Innovative Fly Ash Geopolymer-Epoxy Composites: Preparation, Microstructure and Mechanical Properties
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materials Article Innovative Fly Ash 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 metakaolin. 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 silicate 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 Cement (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 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 macromolecule 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 polymers 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 polymer 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,