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Geopolymer as a coating material: A Review

Abstract

A world towards the concept of sustainable development and environment with low greenhouse gas emissions, zero waste and low energy consumption is an important endeavor. Advantages associated with this material include thermal stability, acid resistance, compact structure, low density and ability to encapsulate hazardous wastes.

Although, most of the research is focused in the field of construction industry, there is other utilization of this material. Catalysis, coating, encapsulation of hazardous waste and separation are some of the other applications.

Geopolymer is an aluminosilicate materials created by dissolving it in highly alkaline solution and then transform into tridimensional tecto-aluminosilicate materials. As an inorganic material, geopolymer has a potential in fire resistant and protective coating for different surfaces including metal and concrete due to their superior mechanical, chemical and thermal resistance properties. With an additional engineering design, in curing and sintering temperature, Si:Al ratio as well as additives used will improve the geopolymer coating properties.

The present paper outlines briefly the potential of geopolymer as a coating material to bring the world towards a better future with a reduced carbon footprint.

Keywords: Geopolymer, Fly ash, Met kaolin, Binder, Coating.

Introduction

HOW should we consider geopolymers? Are they a new material, a new binder or a new cement for concrete? Geopolymers are all of these. They are new materials for coatings and adhesives, new binders for fiber composites, waste encapsulation and new cement for concrete. The properties and uses of geopolymers are being explored in many scientific and industrial disciplines: modern inorganic chemistry, physical chemistry, colloid chemistry, mineralogy, geology, and in all types of engineering process technologies. The wide variety of potential applications includes: fire resistant materials, decorative stone artifacts, thermal insulation, low-tech building materials, low energy ceramic tiles, refractory items, thermal shock refractories, bio-technologies (materials for medicinal applications), foundry industry, cements and concretes, composites for infrastructures repair and strengthening, high-tech composites for aircraft interior and automobile, high-tech resin systems, radioactive and toxic waste containment, arts and decoration, cultural heritage, archaeology and history of sciences.

Geopolymers are inorganic, typically ceramic, materials that form long-range, covalently bonded, non-crystalline (amorphous) networks . Obsidian is an example of naturally-occurring geopolymer. Commercially produced geopolymers may be used for fire- and heat-resistant coatings and adhesives, medicinal applications, high-temperature ceramics, new binders for fire-resistant fiber composites, toxic and radioactive waste encapsulation and as cementing components to make concrete. The properties and uses of geopolymers are being explored in many scientific and industrial disciplines: modern inorganic chemistry, physical chemistry, colloid chemistry, mineralogy, geology, and in other types of engineering process technologies. Raw materials used in the synthesis of silicon- based polymers are mainly rock-forming minerals of geological origin, hence the name: geopolymer. Joseph Davidovits coined the term in 1978 and created the nonprofit French scientific institution (Association Loi 1901) Institut Géopolymère (Geopolymer Institute).

The chemistry background had focused on organic polymer chemistry and in the aftermath of various catastrophic fires in France between 1970–72, which involved common organic plastic, research on nonflammable and noncombustible plastic materials became my objective. In 1972, it is found that private research company Cordi SA, later called Cordi- Géopolymère. On the other hand, the aluminosilicate kaolinite reacts with NaOH at 100–150°C and polycondenses into hydrated sodalite (a tectoaluminosilicate, a feldspathoid), or hydroxysodalite.

From the study of the scientific and patent literature covering the Polycondensation of kaolinite Si2O5,Al2(OH)4 in alkali medium Synthesis of zeolites and molecular sieves essentially in the form of powders — it became clear that this geochemistry had so far not been investigated for producing mineral binders and mineral polymers. Then proceeded to develop amorphous to semi-crystalline three-dimensional silico-aluminate materials, which is called in French "géopolymères", geopolymers (mineral polymers resulting from geochemistry or geosynthesis).

The first applications were building products developed in 1973–1976, such as fire- resistant chip-board panels, comprised of a wooden core faced with two geopolymer nano composite coatings, in which the entire panel was manufactured in a one-step process (Davidovits, 1973). We coined it "Siliface Process". An unusual feature was observed to characterize the manufacturing process: for the first time, the hardening of organic material (wood chips and organic resin based on urea-formaldehyde aminoplast) occurred simultaneously with the setting of the mineral silico-aluminate (Na–poly(sialate) / quartz- nanocomposite), when applying the same thermosetting parameters as for organic resin: 150–180°C temperature (Davidovits, 1976).

Classification of geopolymers

According to T.F. Yen geopolymers can be classified into two major groups: pure inorganic geopolymers and organic containing geopolymers, synthetic analogues of naturally occurring macromolecules. In the following presentation, a geopolymer is essentially a mineral chemical compound or mixture of compounds consisting of repeating units, for example silico-oxide (-Si-O-Si-O-), silico-aluminate (-Si-O-Al-O-), ferro-silico- aluminate (-Fe-O-Si-O-Al-O-) or alumino-phosphate (-Al-O-P-O-), created through a process of geopolymerization. This mineral synthesis (geosynthesis) was first presented at an IUPAC symposium in 1976.

Geopolymers are broadly classified as acid activated and alkali activated geopolymers as shown in Fig.2.

Acid activated geopolymers were recently introduced, having properties comparable to alkali activated materials. It is reported that phosphoric acid activated metakaolin produced 30% higher cold crushing strength than their alkali activated counterparts. The higher porosity of this group of polymers suggested their possible application in waste water treatment and as an adsorbent. Acid activated geopolymers have not been explored and further research in this field is required. Alkali activated geopolymer are materials of special interest in the past four decades due to their superior properties compared to OPC. Based on the alumino silicate matrix they are classified as sialate, sialate silaxo and sialate di silaxo.

Each class offers certain specific properties making them suitable for specific applications e.g. sialate di silaxo showed enhance fire and corrosion resistance due to more siloxane bonding compared to sialate. Formation of geopolymers is controlled by the SiO2 / Al2O3 and water/solid ratios. The chemistry of geopolymers is largely varied with the variation in composition and treatment of raw materials.

Raw materials of Geopolymer

Reaction of starting material with alkali activator in the presence of water is termed as geopolymerization. Starting materials are checked for pozzolanic content i.e.SiO2 + Al2O3, according to ASTM C-618. Now, with the advanced mixing technologies, mix designs comprised of pozzolanic and semi pozzolanic waste materials have been reported. Table 1 shows a review of the very recent literature of different types of starting materials used.

Initially, most of the Geopolymer related studies were focused on use of FA and MK as starting material. To address the environmental issues, caused by volcanic, palm oil, rice husk and municipal incinerator ashes etc., these materials have been successfully utilized as supplementary raw materials, along with FA and MK as major component. Utilization of Nano silica and nano alumina (1-10%) resulted into shortening in setting.

Activating solutions

There are different activating systems are used for geopolymerization i.e. phosphoric acid and alkalies. In this section alkali activation system will be discussed in detail. In the first era of geopolymers technology, sodium hydroxide and sodium silicate were employed as activating solutions. These solutions were combined before mixing with raw materials, in an exothermic reaction. With the development and progress in this technology, novel activating solutions and methodologies have been investigated. Table 2 shows different types of alkali activating solutions used for geopolymerization.

New trends in alkali activation includes addition of nano materials, sodium aluminate and alkali activators synthesized from biomass ashes. Compressive strength attained by rice husk ash derived geopolymers is comparable to the standard alkali activated system. Being economical, ecological and less corrosive, it is suggested that future work may be focused on biomass ash derived alkali activators.

Commercial applications of Geopolymer

There exist a wide variety of potential and existing applications. Some of the geopolymer applications are still in development whereas others are already industrialized and commercialized. See the incomplete list provided by the Geopolymer Institute. They are listed in three major categories:

Geopolymer resins and binders

  • Fire-resistant materials, thermal insulation, foams;
  • Low-energy ceramic tiles, refractory items, thermal shock refractories;
  • High-tech resin systems, paints, binders and grouts;
  • Bio-technologies (materials for medicinal applications);
  • Foundry industry (resins), tooling for the manufacture of organic fiber composites;
  • Composites for infrastructures repair and strengthening, fire-resistant and heat- resistant high-tech carbon-fiber composites for aircraft interior and automobile;
  • Radioactive and toxic waste containment;

Geopolymer cements and concretes

  • Low-tech building materials (clay bricks),
  • Low-CO2 cements and concretes;

Arts and archaeology

  • Decorative stone artifacts, arts and decoration;
  • Cultural heritage, archaeology and history of sciences

Geopolymer materials

The class of geopolymer materials is described by Davidovits to comprise.

  • Metakaolin MK-750-based geopolymer binder : Chemical formula (Na,K)-(Si-O-Al- O-Si-O-), ratio Si:Al=2 (range 1.5 to 2.5)

  • Silica-based geopolymer binder : Chemical formula (Na,K)-n(Si-O-)-(Si-O-Al-), ratio Si:Al>20 (range 15 to 40).

  • Sol-gel-based geopolymer binder (synthetic MK-750) : Chemical formula (Na,K)-(Si- O-Al-O-Si-O-), ratio Si:Al=2

The first geopolymer resin was described in a French patent application filed by J. Davidovits in 1979. The American patent, US 4,349,386, was granted on Sept. 14, 1982 with the title Mineral Polymers and methods of making them. It essentially involved the geopolymerization of alkaline soluble silicate [waterglass or (Na,K)-polysiloxonate] with calcined kaolinitic clay (later coined metakaolin MK-750 to highlight the importance of the temperature of calcination, namely 750°C in this case). In 1985, Kenneth MacKenzie and his team from New-Zealand, discovered the Al(V) coordination of calcined kaolinite (MK-750). This had a great input towards a better understanding of its geopolymeric reactivity. Since 1979, a variety of resins, binders and grouts were developed by the chemical industry, worldwide.

Geopolymer synthesis

Covalent bonding

The fundamental unit within a geopolymer structure is a tetrahedral complex consisting of Si or Al coordinated through covalent bonds to four oxygens. The geopolymer framework results from the cross-linking between these tetrahedral, which leads to a 3- dimensional aluminosilicate network, where the negative charge associated with tetrahedral aluminium is balanced by a small cationic species, most commonly an alkali metal cation. These alkali metal cations are often ion-exchangeable, as they are associated with, but only loosely bonded to, the main covalent network, similarly to the non- framework cations present in zeolites

Geopolymerization starts with oligomers

Geopolymerization is the process of combining many small molecules known as oligomers into a covalently bonded network. The geo-chemical syntheses are carried out through oligomers (dimer, trimer, tetramer, pentamer) which are believed to contribute to the formation of the actual structure of the three-dimensional macromolecular framework, either through direct incorporation or through rearrangement via monomeric species. These oligomers are named by some geopolymer chemists as sialates following the scheme developed by Davidovits, although this terminology is not universally accepted within the research community due in part to confusion with the earlier (1952) use of the same word to refer to the salts of the important biomolecule sialic acid. In 2000, T.W. Swaddle and his team proved the existence of soluble isolated alumino-silicate molecules in solution in relatively high concentrations and high pH, at very low temperatures, as low as −9°C. Indeed, it was discovered that the polymerization at room temperature of oligo-sialates was taking place on a time scale of around 100 milliseconds, i.e. 100 to 1000 times faster than the polymerization of ortho-silicate, oligo-siloxo units. At room temperature or higher, the reaction is so fast that it cannot be detected with conventional analytical equipment.

The image shows 5 soluble oligomers of the K-poly(sialate) / poly(sialate-siloxo) species, which are the actual starting units of potassium-based alumino-silicate geopolymerization.

In geopolymer based production of its product, the emission of CO2 is lesser compared to other material production such as the production of geopolymer industrial structure, railway sleepers (Johnson, 2008) and large concrete columns (Hardjito et. al., 2005). As an inorganic polymer it do not preclude the presence of carbon group. Furthermore it does not pose a threat to the environment throughout their life cycle of production, refinement, application and disposal.

Geopolymer is one of the green materials that exhibit many exceptional properties such as high compressive strength, low shrinkage, acid and fire resistant (Temuujin et. al., 2012) plus an ability to immobilize toxic and radioactive materials (Xu et. al., 2005). Mineral polymer is manifest in nature itself in great abundance. 55% of the volume of Earth's crust is composed of sialate-siloxo, Si-O-Al-O-Si-O and sialates, Si-O-Al-O. The process is based on the ability of the aluminium ion, Al3+ to induce the crystallographic and chemical changes in a silica backbone. These mineral polymers (Davidovits, 1976; Davidovits, 1982) which result from geochemistry or geosynthesis (Davidovits 1988) are latter called geopolymer in 1978 (Davidovits, 1979).

Since 1972, Davidovits, J., showed that the naturally occurring alumino-silicate materials with ceramic like properties such as kaolinite are produced and transformed at low temperature into tridimensional tectoaluminosilicates in a very short time. This geochemistry rules the synthesis of zeolites and molecular sieves (Barrer, 1957). The process yields nanocomposites (amorphous to semi-crystalline three-dimensional silicoaluminate materials). Geopolymerization process takes place when reactive aluminosilicate materials with less or no calcium oxide, CaO component (such as metakaolin, fly ash) is rapidly dissolved in highly alkaline solutions, in the presence of alkali hydroxide (sodium/potassium hydroxide, NaOH/KOH) and silicate solution (sodium/potassium silicate, Na2SiO3/K2SiO3) to form free tetrahedral units, SiO4 and AlO4 in the solution. With the development of reaction, water is gradually split out and these tetrahedral units are alternatively linked to polymeric precursors (-SiO4-AlO4-, or –SiO4-AlO4-SiO4-, or –SiO4-AlO4-SiO4-SiO4-) by sharing oxygen atoms forming amorphous geopolymeric products with a three-dimensional network structure (Zhang et. al., 2009). The geopolymer are then cure at room temperature. The cation (potassium ions, K+ or sodium ions, Na+) present in the framework cavities, balance the negative charge (Davidovits, 1994; Duxson et. al., 2007).

There have been several attempts on formation mechanism has been made since the invention of geopolymers. However, Davidovits believed that the synthesis of geopolymer consist of three steps (Li and Ding, 2001). The first step is dissolution of alumino-silicate under strong alkali solution which includes 8 pathways. In thermodynamic, different pathway can create different ion clusters that directly determine the final properties of geopolymers. Thus, it is very important to understand the actual pathway in order to gain insight into the mechanism of geopolymerization process. The second step is reorientation of free ion cluster and followed by polycondensation process. Up until now, these studies are not done yet. This is because the forming rate of geopolymer is very rapid. As a result, these three steps take place almost at the same time which makes the kinetics of these steps inter-dependent.

According to Geopolymer Chemistry and Applications book (Davidovits, 2011), geopolymerization process with metakaolin, MK-750 involves 3 phases. The first phase is alkaline depolymerization of the poly(siloxo) layer of kaolinite. While the second phase involve the formation of the ortho-sialate (OH)3-Si-O-Al-(OH)3 molecule followed by polymerization (polycondensation) into higher oligomers and polymers.

The geopolymer reaction with NaOH or KOH is as follow. First step is the alkalination and formation of tetravalent Al in the side group sialate -Si-O-Al-(OH)3-Na+. Then, the alkaline dissolution starts with the attachment of the base hydroxide, OH- to the silicon atom, which is able to extend its valence sphere to the penta-covalent state. The subsequent course of the reaction can be explained by the cleavage of the siloxane oxygen in Si-O-Si through the transfer of the electron from Si to O, formation of the intermediate silanol, Si-OH on the one hand, and basic siloxo, Si-O on the other hand. Next step involve further formation of Si-OH groups and isolation of the ortho-sialate molecule, the primary unit in geopolymerization. Follow by the reaction of the basic Si-O with the Na+ and formation of Si-ONa terminal bond. Then, the condensation between ortho-sialate molecules, reactive groups Si-ONa and aluminium hydroxyl OH-Al, with production of NaOH, creation of cyclo-tri-sialate structure takes place, whereby the alkali NaOH is liberated and reacts again and further polycondensation into Na-poly(sialate) nepheline framework. In the presence of waterglass (soluble Na-polysiloxonate) one gets condensation between di- siloxonate and ortho-sialate-molecules, reactive groups Si-ONa, Si-OH and aluminium hydroxyl OH-Al-, creation of orthosialate- disiloxo cyclic structure, whereby the alkali NaOH is liberated and reacts again. Finally, further polycondensation into Na-poly(sialate-disiloxo) albite framework with its typical feldspar crankshaft chain structure takes place as shown in the Figure 4. For chemical designation of geopolymers based on silico- aluminates, the term poly(sialate) that is an abbreviation for silicon-oxo-aluminate has been proposed. Poly(sialates) are chain and ring polymers with Si4+ and Al3+ in 4-fold coordination with oxygen and their general formula is

Mn[-(SiO-2)2-AlO2]n.wH2O Where M = monovalent cation such as K+ or Na+ n = degree of polycondensation z = 1,2,3 or >>3

Chains and rings are formed and cross-linked together always through a sialate, Si-O- Al-O bridge. The amorphous to semi-crystalline three dimensional silico-aluminate structures are illustrated in Figure 5.

Geopolymer is form through geochemical processes of geomolecules during genesis can be classified into two major groups which is pure inorganic and organic containing synthetic anologues of naturally-occuring macromolecules. As we know, geopolymer is proposed as pure inorganic materials but it could be extend to include geomaterials with organic content. The ancient Egyptians used straw and riverine mud containing organics (e.g humic materials) to manufacture construction components of remarkable strength and durability. Roman concretes also contained mud as a binding agent. Therefore, it should be note the existence of crosslink between inorganic and organic species during geopolymerization (Kim et. al., 2006; Davidovits, 1993).

The materials of geopolymer are a source material from solid phase and liquid phase. From solid phase the material itself should be high percentage of silicon,Si and aluminium,Al. This material can act as geopolymer precursor if it can dissolve in an alkaline solution and geopolymerize. Metakaolin (Kamseu et. al., 2010) produced by calcinations of kaolin at 7500C and fly ash (Wan Mastura et. al., 2013) which contain 40-60% of SiO2 and 20- 30% of AlO3 is produced by burning pulverized coal are both use for the production of geopolymers. 16 natural Al-Si minerals have been studied by Xu and van Deventer (Xu and van Deventer, 2000) as the potential source for the production of geopolymers.

Geopolymeric material show significant potential for utilization in a number of areas, essentially as a replacement for OPC and also as a relatively inexpensive yet heat resistant ceramic material (Provis et. al., 2006). Other potential applications of geopolymers include stabilization/immobilization of hazardous wastes, surface capping and stabilization of waste dumps, construction of low permeability base liner in landfills, water control structures, and construction of heap leach pads.

In the mining sector, geopolymerisation due to fast setting and high early strength of the paste may be considered in back-fill or cut-and-fill operations (Van Jaarsveld et. al., 2000; Komnitsas et. al., 2007). Geopolymers can even find applications as biomaterials. They have previously been considered for implant applications, where they have shown to be bioactive with low tendency of ion leakage (Jamstorp et. al., 2010; Oudadesse et. al., 2007; Catauro et. al., 2010). Besides, there are a lot of by-products produced by industry today in huge quatities in every country including mining, metallurgical, municipal, construction and demolition that can also be utilize as a feedstock for geopolymers, such as fly ash, blast furnace slag, bauxite residues and mine tailing. As an inorganic polymer based on synthetic aluminosilicate (Davidovits, 1993) materials, geopolymer has a potential in fire resistant and protective coating for different surfaces including metal (Temuujin et. al., 2010) due to their superior mechanical, chemical and thermal resistance properties (Yong et. al., 2007).

While in Centre of Sustainable Resource Processing, CSRP conference, Rickard showed that geopolymers are one possible solution to improve fire protection of an infrastructures and it is more economical since the raw materials for geopolymers used are made from waste product (Rickard and van Riessen, 2008). This unique properties can be applied in coating as it protect the material or object coated as well as used of geopolymer, an inorganic material will lead towards sustainable environment.

There are several types of coatings that have been tried as surface protection materials such as acrylic, polyurethane, epoxy, etc (Rodrigues et. al., 2000; Almusallam et. al., 2003; Medeiros and Helene, 2009) .However, these organic coating usually covers on the material surface by physical absorption that makes it caduceus. For a good coating application, the coating material need to be coat on the substrate by both physical and chemical adsorption or bonding which will improve the material protection (Balaguru et. al., 2008). In order to improve the adhesion of coating material to substrate, the use of an inorganic coating has been proposed to substitute an organic coating. It is well known that geopolymer is an inorganic polymer or alkali activated binders (Duxson et. al., 2007; Davidovits, 1994) has gained worldwide interests and its high anticorrosion makes it a novel coating material. The coating formulation of geopolymer utilizes by-products that would otherwise be disposed of in landfills at a growing cost and liability. Use of the by-product creates the opportunity for carbon offset credits to further enhance profitability.

The geopolymer coating has valuable uses in municipal water systems, industrial applications, transportation projects and petrochemical plants. The coating also offers an innovative, sustainable solution for maintaining infrastructure in a manner that enhances public image and safety while reducing greenhouse emissions.

Geopolymerization process occurred by synthesizing pozzolanic compounds or aluminosilicate source materials such as fly ash, blast furnace slag, bauxite residues, and mine tailing with alkaline activator liquids (sodium or potassium hydroxide) and sodium or potassium silicate. Then the mixture is subsequently mix and cure at a moderate temperature. The geopolymerization process increase when the alkaline activator used consists of soluble silicate; sodium or potassium silicate, rather than alkaline hydroxide alone (Palomo et. al., 1999). Numbers of research have been done in these past years by using geopolymer based such as metakaolin, fly ash and blast furnace slag in coating (Temuujin et. al., 2012; Zhang et. al., 2010; Cheng and Chiu, 2003). In 2003, Cheng & Chiu fabricated a granulated blast furnace slag based geopolymer for fire resistant application. The geopolymer panel made was exposed to a 1100°C flame, with the measured reverse-side temperature reaching less than 350°C after 35 min (Mustafa et. al., 2011). This research show that geopolymer based material can withstand fire at high temperature thus allowing it to be made as coating material purpose by altering its chemical composition in the reaction system.

Metakaolin ( China National Materials Group Corp.) and granulated blast furnace slag ( Zhujiaqiao cement plant, Anhui, China) were used as coating material for concrete structures which exposed to marine environment. These geopolymer used to inhibit the intrusion of corrosive ions (Zhang et. al., 2010). While Temuujin used metakaolin prepared from Snobrite 65 kaolin from Unimin Corp. Australia (Cheng and Chiu, 2003) and fly ash from Collie thermal power station (Western Australia) (Temuujin et. al., 2010) as a geopolymer based coating on metal substrates. The optimized coating formulation obtained show that these geopolymer have very promising fire resistant characteristics. However, the adhesion properties of metakaolin and fly ash based geopolymer to metal substrate depend on its chemical composition.

In 2012, vermiculite which has a high dehydroxylation temperature up to 800°C were added at 10 mass percent in fly ash based geopolymer coating as a fireproofing agent to geopolymer type composition to improve its thermal resistant properties. For comparison, fine and coarse-ground vermiculite powders sample from South Africa provided by Perlite and Vermiculite Factory (Perth, Western Australia) were prepared and added to the coating formulation. The addition of the coarse-ground vermiculite to the fly ash show greater improvement of fire resistant properties than the addition of the fine-ground vermiculite (Temuujin et. al., 2012).

Davidovits suggested that by changing Si:Al ratio, it is possible to regulate the properties of geopolymers . Furthermore, by increasing the Si:Al ratio, the fire and heat resistant characteristics can be improved. The Si:Al composition optimum for geopolymers prepared from metakaolin and fly ash observed by Temuujin were 2.5 and 3.5 respectively. While Cheng mentioned for silica oxide/aluminium oxide, SiO2/Al2O3 ratio in the range of 3.16-3.46, the geopolymer material showed an optimum compressive strength characteristic. However, as the SiO2/Al2O3 ratio increase up to 3.85, the compressive strength decreased (Cheng and Chiu, 2003).

The chemical composition design is the basic key in gaining high compressive strength of geopolymer coating. It includes the NaOH molarity, Na2SiO3/NaOH ratio, geopolymer/alkaline activator ratio and curing temperature. Mustafa et. al. (2011) in his study revealed that a 12M NaOH solution and the combination of geopolymer/alkaline activator with Na2SiO3/NaOH of 2.0 and 2.5 respectively produced the compressive strength as high as 70 MPa (Mustafa et. al., 2012). This indicate that the chemical composition plays and important role in obtaining desirable compressive strength as well as other properties.

There are a several coating method that can be used such as dipping (Temuujin et. al., 2009), spraying (Temuujin et. al., 2010) and also painting (Balaguru et. al., 2008). However, by spraying a strong uniform coating can be applied to a substrate and futher increase the adhesive strength. The ease with which geopolymer can be applied onto substrate surfaces depend on the water content formulation. The water added into a starting mixture is to increase the flow ability. But, it decreased the geopolymerisation reaction and did not give beneficial thermal-resistant properties. For geopolymer coating preparation, it is important not to increase water content instead it may be better to use additives such as plasticizers to increase the flow ability of the coating material.

Another coating method that can be used is extrusion. For example, JFE Steel Corporation company managed to develop a high quality polyethylene, PE or polypropylene, PP coated steel pipe for the transmission of gas, oil, water and other fluids with strong adhesion, excellent impact resistance, excellent bend ability, easy repair and highly resistant to corrosion from moisture and chemicals. JFE external coating pipe is produced by a process of coating with a primer and PE or PP layer for small and medium diameter by extruded method and for large diameter line pipe by the spiral wrapping method. The tensile strength pipes coated with PE reaches 19 MPa and for PP reaches 33 Mpa .

Figures below show an example of pipe coating method by extrusion process (JFE Steel Corporation company). The pipes coating involved two steps: 

First step is external coating (Figure 6, Figure 7) while the second step involve internal coating (Figure 8).

Testings

The testing method use in previous research was setting time test of fresh geopolymer paste by the Vicad method (ASTM C191) (Zhang et. al., 2010). A Vicat needle was used to measure the geopolymer setting time. Both initial setting time and final testing time were tested. The crystalline phases were identified from X-ray diffraction (XRD) patterns obtained with a Bruker D8 Advance Diffractometer slipped with LynxEye detector using Cu-Kα radiation. Diffraction patterns were collected from 10° to 80° 2θ. The step size was 0.02° 2θ with a scan rate of 0.6° 2θ per minute.

Automated phase identification software (EVA 2, Bruker) was used to analyse the diffraction patterns. Geopolymer fracture surfaces were studied with a Zeiss EVO 40XVP scanning electron microscope with EDS X-ray detector (Energy Dispersive Spectroscopy, Oxford Instrument). The adhesive strength of the coated samples was measured with an Elcometer 106, adhesion tester according to ASTM D4541. Australian standard 1530.4 was used for measuring the heat insulating characteristics of the coatings. Using a custom made gas heating rig, the standard time/temperature curve (Eq. 3) was followed as closely as possible.

T = 345 log10 (8t + 1) + 20

Where, T device temperature in °C at time t (min) form ignition of the heating rig.

Thermal expansion or shrinkage of geopolymer samples was measured with a DI-24 Adamel Lhomargy dilatometer (Roissy En Brie, France). In order to make the sample hard enough for cutting with a diamondtipped blade, the cast sample were cured at 70°C over night. Measurement was conducted according to ASTM E831 over a temperature range of 20°C-900°Cwith a heating rate of 5° C/min. The average of the measurements was used as the representative dilatometric curves, and all results lay within a 15% standard deviation. The compressive strength of the geopolymer coating composition was measured by using an Instron-5500R testing machine.

Given the stone-like property of geopolymer, permeability was measured by Darcy method, which is usually used to test the core in mining drill engineering. Sample of 3d aged was laid in pressure cabin and then distilled water was pressed to permeate through. The surrounding pressure was 3 MPa higher than the driven pressure to avoid water going through along the surface of sample. The permeated water over a period of time was recorded till the permeation was steady. Permeability was calculated according to the following formula

k = 105.Q.μ.L/(A. Δp) in which:

k : permeability coefficient (μm2);

Q : velocity of permeated liquid (ml/s);

m : viscosity of permeated liquid (Pas);

L : length of sample, fixed at 3.50 X 10-2m;

A : sample cross-section area, 4.91 X 10-4m2;

Δp : liquid pressure, set at 2 X 107 Pa.

Anticorrosion was mainly evaluated by the variation of compressive strength under three different curing conditions at 20 ± 2°C: air (RH = 90 ± 5%), seawater and dry-wet cycle. Dry-wet cycle was carried out circulary by 12 h air curing and 12 h emerging in seawater. Concentrated Mocleodon artificial seawater ( 3-fold concentration) was prepared as the reinforced corrosion medium. Specimens were tested on WHY-200 Auto Compressive Resistant Tester ( Shanghai Hualong, China) at different curing ages.

The testing formulas for physical property tests such as water adsorption rate and volumetric density were evaluated according to the Archimedes method. Water resistance of the coated samples were tested by a static immersion test (Wang et. al., 2006) which comprises immersion of the coated samples in distilled water at 25°C and weighing after every 24 h after drying at 40°C for 1 h. Mass change versus immersion time was evaluated up to 72 h immersion.

For fire resistant tests, a 10mm thick geopolymer panel was exposed to an 1100°C flame. The reverse-side temperature of the panel was measured and recorded (Davidovits, 1999b). The thermogravimetric analyzer, TGA measurements were performed on thermogravimetric analyzer/differential scanning calorimetry, TGA/DSC1 Mettler Toledo equipment. Approximately, 20 mg of crushed powder was used for heating up to 1,000°C using a heating rate and starting temperature at 10°C/min and 50°C, respectively. Measurements were performed in air.

The relative effects of mechanical bonding and chemical bonding of substrate coated with geopolymer. The substrates rods were embedded in the center of a mortar cylinder that was 38.1 mm tall and 57.2 mm in diameter. The mortar was prepared from a mixture of water, Type I cement, and sand in the proportions 0.485:1.0:2.75, respectively, following the ASTM C109 guidelines. Fresh mortar was poured around the steel rod to fill up the polypropylene container, and each container was tapped and vibrated to remove entrapped air from the mortar prior to consolidation.

Prior to testing, all samples were kept in a 100% humidity environment at room temperature ( 20°C ) and cured up to 97 days. After curing, each cylinder was removed from its container and the bond strengths were determined using the pin-pull testing configuration. The steel rod was pulled out of the mortar at a strain rate of 50mm per minute in displacement control. Both the applied load and the slip between the rod and mortar were monitored with a built-in load cell and a built-in linear variable differential transformer (LVDT).

These testing used to prove and measure the ability of geopolymer materials to be used as a protective coating on a different substrate.

Properties of geopolymer coating

For coating applications, the adhesive force of the geopolymer based coating to substrate is very important achieving the highest value in order to improve anticorrosion, fire and heat resistant, hardness as well as extending the service life of the material. This is believed to be strongly influenced by the formulation of the geopolymer (Temuujin et. al., 2009) the coating formulation is calculate by taking note the content and composition of the amorphous part of the geopolymer used.

Scanning electron microscopy, SEM observations showed that the geopolymer bonding to steel was strong and likely to be physical adhesion. Heating these geopolymer compositions to 1000°C resulted in formation of sodium aluminosilicate or nepheline (Temuujin et. al., 2009). The expansion of this geopolymer to temperature provided a level of matching with the thermal expansion of the substrate resulting in maintenance of the structural integrity of the coating. In order to achieve high adhesion strength between the substrate and coating material, the bonding needs to be chemical adhesion.

Temuujin et, al. (2010) showed that the best adhesive strength observed was for high silica containing composition (Si:Al = 3.5 ratio) which higher than 3.5 MPa and to achieve a greater fire insulating capacity the coating thicknesses should be increase. The substrate used also affect the adhesion strength of geopolymer coating material. The fly ash sample which is coated onto polished mild steel showed an adhesive strength of 2.7 MPa which is 0.5 MPa lower than non-polished mild steel substrate indicating that surface roughness influences adhesion strength. Microstructure evolution and thermal properties of the optimized coating formulation show that they have very promising fire resistant characteristics.

Curing and sintering conditions also have a significant effect on the development mechanical strength in most cementitious systems. Mustafa et, al. (2013) studied on fly ash from Manjung power station in Lumut, Perak to produce as geopolymer coating paste. The geopolymer paste was sintered using temperature range from 600°C to 1500°C showed an increase in strength up to 40 MPa after 7 days.

According to SEM observation, the geopolymer coating at 600°C show sever damaged of fly ash particles while in the high temperature of sintering which is 1500°C shows an interaction between fly ash fine particles with the alkaline liquids. It is suggest that the coating represents a mixture of semi-reacted amorphous glassy phase and fly ash microsphere (Temuujin et. al., 2011). Therefore, geopolymer materials is possible to be use as a coating in high temperature application as it can withstand a maximum temperature of approximately 1600°C (Cheng and Chiu, 2003).

Geopolymer produced with fly ash/alkaline activator ratios in the range of 1.4-2.3 shows high compressive strengths, ranging from 42-52 MPa (Mustafa et. al., 2011). The geopolymer system is exposed to high temperature (800°C) showed an increased in compressive strength up to 80 MPa. The strengths gain within 1-3 days. High compressive strength of 71.04 MPa is achieved for curing temperature of 60°C. While the porosity or permeability is less than 20% which is lower than OPC (Daniel et. al., 2007). The shrinkage of the geopolymer coating material to concrete structure was controlled by using PP fiber and self-prepared magnesium oxide, MgO expansion agent, later of which could produce a shrinkage compensating effect (Zhang et. al., 2010). A previous research found that appropriate addition of PP fiber decreased not only the compressive modulus but also the large shrinkage of geopolymers (Zhang et. al., 2009b).

In general, geopolymers intended for structural application with low Si:Al ratio, shrink with heating. However, the geopolymer-type coatings with high Si:Al ratio, expand with heating (Provis et. al., 2009). In order to apply geopolymer as an innovative coating material, solving the volume shrinkage is the key work.

Geopolymer is a potassium alumina-silicate matrix which is water based and has mechanical properties similar to Portland cemend concrete. Since the particle size of solids in the matrix is less than 0.5µ, the resin has low viscosity and can be applied as avery thin coating. The material had been investigated as a matrix for high strength advanced composites for aerospace , automobile , and infrastructure applications. The following the summery of the properties that makes this matrix suitable for use as a coating for infrastructure.

  • The matrix is water base and has no toxins. The excess materials can be disposed of as ordinary waste and no fumes are generated during mixing , application and curing.
  • It bonds well with concrete, steel, wood and has bond strength about 1.6ksi. Carbon, steel, glass and ceramic fibers can be mixed to improve the mechanical properties.
  • The basic colour is white and hence pigment can be added to obtain any desired color. UV radiation does not degrade the material.
  • Alumino-silicate materials have been used as a bricks for thousands of years with excellent performance.
  • Filler can be added to obtain very hard surface that cannot scratched even with steel. The composition can also withstand up to 1000°C and hence fire is not problem. 6) As mentioned earlier , a good glossy finish can be obtained to provide a graffiti resistant surface.

Conclusion

As conclusion, geopolymer based materials exhibit an excellent properties which can be used as a protective coating material. This material also offers an innovative, sustainable solution for maintaining or extending the service life of an infrastructure at a moderate cost while reducing greenhouse emissions. However, there is lack of research on the geopolymer coating and it has not been popularize yet. It may still need additional engineering properties such as the use of new additives, processing and sintering temperature as well as geopolymer coating material in nano size in order to improve properties of the coating material. On the other hand, the use of geopolymer as an inorganic and green material will lead the world towards sustainable environment with zero waste.

References

  1. Balaguru, P.N., M. Nazier and M. Arafa, 2008. Field Implementation of Geopolymer Coatings; Dept. of 2) Daniel, L.Y., Kong, Jay, G. Sanjayan and Kwesi, Sagoe-Crentsil, 2007. Comparative Performance of Geopolymers Made with Metakaolin and Fly Ash After Exposure to Elevated Temperatures; Cement and Concrete Research, 37: 1583-1589. 3) Davidovits, J., 1999b. Fire Proof Geopolymer Cements; Geopolymer'99 Proceedings. Second Int. Conference, France, pp: 253-267. 4) Davidovits, J., 1993. From Ancient Concrete to Geopolymers : Art Metiers Mag., 180: 8-16. 5) Davidovits, J., 2008. Geopolyemer, Chemistry and Applications; Institute Geopolymer, 3rd printing, Saint- Quentin, France; pp: 585. 6) Davidovits, J., 1993. Geopolymer Cement to Minimize Carbon Dioxide Greenhouse-Warming; Ceram. Trans., 37: 165-182 7) Davidovits, J., 2011. Geopolymer Chemistry and Application, 3rd Edition. 8) Davidovits, J., 1994. Geopolymers: Inorganic Polymeric New Materials : J. Mater. Edu., 16: 91-139. 9) Geopolymer Technology: The Current State of The Art; J. Mater. Sci., 42: 2917-2933 10) Kamseu, E., A. Rizzuti, C. Leonelli and D. Perera, 2010. Enhanced Thermal Stability in K2O-Metakaolinbased Geopolymer Concretes by Al2O3 and SiO2 Fillers Addition: J. Mater. Sci., 45: 1715-1724. 11) Temuujin, J., A. Minjigmaa, W. Rickard and A. van Riessen, 2012. Thermal Properties of Spray-Coated Geopolymer-Type Compositions ;J. Therm. Anal. Calorim., 107: 287-292. 12) Metakaolin Based Geopolymer Coatings on Metal Substrates as Thermal Barriers; Applied Clay Sci., 46: 265- 270.

Author Details

Dashrath Gangaram Salvi

Kansai Nerolac Paints Limited, Mumbai

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