on the right, in the hypothesis of Na/Al = 1:1) and N-carboxymethyl chitosan macromolecules (in the middle). Dashed lines represent hydrogen bonds [18]. (in the middle). Dashed lines represent hydrogen bonds [18]. In that case, the interaction between the inorganic matrix and the is effective up toIn 0.1% that by case, weight the interactionof N-carboxymethyl between chitosan the inorganic content, matrix while anda further the macromoleculeincrease of the chitosan is effective up toamount 0.1% byproduces weight a of decreaseN-carboxymethyl of the geopolymeriz chitosanation content, degree, while probably a further due increase to an encapsulation of the chitosan amounteffect produces of the fly ash a decrease particles ofthat the become geopolymerization un-reactive [18]. degree, It is worth probably noting duethat, toin ana bid encapsulation to obtain effectadvanced of the fly geopolymer-based ash particles that compos becomeite un-reactivematerials, a [wide18]. Itrange is worth of organic noting that, in a have bid tobeen obtain studied in literature as organic fillers, such as polyvinyl acetate [19], polypropylene [20], polyvinyl advanced geopolymer-based composite materials, a wide range of organic polymers have been alcohol [21], or water-soluble organic polymers [22]. These kind of composites are usually obtained studied in literature as organic fillers, such as polyvinyl acetate [19], polypropylene [20], polyvinyl by blending the with geopolymers, sometimes in the presence of compatibilizers [23–25]. alcoholNevertheless, [21], or water-soluble a worsening effect organic of the polymers mechanical [22]. properties These kind of the of compositesfinal product are [26] usually for very obtained low by blendingconcentrations the polymer (up to ~1 with wt %), geopolymers, similar to that sometimes previously described in the presence in the case ofcompatibilizers of chitosan, has been [23– 25]. Nevertheless,observed. In a worseningthese last cases, effect this of detrimental the mechanical effect propertiesis probably ofdue the to final the use product of organic [26] formaterials very low concentrationspoorly compatible (up to with ~1 wt the %), inorganic similar matrix to that that, previously at high amounts described of the in organic the case components, of chitosan, causes has been observed.a phase In separation these last between cases, thisthe organic detrimental filler and effect the isinorganic probably geopolymer due to the matrix. use of organic materials poorly compatibleVery recently, with we the have inorganic developed matrix an innovative that, at high, easy amounts and cost-effective of the organic synthetic components, strategy causesto a phaserealize separation hybrid composite between materials the organic by using filler metaka and theolin-based inorganic geopolymers geopolymer as matrix.inorganic components andVery different recently, organic we have resins developed up to 25% in an weight innovative, [27–32]. These easy andmaterials cost-effective show significantly synthetic improved strategy to realizephysical hybrid and composite mechanical materials properties by usingand remarkably metakaolin-based reduced geopolymersbrittleness with as inorganicrespect to componentsthe neat geopolymers, still preserving good thermal and fire resistance [30,32]. This approach consists of the and different organic resins up to 25% in weight [27–32]. These materials show significantly improved concurrent co-reticulation of both phases that are mixed together when each polymerization reaction physical and mechanical properties and remarkably reduced brittleness with respect to the neat is already started but is far from being completed, thus allowing for realization of a chemical geopolymers,interaction stillbetween preserving the organic good component thermal andand the fire geopolymeric resistance [30 mixture,32]. This base approachd on the formation consists of the a wide network of hydrogen bonding (Figure 2) [29]. This approach ensures high compatibility

Materials 2016, 9, 461 3 of 15 concurrent co-reticulation of both phases that are mixed together when each polymerization reaction is already started but is far from being completed, thus allowing for realization of a chemical interaction between the organic component and the geopolymeric mixture based on the formation of a wide networkMaterials of 2016 hydrogen, 9, 461 bonding (Figure2)[ 29]. This approach ensures high compatibility between3 of 15 the phases and a very good dispersion even at the nanometric level of the organic phase that can between the phases and a very good dispersion even at the nanometric level of the organic phase that be includedcan be included in the in mixture the mixture up to up about to about 25% 25% by by weight, weight, without without addition of of external external additives additives or or compatibilizerscompatibilizers [30 ].[30].

® FigureFigure 2. Schematic 2. Schematic representation representation of of some some possible possible interactionsinteractions between between the the epoxy epoxy resin resin Epojet Epojet [31]® [31] (in the middle) and the geopolymeric matrix (on the left and the right, in the hypothesis of Na/Al = 1:1). (in the middle) and the geopolymeric matrix (on the left and the right, in the hypothesis of Na/Al = 1:1). Dashed lines represent hydrogen bonds. Dashed lines represent hydrogen bonds. In the present paper, we report on the preparation of novel FA-based geopolymer composites, obtainedIn the present by extending paper, wethe reportsynthetic on themethod preparation developed of novelby us FA-basedin the case geopolymer of more expensive composites, obtainedmetakaolin-based by extending geopolymers the synthetic to raw method waste materi developedals. In bysuch us a in way, the we case have of moresucceeded expensive in metakaolin-basedcombining an easy geopolymers and sustainable to raw synthetic waste materials. approach In with such the a use way, of we industrial have succeeded by-products in combiningfor the an easyrealization and sustainable of novel “eco-friendly” synthetic approach aluminosilicate with the binders use of industrialin order to by-productspreserve the forenvironment the realization and of novellimit “eco-friendly” the cost of construction aluminosilicate materials. binders in order to preserve the environment and limit the cost of constructionThese materials. new promising materials have been characterized by means of several techniques, such as scanning electron microscopy (SEM), thermal analysis (TGA), X-ray diffraction (XRD), and These new promising materials have been characterized by means of several techniques, such as compressive strength tests. Their properties have been compared with neat geopolymer samples and scanning electron microscopy (SEM), thermal analysis (TGA), X-ray diffraction (XRD), and compressive analogous samples obtained from metakaolin [28]. strength tests. Their properties have been compared with neat geopolymer samples and analogous samples2. Experimental obtained from Section metakaolin [28].

2. Experimental2.1. Materials and Section Methods

2.1. MaterialsClass andF coal Methods fly ash employed in this work was supplied by the ENEL S.p.A. power plant located in Brindisi (Southern Italy) and was used as received, without drying treatment. The water content wasClass found F coal to flybe ash4% after employed drying in in this an oven work at was 105 supplied °C until reaching by the ENEL a constant S.p.A. mass. power Its plantchemical located in Brindisicomposition (Southern was obtained Italy) and by was means used of as a received, Perkin-Elmer without Optima drying 2100 treatment. DV ICP-OES The waterapparatus content was(Waltham, found to beMA, 4% USA) after [14,33] drying and in is an reported oven at in105 Table˝C 1. until The water reaching content a constant of fly ash mass. was taken Its chemical into compositionaccount in was the obtainedmix design. by Metakaolin means of a was Perkin-Elmer kindly provided Optima by 2100 Neuchem DV ICP-OES S.r.l. (Milan, apparatus Italy), and (Waltham, its MA,composition USA) [14,33 is] andreported is reported in Table in 1. Table Epojet1.® The epoxy water resin content [34] was of purchased fly ash was by taken Mapei into S.p.A. account (Milan, in the mixItaly). design. Sodium Metakaolin hydroxide was was kindly purchased provided from bySigma Neuchem Aldrich S.r.l.(St. Louis, (Milan, MO, Italy), USA) andand used its composition without is reportedfurther purification. in Table1. The Epojet sodium® epoxy silicate resinsolution [34 was] was supplied purchased by Prochin by Mapei Italia S.r.l. S.p.A. (Naples, (Milan, Italy), Italy). and its composition is reported in Table 1. Sodium hydroxide was purchased from Sigma Aldrich (St. Louis, MO, USA) and used without further purification. The sodium silicate solution was supplied by Prochin Italia S.r.l. (Naples, Italy), and its composition is reported in Table1.

Materials 2016, 9, 461 4 of 15

Table 1. Chemical composition (weight%) of weathered fly ash used in this paper after thermal treatment at 950 ˝C, metakaolin and the sodium silicate solution.

Fly Ash

Al2O3 SiO2 K2O Fe2O3 Na2O MgO CaO others 28.12 53.75 1.89 6.99 0.87 1.59 4.32 2.47 Metakaolin

Al2O3 SiO2 K2O Fe2O3 TiO2 MgO CaO others 41.90 52.90 0.77 1.60 1.80 0.19 0.17 0.67 Sodium Silicate Solution

SiO2 Na2OH2O 27.40 8.15 64.45

The fly ash particle size distribution was determined by means of a Malvern Mastersizer 3000 laser particle analyser (Malvern, UK). Thermogravimetric analyses (TGA) were performed by a Mettler-Toledo TGA/DSC2 STARe SYSTEM (Columbus, OH, USA). The thermographs were obtained with a heating rate of 10 ˝C/min using «10 mg of the powdered sample under air flow. X-ray diffraction patterns were obtained at room temperature with an automatic Rigaku powder diffractometer mod. Miniflex 600 (Tokyo, Japan), operating in the θ/2θ Bragg-Brentano geometry. The phase recognition was carried out by using the PDF-4+ 2014 (International Centre for Diffraction Data®, Tokyo, Japan) database and the Rigaku PDXL2 software (Rigaky, Tokyo, Japan). SEM analysis was carried out by means of a Nova NanoSem 450 FEI Microscope (Hillsboro, OR, USA). The compressive strength was evaluated according to EN 196-1 and measured by testing cubic paste specimens (30 ˆ 30 ˆ 30 mm3) in a Controls MCC8 multipurpose testing machine (CONTROLS s.r.l., Liscate, Milan, Italy) with a capacity of 100 kN. The tests were performed after 28 days of curing at room temperature, and the values reported are the averages of the five compression strength values.

2.2. Specimen Preparation

2.2.1. Preparation of Metakaolin-Based Geopolymer (G-MK) The alkaline activating solution was prepared by dissolving solid sodium hydroxide into the sodium silicate solution. The solution was then allowed to equilibrate and cool for 24 h. The composition of the obtained solution can be expressed as Na2O¨ 1.34SiO2 10.5H2O. Then, the metakaolin was incorporated to the activating solution with a liquid to solid ratio of 1.4:1 by weight and mixed by a mechanical mixer for 10 min at 800 rpm. The composition of the whole geopolymeric system can be expressed as Al2O3 3.5SiO2 1.0Na2O¨ 10.5H2O, assuming that geopolymerization occurred at 100%. Such composition is in good agreement with that actually determined by EDS analyses on the prepared samples.

2.2.2. Preparation of Fly Ash-Based Geopolymer (G-FA) The alkaline activating solution was prepared by mixing the sodium silicate solution with an aqueous solution of sodium hydroxide 10 M. The solution was then allowed to equilibrate and cool for 24 h. The composition of the obtained solution can be expressed as Na2O 0.9SiO2 14.7H2O. Then, fly ash was incorporated into the activating solution with a liquid to solid ratio of 0.66:1 by weight and mixed by a mechanical mixer for 20 min at 800 rpm. The composition of the whole geopolymeric system can be expressed as Al2O3 3.79SiO2 0.66Na2O 8.9H2O, assuming that geopolymerization occurred at 100% (see the related discussion in Section 3.1.4).

2.2.3. Preparation of the EPOXY-Geopolymer Composites Geopolymer-based composites have been obtained by adding Epojet® resin to the freshly-prepared geopolymeric suspension, and were quickly incorporated by controlled mixing (5 min at 1350 rpm). Materials 2016, 9, 461 5 of 15

Epojet® is a commercial two-component epoxy adhesive for injection, which, after mixing, takes on the qualities of a low viscosity liquid. It is obtained by mixing its components in 4:1 ratio in weight, as specified in the technical data sheet, supplied by the manufacturer [34], and it is usable for 40 min at room temperature. Before being added to the geopolymeric mixture, Epojet® was cured at room temperature for 10 min. The resin was added when it was still easily workable and long before its complete crosslinking and hardening (that takes place in about 5–7 h at 23 ˝C). It is worth noting that the addition of the unreacted components of the resins to the inorganic suspension produces phase segregation and, on the contrary, a late mixing of the two components (the cured organic resin and the geopolymer) results in a strongly reduced homogeneity of the final material. Different specimens containing up to 20% w/w of resin were prepared: in particular, G-FA-Ep10 and G-FA-Ep20 specimens were obtained by mixing Epojet® epoxy resin (10% and 20% by weight, respectively) with fly ash-based geopolymer (G-FA); G-MK-Ep10 and G-MK-Ep20 were obtained by mixing Epojet® epoxy resin (10% and 20% by weight, respectively) with metakaolin-based geopolymer (G-MK). All the composites started solidifying in few minutes [28]. The composition of the studied samples is reported in Table2.

Table 2. Composition (wt %) of the samples used in this study.

Mix ID MK FA SS NaOH NaOH soln Resin G-MK 41.6 - 50.0 8.4 - - G-MK-Ep10 37.4 - 45.0 7.6 - 10 G-MK-Ep20 33.3 - 40.0 6.7 - 20 G-FA - 60.2 19.9 - 19.9 - G-FA-Ep10 54.2 17.9 - 17.9 10 G-FA-Ep20 48.2 15.9 - 15.9 20

MK = metakaolin; FA = fly ash; SS = sodium silicate solution; NaOH soln = aqueous sodium hydroxide solution 10 M; Resin = Epojet® Mapei S.p.A. [34].

2.3. Curing Treatments As soon as prepared, MK-based specimens were casted in cubic molds and cured at room temperature (=25 ˝C) in >95% relative humidity conditions for seven days. The evaporation of water was prevented by sealing the top of the molds with a thin plastic film during the curing stage. The specimens were left for a further 21 days in air at room temperature before being characterized. FA-based specimens, as soon as prepared, were casted into cubic molds and cured at 60 ˝C for 48 h in >95% relative humidity conditions (the evaporation of water was prevented by sealing the top of the molds with a thin plastic film during the curing stage) and further five days in >95% relative humidity conditions at room temperature. Finally, the specimens were left for a further 21 days in air at room temperature before being characterized.

3. Results and Discussion

3.1. Characterization

3.1.1. X-ray Diffraction Characterization Figure3 shows the XRD patterns of the used fly ash (FA), the cured neat FA-based geopolymer (G-FA), and the composite sample (G-FA-Ep20) containing 20 wt % of organic resin. The diffraction pattern of the fly ash is characterized by a wide and diffused hump in the interval range 15˝–35˝ 2θ with a maximum at 2θ˝–25˝. Minor crystalline phases such as quartz (JCPDS 01-070-2517), mullite (JCPDS 01-076-2579) and hematite (JCPDS 00-013-0534) may also be identified. This amorphous halo is shifted towards slightly higher angular values (maximum at 2θ˝–30˝) in the G-FA sample, indicating the formation of an alkaline aluminosilicate hydrate gel (N-A-S-H) with a 3D amorphous structure [35]. The crystalline phases detected in the initial material (quartz, mullite and hematite) remain substantially unaltered. In this respect, it is known that, in the fly ash only, the amorphous Materials 2016, 9, 461 6 of 15 aluminosilicate component is reactive in the geopolymerization reaction [36]. Finally, as far as the Materials 2016, 9, 461 6 of 15 composite specimen is concerned, the X-ray diffraction pattern is very similar to that of the FA-based geopolymer,geopolymer, even if even it is if characterized it is characterized by by a a more more pronouncedpronounced amorphous amorphous halo halo due to due theto presence the presence of of the organicthe organic resin. resin.

Figure 3.FigureX-ray 3. X-ray powder powder diffraction diffraction patterns patterns ofof (a)) the the fly fly ash ash (FA; (FA; blackblack line);line); (b) the (b cured) the curedneat neat geopolymergeopolymer (G-FA; (G-FA;red line); red line); and and (c) ( thec) the composite composite sample (G-FA-Ep20; (G-FA-Ep20; blueblue line).line). H = hematite; H = hematite; M = mullite; Q = quartz. M = mullite; Q = quartz. 3.1.2. Thermal Analysis 3.1.2. Thermal Analysis Thermogravimetric analysis was performed on the neat G-FA geopolymer, on the organic resin, Thermogravimetricand on the G-FA-Ep20 analysis composite was (Figure performed 4) after onthe thecuring. neat G-FA geopolymer, on the organic resin, and on the G-FA-Ep20In the case of compositethe neat G-FA (Figure geopolymer4) after specimen, the curing. weight loss starts at =30 °C and is completed at 750°. The overall weight loss is 15% and can be attributed to the removal of water molecules In the case of the neat G-FA geopolymer specimen, weight loss starts at =30 ˝C and is completed at absorbed (up to ≈100 °C) or differently linked (up to ≈200 °C, free water in the pores; at higher ˝ 750 . Thetemperatures, overall weight structural loss water is 15% and and bound can water be attributed in the nanopores) to the removal to the silicate of water molecules molecules [37–39]. absorbed ˝ ˝ (up to «100EpojetC) or® resin differently shows a linkeddegradation (up tomechanism«200 C, involving free water two inmain the steps. pores; The at resin higher is thermally temperatures, structuralstable water up to and about bound 250 °C. water Above in this the temperature, nanopores) a tofirst the degradation silicate molecules step that finishes [37–39 at]. ≈480 °C is Epojetobserved,® resin resulting shows in a degradationa weight loss of mechanism 51%. The second involving degradation two main process steps. is completed The resin at about is thermally stable up650 to °C about and a 250combustion˝C. Above residual this of temperature, about 5% remains. a first degradation step that finishes at «480 ˝C is As far as the G-FA-Ep20 composite specimen, the weight loss shows a complex mechanism observed,involving resulting different in a steps: weight in particular, loss of 51%. a first The step, second corresponding degradation to a weight process loss of is ≈4%, completed is recorded at about ˝ 650 C andup to a ≈ combustion300 °C, while a residual second step, of about characterized 5% remains. by a complex path, is observed from ≈300 °C up to As≈ far700 as°C theand G-FA-Ep20corresponds to composite a further weight specimen, loss equal the weightto ≈22%. lossIt was shows found a[28] complex that the mechanism first involvingdegradation different step steps: is associated in particular, mainly a first with step, the loss corresponding of water of the to geopolymeric a weight loss phase of « while4%, is the recorded up to «300second˝C, one while corresponds a second to the step, degradation characterized of the dispersed by a complex organic path, phase. is The observed combustion from residual«300 ˝C up at 800 °C is about 78%. Moreover, it is worth pointing out that, from the DTA of the curves reported to «700 ˝C and corresponds to a further weight loss equal to «22%. It was found [28] that the first in Figure 4, the peak temperature of water loss for the composite (130 °C) is higher than that of the degradationpure geopolymer step is associated (95 °C): probably, mainly the with polar the groups loss ofof the water resin of interact the geopolymeric with the water phasemolecules, while the second onedelaying corresponds their evaporation to the degradation[28] (see column of numbers the dispersed 4 and 5 of organic Table 3). phase. The combustion residual at 800 ˝C is aboutDegradation 78%. temperatures Moreover, and it is weight worth losses pointing for the out studied that, systems from theare summarized DTA of the in curves Table 3. reported in Figure 4, the peak temperature of water loss for the composite (130 ˝C) is higher than that of the pure geopolymer (95 ˝C): probably, the polar groups of the resin interact with the water molecules, delaying their evaporation [28] (see column numbers 4 and 5 of Table3). Degradation temperatures and weight losses for the studied systems are summarized in Table3. Materials 2016, 9, 461 7 of 15 Materials 2016, 9, 461 7 of 15

Materials 2016, 9, 461 7 of 15

Figure 4. TGA curves of (a) the cured G-FA neat geopolymer sample (black solid line); (b) the pure Figure 4. TGA curves of (a) the cured G-FA neat geopolymer sample (black solid line); (b) the pure epoxy resins (Epojet, blue dashed line); and (c) the G-FA-Ep20 composite specimen (red dotted line). epoxy resins (Epojet, blue dashed line); and (c) the G-FA-Ep20 composite specimen (red dotted line). FigureTable 4.3. TGAThermal curves properties of (a) the of cured the neat G-FA geopolymer neat geopolymer (G-FA), sample pure (epoxyblack solidresins line); (Epojet), (b) the and pure the Tableepoxy 3.compositeThermal resins specimen (Epojet, properties blue(G-FA-Ep20). dashed of the line); neat and geopolymer (c) the G-FA-Ep20 (G-FA), composite pure epoxy specimen resins (red(Epojet), dotted line). and the composite specimen (G-FA-Ep20). Weight Loss Weight Loss Weight Loss Weight Loss Residual Table 3. Thermal properties of the neat geopolymer (G-FA), pure epoxy resins (Epojet), and the Mix ID Starting Ending at 200 °C at 400°C at 800°C composite specimenWeight (G-FA-Ep20). Loss Weight Loss Temperature (°C) Temperature (°C)Weight (wt Loss %) at Weight(wt Loss %) at Residual(wt %) at Mix ID Starting Ending G-FA 30 750 200 ˝C (wt 7.2 %) 400˝C (wt8.7 %) 800˝C85 (wt %) TemperatureWeight Loss (˝C) TemperatureWeight (Loss˝C) Weight Loss Weight Loss Residual MixEpojet ID Starting250 Ending 650 at 200 2.1 °C at 400°C39.4 at 800°C5 G-FA-Ep20G-FA Temperature 3030 (°C) Temperature 750 700 (°C) 7.2(wt 2.6 %) (wt 8.7 9.2 %) (wt 7885 %) Epojet 250 650 2.1 39.4 5 G-FA 30 750 7.2 8.7 85 G-FA-Ep20 30 700 2.6 9.2 78 3.1.3.Epojet Microstructural Analysis250 of Fly Ash 650 2.1 39.4 5 G-FA-Ep20 30 700 2.6 9.2 78 3.1.3. MicrostructuralFigure 5 shows Analysis SEM micrographs of Fly Ash of the used fly ash and their particle distribution. It is apparent that the fly ash consists mostly of glassy cenospheres. Such microstructure is in good agreement with 3.1.3. Microstructural Analysis of Fly Ash Figurethat 5reported shows SEMin the micrographs literature [2,12–15]. of the used Moreov flyer, ash the and dimensions their particle of distribution.the particles and It is their apparent that theaggregates flyFigure ash consists 5range shows between mostlySEM micrographs a of few glassy microns of cenospheres. the to used≈30 microns fly ash Such and have microstructure their been particle detected. distribution. isIn inparticular, good It is agreement apparent the D50 is with that reportedthat30.1 the μm fly in (Figure theash consists literature 5B). mostly [2,12 of–15 glassy]. Moreover, cenosphere thes. Such dimensions microstructure of the particlesis in good andagreement their aggregateswith that reported in the literature [2,12–15]. Moreover, the dimensions of the particles and their range between a few microns to «30 microns have been detected. In particular, the D50 is 30.1 µm aggregates range between a few microns to ≈30 microns have been detected. In particular, the D50 is (Figure30.15B). μm (Figure 5B).

Figure 5. Cont. Materials 2016, 9, 461 8 of 15 MaterialsMaterials 2016 ,2016 9, 461, 9, 461 8 of 15 8 of 15

Figure 5. (A) scanning electron microscope (SEM) micrographs and (B) particle size distribution Figure 5. (A) scanning electron microscope (SEM) micrographs and (B) particle size distribution (volume density vs. size) of the used fly ash. (volumeFigure 5. density (A) scanningvs. size) ofelectron the used microscope fly ash. (SEM) micrographs and (B) particle size distribution 3.1.4.(volume Microstructural density vs. size) Analysis of the of used Geop flyolymers ash. and Geopolymer Based Composites 3.1.4. Microstructural Analysis of Geopolymers and Geopolymer Based Composites 3.1.4. MicrostructuralThe SEM micrographs Analysis of of freshly Geop olymersobtained andfractu Geopolymerre surfaces of Based the MK-based Composites geopolymer and TheFA-based SEM geopolymer micrographs are of reported freshly in obtained Figure 6. fracture surfaces of the MK-based geopolymer and The SEM micrographs of freshly obtained fracture surfaces of the MK-based geopolymer and FA-basedMK geopolymer based-geopolymer are reported (Figure in Figure6A–C) 6is. characterized by a compact morphology that only at FA-basedvery high geopolymer magnification are reported (Figure 6C) in revealsFigure some6. unreacted kaolinite crystals and the presence of MK based-geopolymer (Figure6A–C) is characterized by a compact morphology that only at very smallMK based-geopolymer spheroidal domains, (Figureprobably 6A–C) reminiscen is characterizedt of the gelation by process a compact of the geopolymer.morphology that only at high magnification (Figure6C) reveals some unreacted kaolinite crystals and the presence of small very highThe magnification morphology of (Figure FA-based 6C) geopolymer reveals some (Figure unreacted 6A’–C’) instead kaolinite is dominated crystals and by the the presence presence of spheroidal domains, probably reminiscent of the gelation process of the geopolymer. smallof spheroidal the unreacted domains, FA particles probably that are reminiscen well dispert ofsed the in gelationthe geopolymer process matrix. of the This geopolymer. disaggregated Themorphology morphology is typical of FA-based of fly ash-based geopolymer geopolymers (Figure [1,33,40]6A’–C’) Moreover, instead in is this dominated sample, the by globular the presence The morphology of FA-based geopolymer (Figure 6A’–C’) instead is dominated by the presence of themorphology unreacted FAof the particles geopolymeric that are matrix well is dispersedrather evident in already the geopolymer at 10,000 magnification matrix. This (Figure disaggregated 6B’) of the unreacted FA particles that are well dispersed in the geopolymer matrix. This disaggregated morphologyand, at variance is typical with of the fly MK-based ash-based geopolymer geopolymers for which [1,33, 40the] Moreover,particle dimensions in this sample,are in the the range globular morphology is typical of fly ash-based geopolymers [1,33,40] Moreover, in this sample, the globular morphology20–50 nm of (Figure the geopolymeric 6C), in this last matrix case, matrix is rather globules evident have already bigger ataverage 10,000 diameters, magnification in the (Figurerange 6B’) morphology50–100 nm of (Figure the geopolymeric 6C’). matrix is rather evident already at 10,000 magnification (Figure 6B’) and, at variance with the MK-based geopolymer for which the particle dimensions are in the range and, at varianceThe limited with reactivitythe MK-based of the geopolymer fly ash particles for which causing the particle the non-completeness dimensions are inof thethe range 20–5020–50geopolymerization nm nm (Figure (Figure6 C),6C), in reactionin this this last lastalso case, case, influences matrix matrix globulesthe globules composition havehave bigger biggerof the averageaveragegeopolymer diameters,diameters, matrix that, in the as range 50–10050–100determined nm nm (Figure (Figure by6 EDS C’).6C’). analysis performed on a homogeneous moiety of the surface, is characterized by TheSi:AlThe limited andlimited Na:Al reactivity ratiosreactivity equal of the toof fly 1.11 ashthe and particlesfly 0.16, ash respectively. causing particles the non-completenesscausing the non-completeness of the geopolymerization of the reactiongeopolymerization also influences reaction the composition also influences of the geopolymerthe composition matrix of that, the asgeopolymer determined matrix by EDS that, analysis as performeddetermined on by a homogeneousEDS analysis performed moiety of theon surface,a homogene is characterizedous moiety of by the Si:Al surface, and Na:Al is characterized ratios equal by to 1.11Si:Al and and 0.16, Na:Al respectively. ratios equal to 1.11 and 0.16, respectively.

Figure 6. Cont.

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FigureFigure 6.6. ScanningScanning electron microscope (SEM) (SEM) micrographs micrographs of of MK-based MK-based geopolymer geopolymer (A (A–C–C) )and and FA-basedFA-based geopolymergeopolymer ((A’A’––C’C’).).

InIn FigureFigure7 7,, the the SEM SEM micrographs micrographs ofof thethe FA-basedFA-based geopolymergeopolymer composite containing containing the the epoxy epoxy resinresin (10%(10% byby weight)weight) areare reportedreported (A’–C’) and co comparedmpared to to the the images images of of the the MK-based MK-based samples samples havinghaving thethe samesame resinresin contentcontent (A–C).(A–C). AsAs farfar asas thethe MK-basedMK-based sample, resin particles are are homogeneously homogeneously dispersed dispersed in in the the geopolymer geopolymer matrix as well-defined microspheres with diameters in the range 1–10 μm. No segregation matrix as well-defined microspheres with diameters in the range 1–10 µm. No segregation phenomena phenomena are observed (Figure 7A,B) and a good adhesion between the organic phase (resin) and are observed (Figure7A,B) and a good adhesion between the organic phase (resin) and the inorganic the inorganic one (geopolymer matrix) is apparent (Figure 7C) [28]. one (geopolymer matrix) is apparent (Figure7C) [28]. In the case of the FA geopolymer composite, particles of organic resin strongly interacting with In the case of the FA geopolymer composite, particles of organic resin strongly interacting with the matrix are still observed (Figure 7C’) but, in respect to the MK-based composite sample, an the matrix are still observed (Figure7C’) but, in respect to the MK-based composite sample, an increase increase of the dimensions and a less homogeneous distribution of the particles is observed, probably of the dimensions and a less homogeneous distribution of the particles is observed, probably due to due to the lower reactivity of FA in respect to MK, that allows the coalescence of the drops of resin the lower reactivity of FA in respect to MK, that allows the coalescence of the drops of resin during during the geopolymerization reaction. In addition, several microspheres of unreacted fly ash the geopolymerization reaction. In addition, several microspheres of unreacted fly ash particles are particles are still present. These particles could act as a reinforcing agent of the matrix, improving the still present. These particles could act as a reinforcing agent of the matrix, improving the mechanical mechanical properties with respect to the neat geopolymer (see Section 3.1.5). In order to help properties with respect to the neat geopolymer (see Section 3.1.5). In order to help distinguish between distinguish between these two types of particles (i.e., the organic resin particles containing carbon these two types of particles (i.e., the organic resin particles containing carbon and the unreacted fly and the unreacted fly ash particles), Figure 7D,D’ shows the EDS maps of the elements carbon (in ashred) particles), and silicon Figure (in blue)7D,D’ for shows the G-MK-Ep10 the EDS maps and of G-FA-Ep10 the elements composites. carbon (in It red)is worth and siliconnoting (inthat, blue) in forthe the case G-MK-Ep10 of G-MK-Ep10, and G-FA-Ep10it is evident composites. that the organic It is worth particles noting are that,scratched in the when case ofthe G-MK-Ep10, samples are it isbroken evident to thatprepare the organicthe SEM particlesspecimens are (Figure scratched 7B,C), when while the the samples unreacted are fly broken ash spheroidal to prepare particles the SEM specimenspreserve their (Figure very7B,C), smooth while surface the unreacted(Figure 7B’). fly ash spheroidal particles preserve their very smooth surface (Figure7B’).

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Figure 7. Scanning electron microscope (SEM) micrographs of MK-based geopolymer composites Figure 7. Scanning electron microscope (SEM) micrographs of MK-based geopolymer composites (G-MK-Ep10; (A–C)) and FA-based geopolymer composites (G-FA-Ep10; (A’–C’)). In (D,D’), the EDS (G-MK-Ep10; (A–C)) and FA-based geopolymer composites (G-FA-Ep10; (A’–C’)). In (D,D’), the EDS maps (at 5000 magnification) of silicon (in blue) and carbon (in red) of two representative regions of maps (at 5000 magnification) of silicon (in blue) and carbon (in red) of two representative regions of the G-MK-Ep10 and the G-FA-Ep10 sample are shown, respectively. the G-MK-Ep10 and the G-FA-Ep10 sample are shown, respectively.

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These experimentalexperimental pieces pieces of of evidence evidence reflect reflect the differentthe different interactions interactions of the resinof the and resin FA particlesand FA particleswith the with geopolymeric the geopolymeric matrix, asmatrix, also shownas also inshown Figure in8 Figure. In particular, 8. In particular, the interaction the interaction of the of resin the resinparticles particles with thewith inorganic the inorganic matrix ismatrix very strongis very due strong to the due network to the of network hydrogen of bonds hydrogen between bonds the betweenphases (Figures the phases7C’ and(Figures8C) [ 7C’28]. and On 8C) the contrary,[28]. On th theree contrary, is a clear there gap is between a clear gap unreacted between FA unreacted particles FAand particles geopolymer and geopolymer matrix (Figure matrix8D). (Figure 8D). Figure 88BB reportsreports thethe EDSEDS analysisanalysis takentaken alongalong aa lineline startingstarting onon aa resin particle, passing throughthrough thethe geopolymer geopolymer matrix matrix and and coming coming to to an an FA FA particle particle (the (the arrow arrow is shown is shown in Figure in Figure 8A8 inA yellow). in yellow). By recordingBy recording the thevariation variation of the of chemical the chemical composition composition of carbon, of carbon, silicon silicon and aluminium and aluminium along along this line, this itline, is apparent it is apparent that the that resin the particle resin particle is characterized is characterized as expected as expected by a high by content a high contentof carbon of and carbon by theand presence by the presence of minor of Si minor and Al Si anddue Alto duethe adhesi to theon adhesion of geopolymer of geopolymer matrix matrixon its onsurface. its surface. Both geopolymericBoth geopolymeric matrix matrix and fly and ash fly particle ash particle instead instead show show a negligible a negligible content content of C ofwhile C while Si and Si andAl are Al present.are present. In particular, In particular, in agreement in agreement with with the the chem chemicalical composition composition (see (see Paragraph Paragraph 2.1), 2.1), FA FA particles particles are characterized by a similar content of Si and and Al, Al, while, while, in in the the geopolymer geopolymer matrix, matrix, the the silicon silicon content content isis higher higher than that of aluminium.

Figure 8. SEM image of the sample G-FA-Ep10: ( A): magnification magnification at at 20,000× 20,000ˆ ofof a a part part of of Figure Figure 66B’B’ thatthat shows, followingfollowing thethe directiondirection of of the the yellow yellow arrow: arrow: (i) (i) the the resin resin particle; particle; (ii) (ii) the the matrix matrix and and (iii) (iii) the thenot-reacted not-reacted ash particle;ash particle; (B) % (B distribution) % distribution of C (redof C), Si(red (blue), Si), Al(blue (green), Al) along(green the) along arrow the obtained arrow obtainedby EDS analyses; by EDS (analyses;C,D) 40,000 (Cˆ,Dmagnification) 40,000× magnification image of the image interface of the zone interface between zone the between geopolymer the geopolymermatrix and a matrix resin particle, and a resin and particle, between and the between matrix and the an matrix ash particle, and an respectively.ash particle, respectively.

3.1.5. Compressive Compressive Strength Test The compressive strengths of FA-based FA-based geopolymer geopolymerss (G-FA) and of the corresponding composites containing, respectively, respectively, 10% and 20% by weight of of epoxy epoxy resin resin (G-FA-Ep10, (G-FA-Ep10, G-FA-Ep20) G-FA-Ep20) are reported inin Figure 99 and and comparedcompared withwith thethe mechanical mechanical performancesperformances ofof thethe compositecomposite specimensspecimens obtainedobtained starting from from metakaolin metakaolin and and containing containing the the sa sameme resin resin content content (G-MK, (G-MK, G-MK-Ep10, G-MK-Ep10, G-MK-Ep20). G-MK-Ep20). As expected on the basis of the limited reactivity of the weathered fly-ash with respect to MK- based geopolymers [14,33], G-FA samples show a lower compressive strength than G-MK.

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As expected on the basis of the limited reactivity of the weathered fly-ash with respect to MK-based Materials 2016, 9, 461 12 of 15 geopolymers [14,33], G-FA samples show a lower compressive strength than G-MK. In particular, byby comparingcomparing thethe values of mechanical strength (Figure(Figure9 9),), it itis is possiblepossible toto observeobserve that, in the case of neat geopolymer samples, G-MK mixture is characterized by a noticeable increase of «≈40%40% of of compressive compressive strength strength in in respect respect to to G-FA G-FA mixture.

Figure 9.9. CompressiveCompressivestrength strength (MPa) (MPa) of of G-FA, G-FA, G-FA-Ep10, G-FA-Ep10, G-FA-Ep20 G-FA-Ep20 (in red(in) red and) G-MK,and G-MK, G-MK-Ep10, G-MK- G-MK-Ep20Ep10, G-MK-Ep20 (in blue (in). blue).

As far as composite samples, for both the matrices, the incorporation of the organic resin in the As far as composite samples, for both the matrices, the incorporation of the organic resin in the neat neat geopolymeric material significantly influences their mechanical properties, as the compressive geopolymeric material significantly influences their mechanical properties, as the compressive strength strength increases with the organic content (compare G-FA-Ep10 and G-FA-Ep20 vs. G-FA and G- increases with the organic content (compare G-FA-Ep10 and G-FA-Ep20 vs. G-FA and G-MK-Ep10 MK-Ep10 and G-MK-Ep20 vs. G-MK) and, in both cases, the best mechanical performance was and G-MK-Ep20 vs. G-MK) and, in both cases, the best mechanical performance was obtained for the obtained for the specimen containing 20% by weight of organic resin. As already demonstrated for specimen containing 20% by weight of organic resin. As already demonstrated for the metakaolin the metakaolin composites, this evident improvement of mechanical properties is probably due to composites, this evident improvement of mechanical properties is probably due to the presence of the the presence of the organic resin that acts as reinforcement, thanks to a crack deviation mechanism organic resin that acts as reinforcement, thanks to a crack deviation mechanism and absorbing part of and absorbing part of the load by plastic deformation [28]. the load by plastic deformation [28]. Moreover, it is worth noting that the differences in the mechanical performances of FA-based Moreover, it is worth noting that the differences in the mechanical performances of FA-based composites and MK-based ones strongly decrease as the organic resin content increases. In particular, composites and MK-based ones strongly decrease as the organic resin content increases. In particular, the difference in compressive strength of composite samples is ≈30% for the samples containing 10% the difference in compressive strength of composite samples is «30% for the samples containing 10% by weight of resin (G-MK-Ep10 and G-FA-Ep10) and is ≈5% only in the case of the samples containing by weight of resin (G-MK-Ep10 and G-FA-Ep10) and is «5% only in the case of the samples containing 20% by weight of resin (G-MK-Ep20 and G-FA-Ep20). 20% by weight of resin (G-MK-Ep20 and G-FA-Ep20). These data allow for concluding that the presence of the resin is more effective when it is used These data allow for concluding that the presence of the resin is more effective when it is used with the fly ash-based geopolymer samples, rather than the metakaolin ones. with the fly ash-based geopolymer samples, rather than the metakaolin ones. This observation supports the idea of a valid use of fly ash in place of more expensive raw This observation supports the idea of a valid use of fly ash in place of more expensive raw materials, such as metakaolin, in particular for those applications for which it is important to save materials, such as metakaolin, in particular for those applications for which it is important to save materials and limit the costs. materials and limit the costs. Finally, preliminary data (not reported) indicate that, if compared to neat G-FA, G-FA composite Finally, preliminary data (not reported) indicate that, if compared to neat G-FA, G-FA composite mixtures show an improved adhesion to common construction supports, minimizing pouring mixtures show an improved adhesion to common construction supports, minimizing pouring phenomena and avoiding aggregate segregation. phenomena and avoiding aggregate segregation. 4. Conclusions Through a co-reticulation reaction of commercial epoxy-based organic resins and a fly ash-based geopolymer, new organic–inorganic composite materials with improved mechanical properties were prepared. This strategy allows the organic phase to chemically interact with the geopolymeric mixture during the geopolymerization process through the formation of a wide network of hydrogen bonding due to the presence of several hydroxyl groups. In such a way, a high compatibility between the

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4. Conclusions Through a co-reticulation reaction of commercial epoxy-based organic resins and a fly ash-based geopolymer, new organic–inorganic composite materials with improved mechanical properties were prepared. This strategy allows the organic phase to chemically interact with the geopolymeric mixture during the geopolymerization process through the formation of a wide network of hydrogen bonding due to the presence of several hydroxyl groups. In such a way, a high compatibility between the organic and inorganic phases, even at appreciable concentration of resin (20% w/w), was realized up to micrometric level and a good and homogeneous dispersion (without the formation of agglomerates) of the organic particles was achieved. These new materials show enhanced mechanical properties in respect to the neat geopolymer. In particular, compressive strength of the new composite material is significantly higher than that of the neat fly-ash based geopolymer, being comparable or even superior to that obtained starting from the more expensive metakaolin. It is worth pointing out that, despite the high concentration of organic resin, similarly to the analogous composites containing melamine based resins [30], preliminary data show that these new materials are not flammable and do not produce smoke in significant amounts. Considering the increasing demand for materials with low environmental impact in today’s construction and housing industry, this paper tries to add new results in the field of sustainable building materials with reduced environmental footprint. In fact, the following conclusions can be drawn:

1. the compressive strength of the composite materials is significantly better than that of the neat geopolymer; 2. fly ash-based composite materials represent a valid alternative in place of more expensive raw materials, such as metakaolin, in particular for those applications for which it is important to save materials and limit the costs; 3. the procedure is inexpensive and uses easily available reagents.

Finally, having successfully replaced the metakaolin with fly ash, we can suggest that the novel composites may have all the conditions to be an Environmentally Friendly Material. In order to confirm this hypothesis, a complete LCA (Life Cycle Assessment) study is in progress.

Acknowledgments: The authors thank Neuvendis S.p.A. (San Vittore Olona, Milan, Italy) for the metakaolin supply and Prochin Italia S.r.l. (Naples, Italy) for the silicate solution supply. Thanks for the technical assistance are due to Giovanni Morieri, Ornella D’Andria and Luciana Cimino. Author Contributions: G.R. and O.T. conceived and designed the experiments; G.R., L.R., V.R. and F.C. performed the experiments; G.R., C.F., F.C. and R.C. analyzed and discussed the data; G.R., L.R., and O.T. and C.F. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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