Geopolymerisation: A Sustainable Processing Method for Ceramics and Bricks

Author: Dr Giuliano Tari

www.ceram.com

This work by Ceram is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License

Introduction

Geopolymerisation is an emerging technology in the construction, waste management and ground stabilisation industries. Today, the primary application of geopolymer technology is in the development of reduced CO2 construction materials as an alternative to Portland-based cements (Provis, 2009).

However, geopolymerisation can also be utilised as a processing method in the field of ceramics, offering the potential to improve mechanical properties without the employment of high sintering/firing temperatures. This is, naturally, of great interest to ceramics manufacturers – lowering firing temperatures reduces energy usage and associated costs which, of course, has a positive impact on profitability. A commitment to energy reduction also enhances brand reputation and gives consumers confidence in a business’ sustainability strategy.

Traditionally, ceramic production employs shaping with mixed powders and then firing in kilns with peak temperatures between 1,000-1,250°C. At these temperatures, glass phases or solid state reactions arise to form bonds between the inorganic powder particles. In geopolymerisation, the concept is to form the similar M-O-M bonds (e.g. Si-O-Si or Si-O-Al) between particles by inducing lower temperature (40-180°C) chemical reactions (Figure 1). Locating the sources of silicates that the caustic chemicals driving the chemistry (NaOH or KOH) can readily dissolve is pivotal to successful geopolymerisation.

With firing energy accounting for a significant percentage of ceramic production costs, much interest has been expressed in this technique by a number of sub-sectors, including the traditional high-volume building material and tile producers; reducing firing temperatures in turn reduces energy usage and therefore utility costs. In addition to saving energy, geopolymerisation has the scope to embrace the use of low-cost secondary grade or waste mineral phases. The combination of saving energy and utilising waste improves profitability.

Although geopolymerisation is unlikely to fully match the mechanical properties of conventional ceramics delivered by firing, it does represent a viable option for applications where the requirement for strength, abrasion-resistance, and chemical durability is less demanding. For example, if compressive strength is lacking, bricks may not be a suitable application, whereas roofing tiles (which experience negligible load-bearing) may be.

This white paper reviews selected technologies used in the development of geopolymer systems and examines how, specifically, rheology and material characterisation can improve the final mechanical properties. The paper also discusses how novel shaping routes could possibly play a pivotal role in making geopolymerisation a commercial reality. Examples of bricks and tiles produced by Ceram are also depicted.

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Geopolymerisation Process

The geopolymerisation process is schematically represented in Figure 1 below.

Figure 1. Geopolymerisation process

Geopolymers are formed by partial dissolution of alumino-silicate powders, often characterised by a high degree of amorphous phase and subsequent re-polymerisation. This polymerisation includes reacting into M-OH groups present at the surface of filler particles (Figure 2). The reaction produces an alkali-alumino-silicate gel phase with bonded inclusions of un-reacted solid precursor particles and/or any added fillers.

Figure 2. Condensation reactions involved in forming new M-O-M linkages with expulsion of water

Some of the ingredients used by Ceram in their experiments are shown in Figure 3.

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Figure 3. Geopolymerisation ingredients used by Ceram (NaOH, Na-silicate and fly ash)

Metakaolin and fly ash are the most commonly used alumino-silicate sources in geopolymerisation. Metakaolin is the dehydroxylated form of kaolinite; the elimination of structural water is usually achieved by calcining the kaolin between 600-700°C. Metakaolin usually exhibits a high degree of amorphous phase.

Fly ash is a residue generated from the combustion of coal in power stations. As both the sources of coal and the burning conditions can be very different, the composition of the resulting fly ash can widely fluctuate. In spite of this, all fly ash includes substantial amounts of alumino-silicates (both amorphous and crystalline), iron (III) oxide, unburned carbon, calcium/magnesium oxide and other alkali or amphoteric oxides.

The main variables affecting the physical and mechanical properties of hardened geopolymers, include:

Al/Si ratio: for a given set of experimental parameters, the higher the ratio, the stronger the final material. The resultant networks are rigid and suitable as a concrete, cement or waste encapsulating medium. However, the suspension viscosity tends to be much higher and so more difficult to shape. With low Al/Si ratio, the resultant geopolymer becomes less rigid and more flexible.

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Al/alkali ratio: alkali cations in the geopolymer network are needed to balance the unbalanced charge created by the substitution of Al3+ in the 4-fold co-ordination network. The Na2O+K2O/Al2O3 ratio is generally close to 1. A higher ratio (i.e. a higher concentration of alkali) can lead to the formation of surface salt efflorescence.

A degree of amorphousness associated with the components: the amorphous phase of the raw materials provides the most reactive components for the geopolymerisation process. The crystalline phases have a tendency to act as fillers, therefore enabling reinforcement of the entire system. Thus, the degree of crystalline vs. amorphous of the starting raw materials needs to be optimised for the application.

Particle size and surface area of the feedstock powders: a finer (submicrometric) particle size increases the reactivity and mechanical strength of the final products but usually reduces the viscosity (and so reduces the workability) of the suspension/paste.

Curing temperature (and time): the samples attained a much higher strength in a shorter time (heat accelerates the kinetics of the chemical reactions). In general, the best curing temperature was found to be below 70°C. At higher temperatures, a “muffin top” effect can occur. Furthermore, samples need to be sealed to avoid water loss; otherwise cracks may appear due to drying shrinkage and efflorescence.

Water to solid ratio: a smaller ratio decreases the porosity and increases the mechanical properties, but increases the viscosity and decreases the workability. If the evaporation of the solvent does not occur, the solid loading of the suspension/paste is broadly equal to the density of the artefact.

Ceram’s geopolymerisation studies are focussing on systems featuring metakaolin, chemically treated fly ashes and fly ash as received from the power station (Figure 2).

A large amount of information concerning geopolymerisation experiments is widely available, e.g. Davidovits (2008). However, Ceram is making a breakthrough in the field by optimising the rheology and zeta potential of the suspensions/paste to suit the chosen forming techniques (casting, extrusion, ram pressing of self-supporting products, pressure casting).

For a given set of experimental conditions, the water to solids ratio represents a key parameter in dictating the ultimate mechanical properties and microstructure. Conversely, the high viscosity of the suspension/paste is the limiting factor for using very high solids loading. Through an understanding of the behaviour of inorganic power in aqueous systems (via for example zeta potential and the impact of surfactants), Ceram is able to prepare highly concentrated suspensions whilst keeping the viscosity low enough to be poured, extruded or pressed; proving it is therefore possible that novel shaping routes (such as the Viscous Plastic Processing (VPP) technique) could have a vital role to play in making geopolymerisation a commercial reality.

Examples of tiles and bricks produced by Ceram are illustrated in Figures 4 and 5, while Table 1 shows the range of properties achieved.

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Figure 4. A red tile (10 x 10cm) produced with metakaolin, a black tile (10 x 10cm) obtained with fly ash as received from the power station and a red/black tile (10 x 10cm) showing the excellent bonding between the metakaolin and fly ask layers

Figure 5. A brick (16 x 8 x 3cm) produced with fly ash as received from the coal burning power station

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Table 1. Overview of the range of physical and mechanical properties of the samples produced at Ceram by geopolymerisation

Properties Range of values achieved at Ceram

Water absorption (a measure of open porosity)

15-22wt% (vacuum test)

Modulus of rupture (3 points) 5-10 N/mm2

Compressive strength 10-25 N/mm2

Summary

Geopolymerisation is attracting considerable attention in the construction industry due to the pressure in sourcing and using building materials with low environmental impact (low embedded energy and resource efficient).

The main features of geopolymers are:

Ceramic-like, inorganic polymers, produced at low temperatures and capable of bonding in a matrix to aggregate

Hardened by setting through the geopolymerisation process, not firing

Negligible shrinkage (near-net shape forming)

A micro-porosity or nano-porosity that can be tailored

Fire resistance

The scientific community recognises there are still knowledge gaps that are hindering this promising technology from becoming widely accepted by industry. However, it is believed that geopolymerisation can make a significant contribution to sustainable development in the construction sector. Ceram is currently collaborating with mineral suppliers and manufacturers to support their quest to deliver commercially viable and fit for purpose products created through geopolymerisation.

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References

1. J L Provis and J S J van Deventer (2009). Geopolymers: Structures, processing, properties and industrial applications. Woodhead Publishing.

2. J Davidovits (2008). Geopolymer Chemistry and Applications (2nd ed.). Saint-Quentin, FR: Geopolymer Institute.

Acknowledgement

Samples produced by Ms Amanda Bartkowiak during her placement at Ceram.

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About Ceram

Ceram is an independent expert in innovation, sustainability and quality assurance of materials.

With a long history in the ceramics industry, Ceram has diversified into other materials and other markets including aerospace and defence, medical and healthcare, minerals, electronics and energy and environment.

Partnership is central to how we do business; we work with our clients to understand their needs so that we can help them overcome materials challenges, develop new products, processes and technologies and gain real, tangible results.

Headquartered in Staffordshire, UK, Ceram has approved laboratories around the world.

About the Author

Dr. Giuliano TariOperations Manager

Giuliano holds PhD in Materials Engineering (Aveiro University, Portugal), an M.Eng in Chemical Engineering (Bologna University), and an MBA (Open University).

Giuliano has over 12 year’s sales, operations and financial management experience of high-tech B2B consultancy projects. In addition, he is the technical consultant for rheology, flocculation, sedimentation, and other suspension related development and troubleshooting projects. Giuliano’s service and process knowledge spans across clay-based and technical/advanced ceramics, metal powders, and industrial and retail products. Giuliano has also been Project Manager on several European and DTI/TSB projects.

During his career, Giuliano has been the author or co-author of over 40 papers and reviewer for the J. Am. Ceramic Society and J. Colloid Interface Science. Finally, Giuliano is involved with BSI on the standards committee NTI1 - Nanotechnologies.

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