inorganic polymers for environmental protection applications

Inorganic polymers for environmental protection applications

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2011 IOP Conf. Ser.: Mater. Sci. Eng. 18 172001

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Inorganic polymers for environmental protection applications

KJD MacKenzie

MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600 Wellington, New Zealand

[email protected]

Abstract. Aluminosilicate inorganic polymers have been proposed as low-energy cements since, unlike Portland cement, their production does not require high temperatures or generate large quantities of greenhouse gases. Other environmental protection applications for inorganic polymers are to encapsulate hazardous mining or radioactive wastes for safe long-term storage and as fireproof components for buildings and vehicles. However, newly developed methods for synthesising these materials have opened up the possibility of other novel environmental protection applications. These include porous cladding material for passive cooling of buildings, cost-effective exchange materials for removing heavy metals from wastewater, bacteriocidal materials for purifying polluted drinking water and materials for photodegrading hazardous organic environmental pollutants. The nature and synthesis of inorganic polymers for these environmental applications will be discussed here.

1. Introduction Aluminosilicate inorganic polymers are characteristically X-ray amorphous gel-like materials containing tetrahedal AlO4 and SiO4 units assembled randomly [1,2] with alkali ions providing charge compensation [3]. Conventional synthesis is by reaction of a solid aluminosilicate such as metakaolinite (a kaolinite clay thermally dehydrated at 600-700oC) with alkali silicate solution under highly alkaline conditions [1]. A defining characteristic of these materials is that they set and harden at or just above ambient temperature. Alkali-activated clays have been used in building materials since the early 1950s in the Ukraine; these consisted of kaolinite clay treated with NaOH and autoclaved at 170oC to form a product containing 30-50% hydroxysodalite zeolite giving it a compressive strength of 30-40 MPa [4]. Since dehydrated kaolinite (metakaolinite) is not cheap, other solid waste aluminosilicate sources been successfully used to produce viable alkali-activated products. These include flyash (the inorganic microspherical combustion product from coal-fired boilers) [5] or ground granulated blast furnace slag (GGBS) [6].

In another synthetic approach relevant to some of the more novel applications to be discussed here, the charge-balancing Na+ cations in a conventional inorganic polymer can be exchanged for other cations [4]. This procedure is capable of achieving essentially 100% exchange of Na+ by K+, NH4

+, Ag+ and Pb2+ but less complete exchange by other cations [7].

This paper discusses the three principal current environmental applications for these materials (as a substitute for Portland cement, fireproof components for buildings and vehicles and the remediation of

ICC3: Symposium 11: Advanced Ceramics Surface for Environmental Purification IOP PublishingIOP Conf. Series: Materials Science and Engineering 18 (2011) 172001 doi:10.1088/1757-899X/18/17/172001

c© 2011 Ceramic Society of Japan. Published under licence by IOP Publishing Ltd1

hazardous wastes) and introduces some possible future applications made possible by the new synthetic techniques.

2. Current environmental applications for these materials

2.1. As a substitute for Portland cement The manufacture of Portland cement involves the high-temperature calcination of CaCO3 (limestone) using fossil fuels, producing almost 1 tonne of CO2 per tonne of product [8]. Since the world production of Portland cement in 2008 was 2.86 billion tones, this process is recognized as one of today’s greatest single contributors to the atmospheric CO2 burden, second only to power generation [9]. Since geopolymer cements require much less energy than Portland cement to produce, and do not involve the generation of CO2, this application has attracted the most commercial interest and research funding from cement manufacturers. It is beyond the scope of the present paper to review the extensive literature in this area, so only a few typical papers will be cited. Conventional metakaolinite-based inorganic polymer cements have not proved to be commercially attractive, since although when properly made and cured they can achieve compressive strengths of up to 15 MPa [10], the cost of metakaolinite makes it an uneconomic starting material for this application. The two cheaper alternative raw materials for alkali-activated cements (flyash and granulated ground blast furnace slag, GGBS) are waste products of high-temperature processes, so, unlike kaolinite, do not require further heating. Depending on the type of coal, flyash is classified into two types, Class F with 3-10% CaO content and Class C, with 15-39% CaO content. Reaction of Class F flyash with sodium silicate and NaOH solution produces a cement with compressive strength >100 MPa (Fig. 1A), good freeze-thaw and acid resistance and low chloride permeability [5]. A porous version of this material for insulation purposes can be produced by the addition of aluminium powder [5] which provides pore generating gas by reaction with the alkali (Fig. 1B). This material cures at 60oC to achieve densities of 800-1800 kg.m-3 and strengths of 1-20 MPa [5]. A B

Figure 1. A. Compressive strength of geopolymer cement from Class F flyash cured at different temperatures, compared with conventional Type III high-performance cement (HPC). B. Porous flyash geopolymer cement. Adapted from reference [5] Cements produced from Class C flyash suffer from the drawback of setting too rapidly (flash setting), but the setting time can be retarded to a useful workable time of 60-100 minutes by the addition of boron, in the form of borax or lithium borate (Fig. 2) [10]. The compressive strength of the resulting products (30-80 MPa) is dependent on the amount of added borate, being optimal at 4 wt% of borax, and the boron has been shown by 11B solid-state nuclear magnetic resonance spectroscopy to be located in the tetrahedral network sites [10].


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Figure 2. Effect of borate additions on setting time of a geopolymer prepared from Class C flyash. From reference [10].

Figure 3. Comparison of the compressive strengths of geopolymer cements prepared from metakaolinite, flyash and GGBS by reaction with 10% NaOH. Adapted from reference [11].

Alkali activation of GGBS or mixtures of GGBS and flyash produces strong cements with relatively higher compressive strengths than flyash or metakaolin-based materials [11] (Fig. 3). The strength of these GGBS cements depends on factors such as the concentration of the activating NaOH solution [6], and differences in their chemical composition make then faster setting than cements from Class F flyash [6].

2.2. As fireproof components for buildings and vehicles In a fire, Portland cement becomes mechanically weakened >350oC, and organic insulating materials are extremely flammable. Replacement of both Portland cement components and organic components of buildings and vehicles by inorganic polymers can provide much improved protection from fire. For example, a 10mm thick panel prepared from a mixture of metakaolin, GGBS, KOH and sodium silicate solution, had a compressive strength of 79 MPa [12]. When one face of this panel was exposed to a temperature of 1100oC, the opposite face never exceeded 250oC [12] (Fig. 4), illustrating the potential of this type of material for fire protection in buildings. The need fireproofing of vehicles is illustrated by the complete destruction of resin-bodied cars following quite a minor accident accompanied by a fire. Moulded heat shield panels of inorganic polymers reinforced with carbon fibre have been used in the exhaust areas of specialised vehicles such as Formula 1 racing cars, and can withstand 700oC temperatures for >2-3 hours [13]. However, up till the present, the use of such materials in these applications has been limited to specialised vehicles and has not found more general application.

2.3. As encapsulation media for storage and disposal of hazardous wastes Inorganic polymers based on metakaolin have been shown [13] to be capable of safely containing uranium mining waste, significantly reducing the radioactivity of the acetic acid leachate (pH 5) by locking the wastes into the three-dimensional framework (Fig. 5). Encapsulation of heavy metal-containing mining wastes by metakaolin-based inorganic polymers has been shown to be effectively achieved by a similar mechanism, such that leaching under acid environments will remove almost none of the heavy metals, allowing safe storage of the composite [13].


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Figure 4. Temperature profile on the opposite face of a 10mm inorganic polymer panel exposed to 1100oC flame. Adapted from ref. [12]

Figure 5. 226Ra radioactivity in uranium mining waste, leachate and inorganic polymer encapsulated leachate. From ref. [13]

3. Potential new environmental applications for these materials

3.1. Porous materials for passive cooling of buildings Conventionally-synthesised metakaolin-based inorganic polymers with continuous aligned pores have been produced by extruding the uncured material containing Nylon 66 fibres as the pore formers which are then removed after hardening by gently heating (Fig. 6) [14]. These materials can display a capillary lift of water greater than 1 metre, and if used as cladding for buildings could cool the building by the latent heat of evaporation. This application could be especially important in large cities to counteract their Heat Island effect.

Figure 6. Micrographs of inorganic polymer with aligned pores before curing (upper) and after removal of pore formers (lower). From ref. 14.

Figure 7. Agar plate inoculated with Staph. Aureus. Dark halo around the Ag+ inorganic polymer (left) indicates dead bacteria but not around the Na+ control sample (right). From ref. [7].

3.2. Bacteriocidal filter bed material for water purification


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Total exchange of the Na+ charge-balancing ions for Ag+ in a conventionally synthesised inorganic polymer produces a material with strong antimicrobial properties [7], as demonstrated by a standard test using Staphylococcus Aureus incubated on an agar plate at 37oC for 18hr then killed by autoclaving (Fig. 7). The dark halo around the Ag-exchanged compound indicates extensive destruction of the bacteria, which cannot be attributed to the alkalinity of the samples, since the Na+ precursor has no effect on the bacteria (Fig. 7). This result suggests a potential application as a filter bed material for purification of drinking water.

3.3. Materials for removal of heavy metals from wastewater streams The observation that a conventionally synthesised Na-inorganic polymer can exchange 100% of its charge-balancing Na+ for Pb2+ and 72% for Cd2+ [7] suggests an application as a filter bed material for waste water streams but further research on the effect of temperature and pH on the kinetics of uptake will be required. One limitation to this application is the unfortunate inability of these materials to exchange one of the more serious pollutants, Hg2+ [7].

3.4. Materials for photodegradation of organic pollutants Maghemite, -Fe2O3, has been shown [15] to participate with oxalate ions under UV radiation in a Fenton-like reaction for the photodegradation of harmful organic species in water. The reaction occurs by the formation of hydroxyl radicals via Fe(III)(C2O4)

2- + •(C2O4)- under the action of UV light. The

necessary combination of maghemite, oxalate ions and UV light has been shown by experiments on the photobleaching of methylene blue (Fig. 8), a model compound for a number of organic pollutants. Since the practical application of this reaction to filter bed technology may require the active oxide component to be held in particulate form, a convenient matrix may be an inorganic polymer which would however need to be stable at lower pH. Materials derived from combinations of Class F flyash and GGBS typically possess good acid resistance, and may be suitable for this purpose.

Figure 8. Photobleaching of methylene blue dye by maghemite ( -Fe2O3) in conjunction with 1 mM oxalate under UV illumination. Adapted from ref. 15.

4. Conclusions Inorganic polymers can be synthesised from a number of materials including wastes such as flyash

and blast furnace slag, and possess a range of properties that make them suitable for environmental protection applications. The most thoroughly investigated application to date is their formation of low-energy alkali-activated cements with the potential to reduce greenhouse gas emissions by substituting for Portland cement. However, there are still a number of regulatory hurdles to be overcome before these materials are fully accepted as Portland cement substitutes.

Another application presently under active consideration or in limited use for environmental protection exploits the fireproof properties of these materials for doors in buildings or panels in specialized vehicles. This technology has the potential to be more widely taken up than at present. Similarly, the use of inorganic polymers to immobilize and dispose of hazardous radioactive waste or


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mining wastes has been demonstrated, but the widespread use of this technology has not yet eventuated.

A number of possible future environmental protection applications for inorganic polymers are being active researched. These include the development of materials containing aligned nanopores for evaporative cooling of buildings, silver-containing antimicrobial compounds for water purification filter beds, ion exchange materials for the removal of heavy metal ions from waste water streams, and iron oxide-based materials for photodegradation of hazardous organic compounds. Undoubtedly in the future, further innovative uses will emerge for these interesting and important inorganic materials.

References [1] Davidovits J 1991 J. Thermal Anal. 37 1633 [2] Barbosa VMM, MacKenzie KJD and Thurmaturgo C 2000 Int. J. Inorg. Mater. 2 309 [3] Rowles MR, Hanna JV, Pike KJ, Smith ME and O’Connor BH 2007 Appl. Magn. Reson. 32 663 [4] Berg LG, Demidenko BA, Remiznikova VA and Nizamov MS 1970 Stroit. Mater. 10 22 [5] Brooks R, Bahadory M, Tovia F and Rostami H 2010 Int. J. Sustainable Eng. 3 211 [6] Ravikumar D, Peethamparan S and Neithalath N 2010 Cement Concr. Composites 32 399 [7] O’Connor SC and MacKenzie KJD 2010 J. Mater. Chem. DOI:10.1039/C0JM01254H [8] The cement sustainability initiative progress report 2002, World Business Council for Sustainable Development. [9] Mahasenan N, Smith S, Humphreys K and Kaya T 2003 Proc. 6th Int. Conf. Greenhouse Gas Control Technol. 995 [10] Nicholson CL, Murray BJ, Fletcher RA, Brew DRM, MacKenzie KJD and Schmucker M 2005 Proc. World Geopolymer Conf. Paris 31 [11] Buchwald A, Kaps C and Hohmann M 2003 Proc. 11th Int. Congr. Chem. Cement, Durban, 1238 [12] Cheng TW and Chiu JP 2003 Miner. Eng. 16 205 [13] Davidovits J 1994 Concr. Int. 16 53 [14] Okada K, Ooyama A, T. Isobe T, Kameshima Y, Nakajima A and MacKenzie KJD 2009 J. Eur. Ceram. Soc. 29 1917 [15] Gulshan F, Yanagida S, Kameshima Y, Isobe T, Nakajima A and Okada K 2010 Water Res. 44 2876


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inorganic polymers for environmental protection applications

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