Fly Ash-Based Geopolymer Binder: a Future Construction Material

minerals

Review Fly Ash-Based Geopolymer Binder: A Future Construction Material

Nakshatra B. Singh Department of Chemistry, Sharda University, Greater Noida-201306, India; [email protected]

Received: 15 May 2018; Accepted: 1 July 2018; Published: 12 July 2018

Abstract: A large amount of waste coming out from industries has posed a great challenge in its disposal and effect on the environment. Particularly fly ash, coming out from thermal power plants, which contains aluminosilicate minerals and creates a lot of environmental problems. In recent years, it has been found that geopolymer may give solutions to waste problems and environmental issues. Geopolymer is an inorganic polymer first introduced by Davidovits. Geopolymer concrete can be considered as an innovative and alternative material to traditional Portland cement concrete. Use of fly ash as a raw material minimizes the waste production of thermal power plants and protects the environment. Geopolymer concretes have high early strength and resistant to an aggressive atmosphere. Methods of preparation and characterization of fly ash-based geopolymers have been presented in this paper. The properties of geopolymer cement/mortar/concrete under different conditions have been highlighted. Fire resistance properties and 3D printing technology have also been discussed.

Keywords: geopolymer; ﬂy ash; compressive strength; microstructure; heat evolution; foam concrete

1. Introduction Development of a country may be directly linked with the development of a cement industry. Portland cement is the most widely produced man-made material on earth used for the manufacture of concrete, the most prevalent building material on the planet. Demand for cement is increasing continuously and in the next decades, cement and concrete production is expected to increase further. The production of concrete is used by some economists as a measure of a country’s economic strength [1,2]. Cement manufacture consumes huge amounts of raw materials and energy. Apart from this, a large amount of solid waste and gaseous emissions particularly CO2 are produced [3]. Cement industry is considered to be one of the greenhouse gas (GHG) emitters [4]; 0.83 kg CO2 is produced per kg cement production [5–7] leading to 8% of total global anthropogenic CO2 emissions. There are a number of drawbacks to Portland cement manufacture. Some of the major drawbacks are: consumption of huge amounts of natural resources and energy, release of greenhouse gases, etc. Apart from these, Portland cement is poor in immobilization of contaminants and the concretes made from it are not durable in corrosive atmospheres. Atmospheric CO2 also reacts with the hydration products and deteriorates the structure. Above demerits of OPC compelled researchers to ﬁnd new binders as an alternative to traditional OPC so that a better, less energy and raw materials consuming with less emissions of greenhouse gases can be obtained. Now, one of the most important new binding materials is geopolymer cement [8–16]. Geopolymers are binding materials different from OPC and made by activating source materials containing silica and alumina such as (ﬂy ash) FA with alkali solutions and Na2SiO3. A three-dimensional amorphous aluminosilicate network is formed due to geopolymerization. Geopolymer binders are becoming important because of utilization of waste materials. One such important source material is FA, which has extensively been used for synthesis of geopolymer concrete

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Minerals 2018, 8, x FOR PEER REVIEW 2 of 21 with bettergeopolymerization. mechanical, chemical,Geopolymer thermal binders and are durability becoming propertiesimportant asbecause compared of utilization to OPC concreteof waste [15]. However,materials. there One are such many important other sourcesource material materials is FA, which which have has potentials extensively to been be used for as synthesis replacement or additionof geopolymer to fly ash concrete in making with geopolymers.better mechanical, Geopolymers chemical, thermal made fromand durability different properties source materials as showcompared different properties.to OPC concrete Functionally [15]. However, graded ther geopolymere are many specimens other source have materials also been which fabricated have by consecutivepotentials pouring to be ofused two as different replacement pastes or withaddition fly ashto fly of differentash in making particle geopolymers. size and Si/Al Geopolymers weight ratios. Thesemade showed from some different interesting source resultsmaterials [17 show]. New differ classesent properties. of geopolymers Functionally known graded as boroaluminosilicate geopolymer specimens have also been fabricated by consecutive pouring of two different pastes with fly ash of geopolymers have also been synthesized from mixtures of fly ash and anhydrous borax [18,19]. different particle size and Si/Al weight ratios. These showed some interesting results [17]. New These have entirely different microstructures. classes of geopolymers known as boroaluminosilicate geopolymers have also been synthesized from Themixtures increasing of fly ash knowledge and anhydrous in understanding borax [18,19]. These the have science entirely of geopolymerization different microstructures. is responsible for improvingThe increasing the properties knowledge of geopolymers in understanding particularly the science fly of ash-based geopolymerization geopolymers. is responsible Because for of its uniqueimproving structure, the geopolymer properties isof ageopolymers better substitute particul forarly OPC fly in ash-based different applications.geopolymers. ABecause literature of its survey revealedunique that structure, geopolymers geopolymer possess is a properties better substitute superior for toOPC OPC in different and because applications. of various A literature advantages, geopolymersurvey revealed concretes that are geopolymers used in a number possess of proper areas.ties In superior this article to variousOPC and aspects because of of fly various ash-based geopolymeradvantages, cement, geopolymer mortars concretes and concrete are used have in a been number discussed. of areas. In this article various aspects of fly ash-based geopolymer cement, mortars and concrete have been discussed. 2. Geopolymer Cement 2. Geopolymer Cement Geopolymer may be treated as one of the most promising and important alternative materials and Geopolymer may be treated as one of the most promising and important alternative materials it was first coined by the French scientist Joseph Davidovits [20]. When inorganic aluminosilicate-based and it was first coined by the French scientist Joseph Davidovits [20]. When inorganic materialsaluminosilicate-based react with alkaline materials solutions, react with polycondensation alkaline solutions, reaction polycondensation forming reaction geopolymers forming occur. Geopolymergeopolymers structure occur. Geopolymer may be divided structure into may three be divided basic into forms three which basic forms depend which on depend Si/Al on ratios. The threeSi/Al basicratios. units,The three poly basic (sialate), units, poly (sialate), (sialate-siloxo) poly (sialate-siloxo) and poly (sialate-disiloxo) and poly (sialate-disiloxo) as reported as by Yun-Mingreported et al. by [Yun-Ming13] are shown et al. in[13] Figure are shown1. in Figure 1.

Figure 1. Structure of polysialates [13] (Reproduced with permission from Progress in Materials Figure 1. Structure of polysialates [13] (Reproduced with permission from Progress in Materials Science; Science; published by Elsevier, 2016). published by Elsevier, 2016). On activating aluminosilicate present in FA by alkalies, an inorganic polymer known as Ongeopolymer activating is formed. aluminosilicate The chemical present structure in is FA written by alkalies,as: an inorganic polymer known as geopolymer is formed. The chemicalM structuren{—(SiO2) isq—AlO written2—} as:n where M is an alkali cation, n the degree of polycondensation, and q is the Si/Al ratio. In general, Mn{—(SiO2)q—AlO2—}n geopolymer is a synthetic inorganic polymer and during the chemical reaction under alkaline conditions, a three-dimensional polymeric chain structure is developed. Composition of raw where M is an alkali cation, n the degree of polycondensation, and q is the Si/Al ratio. In general, materials and the nature and concentrations of alkaline solutions control the microstructures and geopolymer is a synthetic inorganic polymer and during the chemical reaction under alkaline mechanical properties of the geopolymers. conditions, a three-dimensional polymeric chain structure is developed. Composition of raw materials and the nature and concentrations of alkaline solutions control the microstructures and mechanical properties of the geopolymers.

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The significance of geopolymer binders in replacing OPC binders is based on the fact that there are Minerals 2018, 8, x FOR PEER REVIEW 3 of 21 huge amounts of waste materials containing aluminosilicates obtained from industries and agriculture and resultMinerals inThe problems2018 significance, 8, x FOR ofPEER theirof REVIEWgeopolymer disposal binders [21]. in replacing OPC binders is based on the fact that 3there of 21 are huge amounts of waste materials containing aluminosilicates obtained from industries and 3. SynthesisagricultureThe of significance Geopolymer and result of in geopolymer problems Cement of binders their disposal in replac [21].ing OPC binders is based on the fact that there are huge amounts of waste materials containing aluminosilicates obtained from industries and Geopolymersagriculture3. Synthesis and of are Geopolymerresult normally in problems Cement synthesized of their disposal by mixing [21]. source materials having alumino-silicate and the alkaline solutions. Source materials used are kaolinite, clays, zeolite, fly ash, silica fume, slag, Geopolymers are normally synthesized by mixing source materials having alumino-silicate and 3. Synthesis of Geopolymer Cement POFA, rice-huskthe alkaline ash, solutions. red mud, Source etc. materials The most used common are kaolinite, alkaline clays, liquid zeolite, used fly ash, in geopolymerizationsilica fume, slag, is a combinationPOFA,Geopolymers of rice-husk NaOH/KOH ash, are red normally andmud, sodium etc. synthesized The silicate.most bycommon mixing When alkaline source any of materials liquid the above used having in source geopolymerization alumino-silicate materials (for and is a example fly ash (FA)thecombination alkaline in solid solutions.of form) NaOH/KOH areSource mixed andmaterials withsodium used alkali silicate. are solutions kaolinite, When any ofclays, appropriateof zeolite,the above fly concentrationash,source silica materials fume, and slag,(for sodium silicate,POFA,example geopolymers rice-husk fly ash (FA) areash, formed. inred solid mud, form) It etc. has Theare been mixedmost reported common with alkali byalkaline solutions Xiao liquid et al.of usedappropriate [7] that in geopolymerization during concentration the polymerization andis a combination of NaOH/KOH and sodium silicate. When any of the above source materials (for reactionsodium of fly ash,silicate, the geopolymers following processesare formed. may It has take been place reported (Figure by 2Xi).ao et al. [7] that during the examplepolymerization fly ash reaction(FA) in solid of fly form) ash, the are following mixed with processes alkali solutions may take of place appropriate (Figure concentration2). and sodium silicate, geopolymers are formed. It has been reported by Xiao et al. [7] that during the polymerization reaction of fly ash, the following processes may take place (Figure 2).

Figure 2. Processes of geopolymerization from FA [7] (Reproduced with permission from Journal of Figure 2. Processes of geopolymerization from FA [7] (Reproduced with permission from Journal of Cleaner Production; published by Elsevier, 2016). CleanerFigure Production; 2. Processes published of geopolymerization by Elsevier, from 2016). FA [7] (Reproduced with permission from Journal of CleanerSometimes Production; the alkali published activator by Elsevier,in solid 2016).form is milled together with source materials (e.g., FA) Sometimeswith a certain the composition, alkali activator resulting in solid in a fine form grain is milledsimilar to together cement. withThis is source then mixed materials with an (e.g., FA) appropriateSometimes amount the alkali of water activator at the in time solid of form its useis mi [13,22].lled together Diagrammatical with source representation materials (e.g., of FA)the with a certain composition, resulting in a ﬁne grain similar to cement. This is then mixed with an withmethod a certain as reported composition, [13] can resultingbe seen in in Figure a fine 3. grain similar to cement. This is then mixed with an appropriateappropriate amount amount of water of water at the at time the oftime its of use its [13use,22 [13,22].]. Diagrammatical Diagrammatical representation representation ofof thethe method as reportedmethod [13 as] canreported be seen [13] incan Figure be seen3 .in Figure 3.

Figure 3. Geopolymerization by dry mixing [13] (Reproduced with permission from Progress in

Materials Science; published by Elsevier, 2016). MineralsMinerals2018, 82018, 299, 8, x FOR PEER REVIEW 4 of 21 4 of 21

Figure 3. Geopolymerization by dry mixing [13] (Reproduced with permission from Progress in 4. GeopolymerMaterials Concrete Science; published Printing by Elsevier, 2016).

Use4. Geopolymer of group of Concrete techniques Printing in making 3D structures straightway from a digital model is known as additive manufacturing (AM). This may (1) reduce the number of workers resulting into cost Use of group of techniques in making 3D structures straightway from a digital model is known reduction and safety; (2) reduce construction time; (3) reduce the chances of mistakes and (4) increase as additive manufacturing (AM). This may (1) reduce the number of workers resulting into cost architectural freedom [23,24]. Now, 3D printing of concrete is developed more in comparison to reduction and safety; (2) reduce construction time; (3) reduce the chances of mistakes and (4) conventionalincrease methodsarchitectural of castingfreedom concrete [23,24]. intoNow, formwork. 3D printing Layering of concrete processes is developed in geopolymers more in result in an increasecomparison in ﬂexural to conventional strength [methods25]. of casting concrete into formwork. Layering processes in Three-Dgeopolymers printing result processesin an increase increased in flexural compressive strength [25]. strengths of geopolymer mortars made from variety ofThree-D aluminosilicate printing processes materials increased including compressive FA. It strengths was found of geopolymer that there mortars was anisotropy made from in the compressivevariety of strengths aluminosilicate due to materials layered depositionincluding FA. of theIt was geopolymer found that mortarsthere was [26 anisotropy]. in the compressive strengths due to layered deposition of the geopolymer mortars [26]. 5. Reactions during Geopolymerization 5. Reactions during Geopolymerization The process of geopolymerization is a chemical reaction between an alkali solution and source The process of geopolymerization is a chemical reaction between an alkali solution and source materialmaterial containing containing aluminosilicate aluminosilicate (FA)(FA) andand gives a athree-dimensional three-dimensional polymeric polymeric chain chainand ring and ring structurestructure consisting consisting of Si–O–Al–Oof Si–O–Al–O bonds, bonds, asas reported by by Scheme Scheme 1 [27].1[ 27 The]. Thereactions reactions can occur can at occur at roomroom temperature. temperature. Therefore, Therefore, it it can can be be considered considered as as energy energy and and source source efficient efﬁcient and andmuch much cleaner. cleaner.

SchemeScheme 1. 1.Reactions Reactions during geopolymerization geopolymerization [27]. [ 27].

The following steps may be involved during the geopolymerization reaction [28,29]: The following steps may be involved during the geopolymerization reaction [28,29]:  Si and Al atoms present in the fly ash may dissolve by the action of hydroxide ions. • Si andPrecursor Al atoms ions present may be inconverted the ﬂy ashinto maymonomers. dissolve by the action of hydroxide ions. • Precursor Polycondensation ions may be of convertedmonomers into polymeric monomers. structures. • PolycondensationThese three steps of can monomers occur almost into si polymericmultaneously structures. and may also overlap. Reaction (2) indicates that during polymerization, water is eliminated. On the other hand, Theseduring three hydration steps of can OPC, occur water almost is consumed. simultaneously and may also overlap. ReactionFurthermore, (2) indicates due thatto different during sizes polymerization, and charge waterdensities, is eliminated.different alkali On cations the other affect hand, the during hydrationnucleation of OPC, and watergrowth is of consumed. aluminosilicate chains in different ways and, as a result, the rate and the + Furthermore,extent of polymerization due to differentchange differently sizes and [12,30]. charge For example, densities, K differentcation (1.33 alkali Å) having cations a larger affect the size and lower charge density than that of Na+ cation (0.97 Å) leads to a higher degree of nucleation and growth of aluminosilicate chains in different ways and, as a result, the rate and polymerization of geopolymer matrix [12,31]. The exact mechanism of setting and hardening of the the extent of polymerization change differently [12,30]. For example, K+ cation (1.33 Å) having a larger size and lower charge density than that of Na+ cation (0.97 Å) leads to a higher degree of polymerization of geopolymer matrix [12,31]. The exact mechanism of setting and hardening of the geopolymer material is not well understood. However, based on the ﬁndings of Marios et al. [32], tentative mechanism of geopolymerization can be proposed by Scheme2[32]. Minerals 2018, 8, x FOR PEER REVIEW 5 of 21 Minerals 2018, 8, x FOR PEER REVIEW 5 of 21 geopolymer material is not well understood. However, based on the findings of Marios et al. [32], tentativegeopolymer mechanism material of is geop not olymerizationwell understood. can However, be proposed based by on Scheme the findings 2 [32]. of Marios et al. [32], Mineralstentative2018 ,mechanism8, 299 of geopolymerization can be proposed by Scheme 2 [32]. 5 of 21

SchemeScheme 2. 2. Proposed ProposedProposed mechanismmechanism ofof geopolymerization. geopolymerization.

WhenWhen ﬂyfly fly ash ash ash reacts reactsreacts with withwith alkalies alkaliesalkalies forming forming geopolymers, geopolymers,geopolymers, its surface itsits surface getssurface corroded gets gets corroded andcorroded amorphous and and amorphousreactionamorphous products reaction reaction are products formed.products Ryuare are etformed.formed. al. proposed Ryu et aal. model proposedproposed (Figure a a model model4)[ 33 (Figure]. (Figure The reaction 4) 4) [33]. [33]. The products The reaction reaction make products make the structure dense. productsthe structure make dense. the structure dense.

Figure 4. Interaction of fly ash with alkali activator [33] (Reproduced with permission from FigureConstruction 4. InteractionInteraction and Building of of fly fly Materi ash ash als;with with published alkali alkali activato activatorby Elsevier,r [33] [33 2013).] (Reproduced (Reproduced with with permission from Construction and Building Materi Materials;als; published by Elsevier, 2013).2013). 6. The Structure of Geopolymers 6. The Structure of Geopolymers 6. TheIn Structure spite of oflarge Geopolymers number of studies, a definite structural model could not be proposed for geopolymers.InIn spite of Itlarge may be number due to of its amorphous studies, a definidefinitenature.te Smallstructural molecules model (oligomers) could could not not combine be proposed together for geopolymers.forming 3D networkIt It may may be be during due due to geopol its its amorphous ymerization. nature. Spectroscopic Small Small molecules molecules method s(oligomers) (oligomers)have been used combine to propose together formingformingstructure 3D to network polymers. during Theoretical geopol geopolymerization. modelsymerization. have been Spectroscopic Spectroscopic proposed andmethod methods ab initios have have DFT been been calculations used used to propose were structuremade by to Kole polymers. żyński etTheoretical Theoretical al. [14]; the models models proposed have have structural been been proposed model for and geopolymer ab initio DFT is given calculations in Figure were5. madeUsing by solid Kole stateżzyńskiet˙yński MAS et al.NMR, al. [14]; [14 ];a newthe the proposed proposedmodel was structural structural also proposed mo modeldel for for forN–A–S–H geopolymer geopolymer gel [15]. is is given given in in Figure Figure 55.. Using solid state MAS NMR, a new model was also proposed for N–A–S–H gel [[15].15].

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Figure 5. Structural model of geopolymer with 800 (Si:Al = 2.81) [14] (Reproduced with permission Figure 5. Structural model of geopolymer with 800 (Si:Al = 2.81) [14] (Reproduced with permission from Journal of Molecular Structure; published by Elsevier, 2018). from Journal of Molecular Structure; published by Elsevier, 2018). 7. Casting and Curing of Test Specimens 7. Casting and Curing of Test Specimens Although there is no standard method prescribed, geopolymer concrete is made in a similar Althoughway as conventional there is no Portland standard cement method concrete. prescribed, First of geopolymer all, activator concrete solutions is madeas given in ahere similar are way as conventionalprepared. The Portland required cement quantity concrete. of NaOH First pellets of all, is activatordissolved solutionsin water to as have given to herethe required are prepared. The requiredconcentration. quantity For example, of NaOH to pelletsprepare isandissolved 8 M NaOH in solution, water to320 have g of NaOH to the pellets required are dissolved concentration. in one liter of solution since the molecular weight of NaOH is 40. This solution is then mixed with For example, to prepare an 8 M NaOH solution, 320 g of NaOH pellets are dissolved in one liter of the required quantity of Na2SiO3 solution to fix the ratio of alkaline activator solution as 1.5, 2.0 and solution2.5. sinceFly ash, the sand molecular and the weightaggregates of NaOHare first is mixed 40. This together solution in dry is thenstate and mixed then with mixed the with required quantityalkaline of Na liquid2SiO and3 solution a small todose ﬁx of the the ratio super of plasticizer. alkaline activator Extra water solution may be as added 1.5, 2.0 if needed. and 2.5. The Fly ash, sand andfreshthe concrete aggregates was cast are and ﬁrst compacted mixed together in the mold in drys of statestandard and size. then Generally, mixed with heat-curing alkaline (60–90 liquid and a small°C) dose of geopolymer of the super concrete plasticizer. is done.Extra It assists water the maychemical be addedreaction if that needed. occurs The in the fresh geopolymer concrete was cast andpaste. compacted Microwave in heating the molds can al ofso standardbe used for size. curing Generally, [34]. heat-curing (60–90 ◦C) of geopolymer concrete is done. It assists the chemical reaction that occurs in the geopolymer paste. Microwave 8. Factors Affecting Geopolymerization heating can also be used for curing [34]. Some of the important factors which affect the process of geopolymerization are given below. 8. Factors Affecting Geopolymerization I. Type of raw materials containing aluminosilicate SomeII. of theSurface important area of solid factors raw which materials affect the process of geopolymerization are given below. III. Glassy phase content in the raw material I. IV.Type ofAmount raw materials of aluminum containing and reactive aluminosilicate silicon II. SurfaceV. Presence area of of solid iron, raw calcium, materials and inert particles in FA

III. VI.GlassyCuring phase temperature content in the and raw pressure material VII. Duration of curing IV. VIII.Amount Type of aluminumof curing (conventional and reactive heating silicon or microwave heating) V. PresenceIX. Type of and iron, concentration calcium, and of inertalkalies particles in FA VI. CuringX. Alkaline temperature liquid-to-raw and pressure material ratio VII. DurationXI. H2O of to curing Na2O molar ratio XII. Water to Geopolymer solids ratio VIII. Type of curing (conventional heating or microwave heating) XIII. Na2O to SiO2 ratio IX. Type and concentration of alkalies XIV. SiO2 to Al2O3 ratio X. Alkaline liquid-to-raw material ratio

XI.H 2O to Na2O molar ratio

XII. Water to Geopolymer solids ratio

XIII. Na2O to SiO2 ratio XIV. SiO2 to Al2O3 ratio Minerals 2018, 8, 299 7 of 21

Minerals 2018, 8, x FOR PEER REVIEW 7 of 21 In general, geopolymerization increase with curing time and curing temperature (up to 90 ◦C). In general, geopolymerization increase with curing time and curing temperature (up to 90 °C). HigherHigher concentrations concentrations of of alkalies alkalies enhanceenhance thethe geopolymerization,geopolymerization, whereas whereas the the type type of ofalkalies alkalies also also affect the process. With the increase of H O-to-Na O, Water/Geopolymer Solids, Na O/SiO ratios, affect the process. With the increase of H2 2O-to-Na22O, Water/Geopolymer Solids, Na2O/SiO2 2 ratios,2 compressivecompressive strength strength decreases decreases indicated indicated lesserlesser geopolymerization.geopolymerization. TheThe variation variation of of compressive compressive strength strength withwith SiOSiO22/Na2O2O /Al /Al2O23/NaO3/Na2O ratio2O ratio as reported as reported by Ryu by et Ryu et al. [33] [33] is is shown shown in in Figure Figure 6.6 .As As the the ratio ratio increased, increased, the the compressive compressive strength strength decreased. decreased.

FigureFigure 6. 6.Compressive Compressive StrengthStrength vsvs SiOSiO2/Na2/Na2OO and Al 22OO3/Na3/Na2O2 Oratio ratio [33] [33 (Reproduced] (Reproduced with with permissionpermission from from Construction Construction and and Building Building Materials;Materials; published by by Elsevier, Elsevier, 2013). 2013).

Barbosa et al. [35] found that the optimum Na2O/SiO2, H2O/Na2O and SiO2/Al2O3 ratios were Barbosa et al. [35] found that the optimum Na2O/SiO2,H2O/Na2O and SiO2/Al2O3 ratios found to be 0.25, 10.0, and 3.3 for better performance [36]. In order to have an idea about the effect of were found to be 0.25, 10.0, and 3.3 for better performance [36]. In order to have an idea about the different parameters on geopolymerization, setting times were determined. Experiments were effect of different parameters on geopolymerization, setting times were determined. Experiments designed using the Taguchi model [37]. Furthermore, when geopolymers are subjected to high were designed using the Taguchi model [37]. Furthermore, when geopolymers are subjected to temperature (350 °C) and high pressure known as hot pressing the mechanical properties are high temperature (350 ◦C) and high pressure known as hot pressing the mechanical properties are enhanced [38]. enhanced [38]. 9. Characterization of Geopolymers 9. Characterization of Geopolymers There are a large number of techniques which have been used for the characterization of geopolymerThere are cements. a large Some number of the of techniques techniques have which been have discussed been below. used for the characterization of geopolymer cements. Some of the techniques have been discussed below. 9.1. X-ray Diffraction Technique 9.1. X-ray Diffraction Technique XRD patterns of fly ash and geopolymer as recorded by Hanjitsuwan et al. [39] are shown in FigureXRD 7. patterns The fly ofash ﬂy consists ash and of geopolymeran amorphous as phase recorded with by broad Hanjitsuwan hump around et al. 2 [θ39 =] 20–38°. are shown The in ◦ Figureother7. peaks The ﬂy are ash due consists to some of crystalline an amorphous phases phase present. with In broad geopolymer hump aroundthe hump 2 θis= not 20–38 changed.. The other peaks are due to some crystalline phases present. In geopolymer the hump is not changed.

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Figure 7.FigureXRD 7. pattern XRD pattern of ﬂy ashof fly and ash geopolymerand geopolymer [39 ][39] (Reproduced (Reproduced with with permissionpermission from from Cement Cement and Concreteand Composites; Concrete Composites; published published by Elsevier, by Elsevier, 2014). 2014).

9.2. FigureFTIR Spectra 7. XRD of pattern Geopolymers of fly ash and geopolymer [39] (Reproduced with permission from Cement 9.2. FTIR Spectraand Concrete of Geopolymers Composites; published by Elsevier, 2014). FTIR spectra of geopolymer prepared in the presence of different concentrations of NaOH and FTIR9.2.cured spectraFTIR at Spectra60 of°C geopolymeras of reportedGeopolymers by preparedNath et al. [40] in the are presenceshown in Figure of different 8. Bands concentrations at 460 cm−1 and 550 ofNaOH cm−1 and respectively◦ are assigned to in plane and bending vibrations of Al–O/Si–O [39]. Geopolymer−1 with −1 cured at 60 FTIRC as spectra reported of geopolymer by Nath et prepared al. [40] in are the shown presence in of Figure different8. Bands concentrations at 460 cm of NaOHand and 550 cm 8M NaOH showed a much broader peak, may be due to the presence of large amount of structural respectivelycured areat 60 assigned °C as reported to in by plane Nath andet al. bending[40] are shown vibrations in Figure of 8. Al–O/Si–O Bands at 460 [cm39].−1 and Geopolymer 550 cm−1 with water. Appearance of a band at ~1456 cm−1 may be due to C=O vibrations, confirming the presence respectively are assigned to in plane and bending vibrations of Al–O/Si–O [39]. Geopolymer with 8M NaOHof carbonate showed groups a much [40]. broader peak, may be due to the presence of large amount of structural water. Appearance8M NaOH showed of a band a much at ~1456broader cm peak,−1 maymay be be due due to to the C=O presence vibrations, of largeconﬁrming amount of structural the presence of water. Appearance of a band at ~1456 cm−1 may be due to C=O vibrations, confirming the presence carbonate groups [40]. of carbonate groups [40].

Figure 8. FTIR spectra of geopolymer [40] (Reproduced with permission from Construction and Building Materials; published by Elsevier, 2016).

9.3. FigureNMR Spectroscopic 8. FTIR spectra Technique of geopolymer [40] (Reproduced with permission from Construction and Figure 8.BuildingFTIR Materials; spectra published of geopolymer by Elsevier, [40 ]2016). (Reproduced with permission from Construction and BuildingNMR Materials; is a technique published to derive by Elsevier, structural 2016). and compositional features (or changes) of most of the 9.3.compact NMR Spectroscopicor porous Technique(zeolitic) natural and synthetic aluminosilicates [41]. 29Si-NMR allows 9.3. NMR SpectroscopicNMR is a technique Technique to derive structural and compositional features (or changes) of most of the compact or porous (zeolitic) natural and synthetic aluminosilicates [41]. 29Si-NMR allows NMR is a technique to derive structural and compositional features (or changes) of most of the compact or porous (zeolitic) natural and synthetic aluminosilicates [41]. 29Si-NMR allows straightforward determination of the framework Si/Al ratios and the resolution of crystallographically non-equivalent sites. The distribution of Al atoms on the various sites can either be random or MineralsMinerals2018, 82018, 299, 8, x FOR PEER REVIEW 9 of 219 of 21

straightforward determination of the framework Si/Al ratios and the resolution of specific.crystallographically27Al-NMR characterizes non-equivalent Al speciessites. The in distribution different coordination.of Al atoms on the MAS various NMR sites spectroscopy can either is an importantbe random tool or forspecific. the study27Al-NMR of inorganic characterizes solids Al suchspecies as in minerals, different zeolites,coordination. clays, MAS ceramics NMR and cementitiousspectroscopy systems is an with important29Si and tool27 forAl the representing study of inorganic some of solids the most such importantas minerals, nuclei. zeolites, In clays,29Si MAS ceramics and cementitious systems with 29Si and 27Al representing some of the most important NMR, the chemical shift (δ(29Si)) depends on the local environments of the 29Si nucleus. nuclei. In 29Si MAS NMR, the chemical shift (δ(29Si)) depends on the local environments of the 29Si The exploratory investigations on silicates with known molecular structure showed that 29Si nucleus. chemicalThe shift exploratory mainly depends investigations on the on degree silicates of with condensation known molecular of SiO 4structuretetrahedra showed with that increasing 29Si 29 condensationchemical correspondingshift mainly depends to a high-field on the degree shift, i.e.,of condensation more negative of δSiO( 4Si). tetrahedra Furthermore, with increasing it was shown 29 that eachcondensation AlO4 tetrahedron corresponding connected to a high-field to a SiO shift,4 group i.e., more increases negativeδ( δSi)(29Si). by Furthermore, approximately it was 5 ppm. 29 In addition,shown that the each NMR AlO intensity4 tetrahedron is directly connected proportional to a SiO4 group to the increases number δ of(29Si)Si by nuclei approximately present which5 allowsppm. quantitative In addition, determination the NMR intensity of Si is components. directly proportional Thus, 29toSi the MAS number NMR of represents29Si nuclei present a valuable techniquewhich for allows studying quantitative amorphous determination systems, and of inSi thecomponents. area of cementitious Thus, 29Si MAS systems NMR to obtainrepresents structural a and kineticvaluable information. technique for NMR studying was amorphous used for the systems, first time and in in the the 1980s area toof investigatecementitiousthe systems structure to of obtain structural and kinetic information. NMR was used for the first time in the 1980s to investigate metakaolin-based geopolymer by Davidovits. the structure of metakaolin-based geopolymer by Davidovits. 29Si MAS NMR results showed that fraction of aluminum-rich structural units were found more 29Si MAS NMR results showed that fraction of aluminum-rich structural units were found comparedmore tocompared silicon-rich to silicon-rich units with units increasing with increasi reactionng reaction times. Thetimes. increase The increase in strength in strength was duewas to (i) moredue aluminum to (i) more rich aluminum structural rich units; structural (ii) higher units; cross(ii) higher linking cross and linking (iii) higherand (iii) compactness higher compactness [42]. [42]. 9.4. Thermal Methods of Characterization of Geopolymer 9.4. Thermal Methods of Characterization of Geopolymer With heat treatments, shrinkage and weight loss of Na-geopolymers at 1.15 ≤ Si/Al ≤ 2.15 occurred [With40,41 heat]. The treatments, shrinkage shrinkage can be influencedand weight byloss the of Si/AlNa-geopolymers ratio. TGA at of1.15 the ≤ geopolymersSi/Al ≤ 2.15 as reportedoccurred by Nath [40,41]. et al.The [40 shrinkage] are shown can inbe Figureinfluenced9. Loss by the in weightSi/Al ratio. between TGA of 100 the◦ Cgeopolymers and 700 ◦C as could be duereported to structurally by Nath et bonded al. [40] are water shown in in the Figure N–A–S–H 9. Loss gel.in weight This between indirectly 100 gave°C and an 700 idea °C aboutcould the be due to structurally bonded water in the N–A–S–H gel. This indirectly gave an idea about the reaction product. reaction product.

Figure 9. TGA of geopolymer [40] (Reproduced with permission from Construction and Building FigureMaterials; 9. TGA published of geopolymer by Elsevier, [40] 2016). (Reproduced with permission from Construction and Building Materials; published by Elsevier, 2016). 9.5. Heat Evolution during Geopolymerization 9.5. Heat EvolutionSun and Vollpracht during Geopolymerization [43] studied in detail the heat evolution of FA geopolymerization in the Sunpresence and of Vollpracht NaOH (Figures [43] studied10–12). Sample in detail designatio the heatn with evolution different ofparameters FA geopolymerization is given in Table 1. in the presenceEffect of of NaOH NaOH (Figures concentration 10–12 on). Sampleheat evolution designation and cumulative with different heat at parameters 20 °C is given is in given Figure in 10. Table 1. As soon as the activator solution is mixed with fly ash, an exothermic peak due to wetting appeared Effect of NaOH concentration on heat evolution and cumulative heat at 20 ◦C is given in Figure 10. [44]. As soon as the activator solution is mixed with ﬂy ash, an exothermic peak due to wetting appeared [44].

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Minerals 2018, 8, x FOR PEER REVIEW 10 of 21 Minerals 2018, 8, x FOR PEER REVIEW 10 of 21 Table 1. Oxide molar ratio of NaOH activated Fly Ash with SiO2/Al2O3 = 4.6 [43]. Table 1. Oxide molar ratio of NaOH activated Fly Ash with SiO2/Al2O3 = 4.6 [43]. Table 1. Oxide molar ratio of NaOH activated Fly Ash with SiO2/Al2O3 = 4.6 [43]. Sample IDSample SiOID 2/Na SiO2O2/Na2O SiOSiO2/(Na2/(Na2O2O + + CaO) CaO) H H2O2/O/NaNa2O2 OHH2O/(Na2O/(Na2O + CaO)2O + CaO) Sample ID SiO2/Na2O SiO2/(Na2O + CaO) H2O/Na2O H2O/(Na2O + CaO) FA-6-0.45FA-6-0.45 4.34.3 2.82.8 13.1 13.1 8.8 8.8 FA-6-0.45 4.3 2.8 13.1 8.8 FA-9-0.45FA-9-0.45 3.53.5 2.52.5 10.0 10.0 7.1 7.1 FA-12-0.45FA-9-0.45FA-12-0.45 3.23.53.2 2.32.52.3 10.08.8 8.8 7.16.4 6.4 FA-9-0.4FA-12-0.45FA-9-0.4 3.83.23.8 2.62.32.6 8.89.7 9.7 6.46.7 6.7 FA-9-0.5FA-9-0.4FA-9-0.5 3.23.83.2 2.42.62.4 9.710.2 10.2 6.77.4 7.4 FA-9-0.6FA-9-0.5FA-9-0.6 2.83.22.8 2.12.42.1 10.210.6 10.6 7.48.0 8.0 FA-9-0.7FA-9-0.6FA-9-0.7 2.52.82.5 1.92.11.9 10.610.9 10.9 8.08.5 8.5 FA-9-0.7 2.5 1.9 10.9 8.5

FigureFigure 10. Effect 10. ofEffect NaOH of NaOH concentration concentrat onion heat on evolution. heat evolution. (a) Heat (a ﬂow) Heat in mW/gflow in FA,mW/g (b) CumulativeFA, (b) heatFigure inCumulative J/g FA10. [43Effect heat] (Reproduced inof J/gNaOH FA [43] concentrat with (Reproduced permissionion on with heat from permission evolution. Cement from and(a) CementHeat Concrete flow and Research;in Concrete mW/g publishedResearch;FA, (b) by Cumulative heat in J/g FA [43] (Reproduced with permission from Cement and Concrete Research; Elsevier,published 2018). by Elsevier, 2018). published by Elsevier, 2018). Figure 11 gives the effect of liquid/solid ratio on heat of reaction during activation of FA by FigureNaOH.Figure 11 A lowgives11 gives liquid/solid the the effect effect ratio of of liquid/solidretards liquid/solid the process ratio ratio onof on geopolymerizationheat heat of of reacti reactionon during [45]. during activation activation of FA of by FA by NaOH.NaOH. A low A low liquid/solid liquid/solid ratio ratio retards retards thethe process of of geopolymerization geopolymerization [45]. [45 ].

Figure 11. Heat evolution in the presence of varying liquid/solid ratio. (a) Heat flow in mW/g FA, (b) FigureFigureCumulative 11. Heat11. Heat evolution heat evolution in J/g inFA in the the[43]presence presence (Reproduced of of varying varying with permissionliquid/solid liquid/solid fromratio. ratio.Cement (a) Heat (a )and flow Heat Concrete in ﬂow mW/g in Research; FA, mW/g (b) FA; (b) CumulativeCumulativepublished heatbyheat Elsevier, inin J/g FA 2018). FA [43] [43 ](Reproduced (Reproduced with with permission permission from from Cement Cement and andConcrete Concrete Research; Research; publishedpublished by Elsevier, by Elsevier, 2018). 2018). Temperature affects the heat evolution strongly. The increase of curing temperature converts a singleTemperature peak into affectsa number the heatof peaks evolution (Figure strongly. 12). This The indicatesincrease ofthat curing multiple temperature chemical converts steps area Temperature affects the heat evolution strongly. The increase of curing temperature converts a singleinvolved peak during into ageopolymerization. number of peaks (Figure 12). This indicates that multiple chemical steps are singleinvolved peak into during a number geopolymerization. of peaks (Figure 12). This indicates that multiple chemical steps are involved during geopolymerization.

Minerals 2018, 8, 299 11 of 21 Minerals 2018, 8, x FOR PEER REVIEW 11 of 21

Minerals 2018, 8, x FOR PEER REVIEW 11 of 21

FigureFigure 12. Heat 12. Heat evolution evolution at different at different temperatures. temperatures. (a ()a Heat) Heat ﬂow flow in in mW/g mW/g FA, FA, ( (bb)) Cumulative Cumulative heat heat in J/g FAin [ 43J/g] (ReproducedFA [43] (Reproduced with permission with permission from Cement from Cement and Concrete and Concrete Research; Research; published published by Elsevier, by 2018).Elsevier,Figure 12.2018). Heat evolution at different temperatures. (a) Heat flow in mW/g FA, (b) Cumulative heat in J/g FA [43] (Reproduced with permission from Cement and Concrete Research; published by 9.6. SEMElsevier, of Fly 2018). Ash-Based Geopolymer 9.6. SEM of Fly Ash-Based Geopolymer 9.6.Nath SEM etof Flyal. Ash-Based[40] performed Geopolymer detailed microscopic studies. Figure 13a–i shows SEM pictures of Nathgeopolymers. et al. [40 With] performed change in detailed alkali concentrations microscopic and studies. curing Figure temperature, 13a–i shows the microstructures SEM pictures of geopolymers.changedNath [40]. With et al. change [40] performed in alkali detailed concentrations microscopic andstudies. curing Figure temperature, 13a–i shows SEM the pictures microstructures of changedgeopolymers. [40]. With change in alkali concentrations and curing temperature, the microstructures changed [40].

FigureFigure 13. 13. SEM SEM pictures pictures of of FA-basedFA-based geopolymers ( a(a) )6M27; 6M27; ( b()b 6M45;) 6M45; (c )( c6M60;) 6M60; (d )( d8M27;) 8M27; (e) (8M45;e) 8M45; Figure(f 13.)( f8M60;) 8M60;SEM (g ( pictures)g 10M27;) 10M27; (of h(h) )FA-based10M45; 10M45; ((ii)) geopolymers10M60 [40][40] (Reproduced(Reproduced (a) 6M27; with with (b )perm 6M45;permissionission (c )from 6M60; from Construction Construction (d) 8M27; and (e and ) 8M45; (f) 8M60;BuildingBuilding (g) 10M27; Materials; Materials; (h) published 10M45;published ( i byby) 10M60 Elsevier, [40 2016).2016).] (Reproduced with permission from Construction and Building Materials; published by Elsevier, 2016).

Minerals 2018, 8, 299 12 of 21

10. Properties of Geopolymer

10.1. CompressiveMinerals 2018, Strength 8, x FOR PEER REVIEW 12 of 21

Compressive10. Properties strength of Geopolymer of geopolymer mortars and concretes is an important property. The strength change in FA geopolymer concrete is due to shape, size distribution, calcium content and high 10.1. Compressive Strength amorphous microstructure of raw material [46]. The silicon and aluminum components of FA are activated byCompressive alkali solution strength which of polymerizes geopolymer intomortars molecular and concretes chains is and an becomesimportant a property. binder which The binds the coarsestrength and finechange aggregates in FA geopolymer into a homogeneous concrete is due to mass shape, and size attains distribution, optimum calcium strength content at and elevated high amorphous microstructure of raw material [46]. The silicon and aluminum components of FA temperatures. Calcium content of FA contributes very significantly in the strength development of are activated by alkali solution which polymerizes into molecular chains and becomes a binder geopolymerwhich concrete. binds the coarse However, and fine low-calcium aggregates into FA a homogeneous is preferable mass for and high attains binding optimum properties strength since a geopolymerat elevated binder temperatures. depends more Calcium on high content content of FA of aluminosilicatecontributes very mineralssignificantly for in performance. the strength Alkali concentrationsdevelopment also of affect geopolymer the strength concrete. of geopolymerHowever, low-calcium concrete. FA As is thepreferable concentration for high binding of alkali was increased,properties the strength since a also geopolymer increased. binder Blending depends FA, more kaolin on high and content SF in of geopolymer aluminosilicate concrete minerals increase for the performance. Alkali concentrations also affect the strength of geopolymer concrete. As the compressive strength up to a certain limit. With the increase of kaolin, compressive strength decreased. concentration of alkali was increased, the strength also increased. Blending FA, kaolin and SF in Furthermore,geopolymer the additionconcrete increase of slag the increased compressive the streng compressiveth up to a strength. certain limit. This With is asthe aincrease result of closer packingkaolin, of fine compressive particles of strength slag which decreased. fills Furthermore, the pore spaces the addition in the compositeof slag increased mixture. the compressive This resulted in fine surfacestrength. texture This andis as increasea result of incloser compressive packing of fine strength particles [46 of]. slag which fills the pore spaces in the Okoyecomposite et al. mixture. [47] reported This resulted that in strength fine surface increases texture and up increase to certain in compressive temperatures strength but [46]. after that, it decreases (FigureOkoye et 14 al.). [47] reported that strength increases up to certain temperatures but after that, it decreases (Figure 14).

Figure 14. Change in compressive strength of FA-based geopolymer concrete in presence of kaoline Figure 14.at Changedifferent intemperatures compressive [47] strength (Reproduced of FA-based with permission geopolymer from concrete Construction in presence and Building of kaoline at differentMaterials; temperatures published [47 ]by (Reproduced Elsevier, 2015). with permission from Construction and Building Materials; published by Elsevier, 2015). Silica fume addition decreases the compressive strength of the paste whereas in case of mortar, strength is increased. In the case of paste, porosity is increased, whereas in the case of mortars, Silicaporosity fume is addition decreased decreases in the presence thecompressive of silica fume. strengthIn the case of of themortars/concretes, paste whereas in inaddition case of tomortar, strengthreaction is increased. of silica In thefume case with of paste,alkalies, porosity fine particles is increased, also enter whereas into the in pores the case and of increase mortars, the porosity is decreasedcompactness. in the presenceAccording ofto silicaOkoye fume.et al. [48] In in the mort casears and of mortars/concretes, concretes, increase in strength in addition was found to reaction of silicaeven fume up withto 40% alkalies, silica fume ﬁne addition particles (Figure also 15). enter into the pores and increase the compactness. According to Okoye et al. [48] in mortars and concretes, increase in strength was found even up to 40% silica fume addition (Figure 15).

Minerals 2018, 8, 299 13 of 21 Minerals 2018, 8, x FOR PEER REVIEW 13 of 21

Figure 15. Effect of silica fume on compressive strength [48] (Reproduced with permission from Figure 15. CeramicsEffect International; of silica fume published on compressiveby Elsevier, 2016). strength [48] (Reproduced with permission from Ceramics International; published by Elsevier, 2016). Saxena et al. [49] reported that similar to silica fume, Alccofine powder also increased the Saxenacompressive et al. [49strength.] reported Curing that can similar alternatively to silica be fume, done Alccoﬁneby microwave powder heating, also increasedwhere the electromagnetic energy is converted to thermal energy and heat uniformly. This rapidly accelerates compressivethe strength strength. gain Curing [50]. Compressive can alternatively strength be of donegeopolymer by microwave mortars cured heating, by conventional where electromagnetic and energymicrowave is converted heating to thermalare given in energy Table 2 and[34]. The heat data uniformly. show that 30 This min rapidlyheating in accelerates a microwave the oven strength gain [50gives]. Compressive higher strength strength than 12 of h geopolymerof conventional mortars heating curedat 80 °C. by conventional and microwave heating are given in Table2[ 34]. The data show that 30 min heating in a microwave oven gives higher strength than 12 h ofTable conventional 2. Mix design heating with sodium at 80 ◦ C.hydroxide (14 M), sodium silicate and lithium silicate and compressive strength by conventional and microwave heating [34].

Pond Natural NaOH Compr.Str. (MPa) Compr.Str. (MPa) Table 2. Mix design with sodium hydroxideSodium (14 M), sodiumLithium silicate and lithium silicate and compressive MIX Fly Ash Sand Solution Conventional Curing MW Curing (30 Silicate (g) Silicate (g) strength by conventional(g) (g) and microwave(g) heating [34]. 80 °C (12 h) min) Mix-4 200 600 40 80 0 39.0 40.0 Mix-8 Pond200 Natural600 NaOH40 0 80 Compr.Str.32.6 (MPa) Compr.Str.39.0 Sodium Lithium MIXMix-12 Fly Ash200 Sand600 Solution40 80 0 Conventional40.6 (MPa)42.0 MW Silicate (g) Silicate (g) Mix-16 (g)200 (g)600 (g)40 0 80 Curing32.0 80 ◦C (12 h) Curing39.9 (30 min) Mix-4 200 600 40 80 0 39.0 40.0 Mix-810.2. Flexural 200 and Split 600ting Tensile 40 Strength 0 80 32.6 39.0 Mix-12Normally, 200 a fly 600 ash-based 40 geopolymer 80 has low tensile 0 strength. 40.6 It also has low 42.0fracture Mix-16 200 600 40 0 80 32.0 39.9 toughness. Because of these properties, fly ash-based geopolymers suffer from brittle failure. These properties can be enhanced if the geopolymer is embedded with chitosan or fibers of steel (ST), 10.2. Flexuralpolyvinyl and alcohol Splitting (PVA), Tensile sweet Strength sorghum and cotton.

Normally,10.3. Electrical a fly Properties ash-based of Geopolymer geopolymer Pastes has low tensile strength. It also has low fracture toughness. Because of these properties, fly ash-based geopolymers suffer from brittle failure. These properties can Electrical properties of FA-based geopolymer pastes at different NaOH concentrations and be enhancedfrequencies if the were geopolymer measured by is embeddedHanjitsuwan withet al. chitosan[39] and are or show fibersn ofin Figure steel (ST),16. The polyvinyl dielectric alcohol (PVA), sweetconstant sorghum (16a) decreased and cotton. and electrical conductivity increased with frequency (16b).

10.3. Electrical Properties of Geopolymer Pastes Electrical properties of FA-based geopolymer pastes at different NaOH concentrations and frequencies were measured by Hanjitsuwan et al. [39] and are shown in Figure 16. The dielectric constant (16a) decreased and electrical conductivity increased with frequency (16b).

Minerals 2018, 8, 299 14 of 21 MineralsMinerals 2018 2018, ,8 8, ,x x FOR FOR PEER PEER REVIEW REVIEW 1414 of of 21 21

FigureFigureFigure 16. 16.16.( a((a)a)) Dielectric DielectricDielectric constantconstantconstant andand ( (bb))) AC ACAC conductivity conductivityconductivity ((σ (σσacacac)) of)of of FAGPFAGP FAGP [39][39] [39 (Reproduced](Reproduced (Reproduced withwith with permissionpermissionpermission from from from Cement Cement Cement and and and ConcreteConcrete Concrete Composites;Composites; Composites; published publishedpublished by by Elsevier, Elsevier, Elsevier, 2014). 2014). 2014).

10.4.10.4.10.4. Fire FireFire Retardant RetardantRetardant Properties PropertiesProperties GeopolymersGeopolymersGeopolymers have havehave low lowlow thermalthermalthermal conductivity conductivity andand and theythey they dodo do notnot not emitemit emit toxictoxic toxic fumesfumes fumes whenwhen when heated.heated. heated. Geopolymer concretes are one of the most important building materials which can withstand high GeopolymerGeopolymer concretes concretes areare oneone ofof thethe mostmost important building building materials materials which which can can withstand withstand high high temperature. Geopolymer concrete is more porous and favors the escape of internal steam pressure temperature.temperature. Geopolymer Geopolymer concreteconcrete is more porous porous and and favors favors the the escape escape of of internal internal steam steam pressure pressure duringduring heatingheating andand asas aa resultresult becomesbecomes aa goodgood fire-resistantfire-resistant material.material. during heating and as a result becomes a good ﬁre-resistant material. SaxenaSaxena etet al.al. [51][51] measuredmeasured compressivecompressive strestrengthngth ofof FA-basedFA-based geopolymergeopolymer mortarsmortars inin thethe Saxena et al. [51] measured compressive strength of FA-based geopolymer mortars in the absence absenceabsence and and presence presence of of silica silica fume fume at at different different cu curingring temperatures. temperatures. Results Results (Figure (Figure 17) 17) [51] [51] showed showed and presence of silica fume at different curing temperatures. Results (Figure 17)[51] showed that thatthat inin thethe presencepresence andand absenceabsence ofof SFSF inin geopolgeopolymerymer mortars,mortars, thethe compressivecompressive strengthstrength increasedincreased in the presence and absence of SF in geopolymer mortars, the compressive strength increased up to upup toto 600600 °C°C butbut afterafter that,that, therethere waswas aa continuouscontinuous decrease.decrease. 600 ◦C but after that, there was a continuous decrease.

Figure 17. Compressive strength at different temperatures [51] (Reproduced with permission from FigureFigure 17.17. CompressiveCompressive strengthstrength atat differentdifferent temperaturestemperatures [51][51] (Reproduced(Reproduced withwith permissionpermission fromfrom Materials Today: Proceedings; published by Elsevier, 2017). MaterialsMaterials Today: Today: Proceedings; Proceedings; published published by by Elsevier, Elsevier, 2017). 2017).

Minerals 2018, 8, x FOR PEER REVIEW 15 of 21

Minerals10.5. Properties2018, 8, 299 in Presence of Nanomaterials 15 of 21 In the presence of nano-silica, the compressive strength of high volume fly ash mortars at room 10.5.temperature PropertiesMinerals 2018curing in, Presence8, x FOR is PEERreported of REVIEW Nanomaterials to be improved significantly. The nano-silica enhances15 of 21 the polymerization process because of its high specific area and amorphous nature [52]. In10.5. the presenceProperties in of Presence nano-silica, of Nanomaterials the compressive strength of high volume fly ash mortars at roomWhen temperature graphene curing (GR) iswas reported incorporated to be improvedinto the matrix significantly. of fly ash-based The nano-silica geopolymer enhances (FAG), the electroconductiveIn the presence GR/FAG of nano-silica, composite the was compressive obtained. strength Photocatalytic of high volume degradation fly ash mortars of indigo at room carmine polymerizationtemperature process curing because is reported of its to high be specificimproved area significantly. and amorphous The nano-silica nature [ 52enhances]. the over 1.0GR/FAG composite studied by Zhang et al. [53] is shown in Figure 18. Fe2O3 present in FA Whenpolymerization graphene process (GR) wasbecause incorporated of its high specific into area the and matrix amorphous of fly nature ash-based [52]. geopolymer (FAG), and in turn in geopolymer is wrapped inside and 1.0GR/FAG composite in presence of sunlight acts electroconductiveWhen graphene GR/FAG (GR) composite was incorporated was obtained. into the Photocatalyticmatrix of fly ash-based degradation geopolymer of indigo (FAG), carmine as a photocatalyst and degrades the dye. over 1.0GR/FAGelectroconductive composite GR/FAG studied composite by Zhangwas obtained. et al. [Photocatalytic53] is shown degradation in Figure 18 of. indigo Fe2O3 carminepresent in FA and in turnover in1.0GR/FAG geopolymer composite is wrapped studied insideby Zhang and et 1.0GR/FAGal. [53] is shown composite in Figure 18. in presenceFe2O3 present of sunlightin FA acts and in turn in geopolymer is wrapped inside and 1.0GR/FAG composite in presence of sunlight acts as a photocatalyst and degrades the dye. as a photocatalyst and degrades the dye.

Figure 18.Figure Photocatalytic 18. Photocatalytic degradation degradation of of indigo indigo carmincarminee over over GR/FAG GR/FAG composite composite [53] (Reproduced [53] (Reproduced Figure 18. Photocatalytic degradation of indigo carmine over GR/FAG composite [53] (Reproduced with permissionwith permission from fromChem Chemicalical Engineering Engineering Journal; Journal; publishedpublished by by Elsevier, Elsevier, 2018). 2018). with permission from Chemical Engineering Journal; published by Elsevier, 2018). 11. Durability 11. Durability 11. DurabilityDurability of geopolymers (resistance to chloride, sulfate, acid, freeze-thaw, thermal and Durability of geopolymers (resistance to chloride, sulfate, acid, freeze-thaw, thermal and Durabilityefflorescence) of geopolymersdepends on the (resistancemicrostructure to and chloride, the movement sulfate, of ions acid, within freeze-thaw, the structure thermal [54]. and efflorescence)Geopolymer depends made on by theNaOH micr isostructure found to be and mo rethe crystalline movement and of hence ions more within stable, the whereasstructure [54]. efﬂorescence) depends on the microstructure and the movement of ions within the structure [54]. Geopolymergeopolymers made made by NaOHwith the sodiumis found silicate to beactivato morer is crystallineamorphous and and less hence stable inmore acidic stable, medium. whereas Geopolymer made by NaOH is found to be more crystalline and hence more stable, whereas+ geopolymersActive madesites present with onthe th sodiume surface silicate of alumnosilcate activato gelr is are amorphous also responsible and lessfor chemical stable in stability. acidic Kmedium. geopolymersions also made affect with it. If thegeopolymer sodium deteriorates silicate activator in acidic is medium, amorphous it may and be due less to stable depolymerization in acidic medium. Active sites present on the surface of alumnosilcate gel are also responsible for chemical stability. K+ Active sitesand liberation present of on silicic the acid. surface Na+ ofcations alumnosilcate may be replaced gel areby hydrogen also responsible ion as given for below chemical (Figure stability. ions also affect it. If geopolymer deteriorates in acidic medium, it may be due to depolymerization K+ ions19). also affect it. If geopolymer deteriorates in acidic medium, it may be due to depolymerization and liberation of silicic acid. Na+ cations may be replaced by hydrogen ion as given below (Figure and liberation of silicic acid. Na+ cations may be replaced by hydrogen ion as given below (Figure 19). 19).

Figure 19. Replacement of Na+ ions by H+ ions during depolymerization in acidic medium.

When geopolymer is attacked by acids, Si–O–Al bonds are broken and as a result Si–OH and Al–OH groups get increased in the structure. As a result, the amount of silicic acid and dimmers get Figure 19. Replacement of Na+ ions by H+ ions during depolymerization in acidic medium. increasedFigure 19. in theReplacement solution leading of Na +toions mass by loss. H+ Becauseions during of this, depolymerization mechanical properties in acidic are decreased. medium. Acid resistance of geopolymer increases with curing temperature. FA-based geopolymer cured Whenat 80 geopolymer °C for 10 h when is attacked immersed by in acids,HCl solution Si–O–Al did notbonds deteriorate are broken much and[55]. Microwaveas a result cured Si–OH and Al–OHWhen groups geopolymer get increased is attacked in the structure. by acids, As Si–O–Al a result, bonds the amount are broken of silicic and asacid a resultand dimmers Si–OH and get

Al–OHincreased groups in the get solution increased leading in the to structure.mass loss. AsBecause a result, of this, the amount mechanical of silicic properties acid and are dimmers decreased. get increasedAcid inresistance the solution of geopolymer leading to massincreases loss. with Because curing of this,temperature. mechanical FA-based properties geopolymer are decreased. cured at 80Acid °C for resistance 10 h when of geopolymerimmersed in increases HCl solution with curingdid not temperature. deteriorate much FA-based [55]. geopolymerMicrowave cured at 80 ◦C for 10 h when immersed in HCl solution did not deteriorate much [55]. Microwave cured

Minerals 2018, 8, 299 16 of 21

FA-based geopolymer gave enhanced densification comparable to the conventional curing and as a result more durable in acidic environment [56]. Chloride diffusion into the concretes promotes the corrosion of steel bars in the structure. Chloride diffusion can be minimized by using higher concentrations of NaOH. This refines the pore structure due to polycondensation reactions. Geopolymer when exposed to fire has the tendency for shrinkage and cracking. However, fly ash-based geopolymer has much less effect of fire as compared to OPC-based concretes [57]. There occurs expansion, cracking and scaling mass loss due to freeze-thaw attack [58]. In geopolymers excess of hydroxide solution remain in the pores and excess sodium oxide in pore network. This in contact to atmospheric CO2 degrade the geopolymer [59,60]. Efflorescence depends on alkali activator, curing temperature and calcium content. KOH instead of NaOH as activator reduces the efflorescence of geopolymer [61]. It is reported that heat curing with surface sealing can prevent the early carbonation of FA-based geopolymer paste by increasing polycondensation reaction [62].

12. Foamed Geopolymer Concrete Geopolymer foams (GFs) show a large number of properties and are eco-building materials [63]. Foamed geopolymer concretes are formed by adding foaming agents to a geopolymer and have a porous structure. Because of the foaming agents, gases are evolved and entrapped in the structure before the gel hardens. The gel contains different sizes and types of pores [64]. Chemical and mechanical foaming methods are generally used. In chemical methods, density can be reduced but large voids are generated [65]. When foaming agents are added to FA slurry in alkali activator solutions, geopolymer foam is formed. The number of compounds is known which produce gases on addition to geopolymers and are then entrapped to produce a foamed microstructure in the hardened material [66]. When foaming agents are mixed, chemical reactions liberate different gases which are entrapped in the structure. Because it is highly reactive, when aluminum metal is kept in alkaline solution, H2 gas is evolved and aluminum hydroxide is formed. Following reactions (Equations (1)–(3)) occur [67].

2Al + 6H2O + 2NaOH ¡ 2NaAl(OH)4 + 3H2 (1)

2NaAl(OH)4 ¡ NaOH + Al(OH)3 (2)

2Al + 6H2O ¡ 2Al(OH)3 + 3H2 (3) Hydrogen peroxide reacts, evolving oxygen gas, which is entrapped in the paste, giving foamed structure. Equations (4) and (5) represent the reactions [66].

− − H2O2 + OH ¡ HO2 + H2O (4) − − HO2 + H2O2 ¡ H2O + O2 + OH (5) NaOCl also liberates gases as per Equations (6) and (7) and makes foamed geopolymer concrete [68].

NaOCl ¡ NaCl + 1/2 O2 (6)

2NaOCl + C ¡ 2NaCl + CO2 (7) Silica fume when added, during geopolymerization, silicon of silica fume is oxidized by water (Equation (8)) liberating H2 gas [64].

4H2O + Si ¡ 2H2 + Si(OH)4 (8) Minerals 2018, 8, 299 17 of 21

Sodium dodecyl benzene sulfonate and gluten have also been used as foaming agent using ﬂy ash and sheet glass powder as solid materials. The appearance and pore structure of the sintered geopolymer foam studied by Zuhua et al. [64] are shown in Figure 20. Minerals 2018, 8, x FOR PEER REVIEW 17 of 21

Figure 20. (a) Sintered geopolymer foam and (b) SEM image [64] (Reproduced with permission from Figure 20. (a) Sintered geopolymer foam and (b) SEM image [64] (Reproduced with permission from Construction and Building Materials; published by Elsevier, 2014). Construction and Building Materials; published by Elsevier, 2014). 13. Carbon Foot Print in Production and Construction of Portland Cement and Geopolymer 13. CarbonConcretes Foot Print in Production and Construction of Portland Cement and Geopolymer Concretes CO2 emissions from all the processes involved in the manufacture of 1 m3 of geopolymer and Portland cement concrete were evaluated by Turner and Collins [69] and are shown3 in Figure 21. CO2 emissions from all the processes involved in the manufacture of 1 m of geopolymer and The total emissions from the OPC and geopolymer concrete mixes were estimated as 354 kg CO2 Portland cement concrete were evaluated by Turner and Collins [69] and are shown in Figure 21. e/m3and 320 kg CO2 e/m3 respectively, showing 9% difference. The total emissions from the OPC and geopolymer concrete mixes were estimated as 354 kg CO2 3 3 e/m and 320 kg CO2 e/m respectively, showing 9% difference.

Minerals 2018, 8, 299 18 of 21 Minerals 2018, 8, x FOR PEER REVIEW 18 of 21

Figure 21. CO2-e for concrete mixtures with OPC and geopolymer [69] (Reproduced with permission Figure 21. CO2-e for concrete mixtures with OPC and geopolymer [69] (Reproduced with permission from Construction and Building Materials; published by Elsevier, 2013). from Construction and Building Materials; published by Elsevier, 2013). 14. Economic Benefits of Geopolymer Concrete 14. Economic Benefits of Geopolymer Concrete There are a number of economic benefits of Fly ash-based geopolymer concrete over Portland Therecement are concrete. a number An approximate of economic cost benefits of geopolymer of Fly concrete ash-based of one geopolymer cubic meter concrete is reported over to be Portland 45 cementUSD concrete. whereas Anthat approximate for Portland cements cost of geopolymerconcrete, it is concrete60 USD. Additional of one cubic economic meter benefits is reported may to be 45 USDalso whereas be due thatto low for drying Portland shrinkage cements and concrete, creep, excellent it is 60 USD. resistance Additional to sulfate economic attack benefitsand fire may also beresistance due to lowoffered drying by geopolymer shrinkage concrete and creep, when excellent it is used resistance in infrastructure to sulfate applications attack and [46]. fire resistance offered by geopolymer concrete when it is used in infrastructure applications [46]. 15. Conclusions and Future Prospects 15. ConclusionsProduction and of Future geopolymer Prospects cement and concretes using fly ash is a better alternative to conventional OPC concrete because it gives high early strength, is durable, economical and emits Production of geopolymer cement and concretes using fly ash is a better alternative to conventional less carbon. At the same time it minimizes waste generation. The properties of geopolymer cement OPCconcrete depend because on it givesvarious high factors. early strength,However, is the durable, most economicalimportant one and is emits curing less conditions, carbon. At the sametemperature time it minimizes and duration. waste generation.It can be used The as propertiesa fire resistant of geopolymermaterial in the cement construction concrete industry. depend on variousBecause factors. of However,variations theof composition most important of fly one ash, is it curing is difficult conditions, to have temperature standards for and this duration. concrete. It can be usedAttempts as a fire should resistant be made material to have in the solid construction geopolymers industry. which can Because be easily of variations mixed on ofthe composition site of of flyconstruction. ash, it is difficult Further to systems/additives have standards should for this be concrete. found out Attempts which can should give high be strength made to even have at solid geopolymersroom temperature which can curing be easily in a mixedshorter onperiod the siteof ti ofme. construction. Detailed investigations Further systems/additives are required so that should be foundgeopolymer out which can become can give an highalternative strength to Portland even at cement room concrete. temperature curing in a shorter period of time. Detailed investigations are required so that geopolymer can become an alternative to Portland Funding: This research received no external funding. cement concrete. Conflicts of Interest: The author declares no conflict of interest. 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