Les Vickers

In Situ Elevated Temperature Testing of Fly Ash Based Geopolymer Composites

In situ elevated temperature investigations using fly ash based geopolymers filled with alumina aggregate were undertaken. Compressive strength and short term creep tests were carried out to determine the onset temperature of viscous flow. Fire testing using the standard cellulose curve was performed. Applying a load to the specimen as the temperature increased reduced the temperature at which viscous flow occurred (compared to test methods with no applied stress). Compressive strength increased at the elevated temperature and is attributed to viscous flow and sintering forming a more compact microstructure. The addition of alumina aggregate and reduction of water content reduced the thermal conductivity. This led to the earlier onset and shorter dehydration plateau duration times. However, crack formation was reduced and is attributed to smaller thermal gradients across the fire test specimen.

Fire-Resistant Geopolymers

Overview.- History of Geopolymers.- Portland Cement (OPC) and Concrete.- Geopolymer Applications.- Precursors and Additives for Geopolymer Synthesis.- Geopolymer Chemistry.- Fibres: Technical Benefits.- Thermal Properties of Geopolymers.- Fire Resistance of OPC and geopolymer.- Conclusion.

Precursors and Additives for Geopolymer Synthesis

The raw materials typically used in geopolymer manufacturing are described. The use of metakaolin, coal derived fly ash and slag as aluminosilicate sources is presented. The use of readily available coal fly ash in geopolymer products offers sustainability and technical benefits, especially in niche applications such as fire resistant products. There are national standards for the use of fly ash in concrete and these have some applicability to geopolymer production. For instance, beneficiation of fly ash by screening and milling can significantly improve physical properties of resultant geopolymer products. Activation of high calcium ground blast furnace slag and other slags result in more complex microstructures and are not as suitable for fire resistant products. Metakaolin is a purer source of aluminosilicate than fly ash. It is capable of ambient cures but has the penalty of high water demand which influences property development. The investigation of volcanic ashes as aluminosilicate sources has grown in importance recently. Alkaline hydroxides and silicates are used to activate the aluminosilicate sources and the sodium and potassium derivatives are commercially available. The speciation of alkali metal silicates is briefly outlined together with the influence of the alkali metal cation on this speciation. The use of sodium aluminate derivatives is examined, in particular the use of Bayer liquor, a by-product of alumina extraction from bauxite. The structure of sodium aluminate solutions is also reviewed. A final section describes the admixtures, typically superplasticisers and fibres, and fillers employed to modify the plastic and cured properties of geopolymers with emphasis on optimisation of fire resistant properties.

Mechanical properties of cotton fabric reinforced geopolymer composites at 200–1000 ℃

Geopolymer composites containing woven cotton fabric (0–8.3 wt%) were fabricated using the hand lay-up technique, and were exposed to elevated temperatures of 200 °C, 400 °C, 600 °C, 800 °C and 1000 °C. With an increase in temperature, the geopolymer composites exhibited a reduction in compressive strength, flexural strength and fracture toughness. When heated above 600 °C, the composites exhibited a significant reduction in mechanical properties. They also exhibited brittle behavior due to severe degradation of cotton fibres and the creation of additional porosity in the composites. Microstructural images verified the existence of voids and small channels in the composites due to fibre degradation.

Chemistry of Geopolymers

This chapter focuses on how the aluminosilicate sources react with the alkaline activating solutions. The application of model systems based on metakaolin is explored in detail and a simplified geopolymerisation model is shown. The critical influence of water content on the outcome of geopolymerisation is clearly demonstrated. The wide range of analytical techniques employed to characterise the precursors and geopolymer microstructure is addressed. In the case of fly ash based geopolymers the interpretation of kinetic and mechanistic results is more difficult due to the presence of crystalline materials. Variation in fly ashes sourced from different plants also contributes to these difficulties. The alkali reactive glassy aluminosilicates may not be readily accessible for alkali dissolution reactions, being shielded by non-reactive phases. An alkali activation of fly ash model is described. The presence of soluble calcium entities changes the kinetics of fly ash geopolymerisation. It is shown that increasing CaO content generally increases compressive strength, reduces setting times and facilitates ambient curing.

Fire Resistance of OPC and Geopolymers

Geopolymer based systems have inherently superior fire resistance compared to Portland cement based and organic polymer systems. Geopolymer systems are substantially inorganic based and are considered incombustible, emitting no toxic fumes when exposed to fire. Compared to Portland cement based systems geopolymers retain a significant level of structural stability after exposure to fire events and show little if any explosive spalling. Spalling may be controlled by the addition of organic fibres which vaporise leaving a network of channels which facilitate water escape. Standard fire testing curves for the evaluation of cementitious materials are described together with the outcomes of using these test standards with both geopolymer and OPC components. The replacement of organic binders by geopolymer in woven fabric reinforced composites lead to systems meeting the Federal Aviation Authority (FAA) requirements. An extension of this work has lead to the development of lightweight, fire resistant coatings. A brief overview of passive fire systems for tunnels is included.

Fibres: Technical Benefits

The benefits of metallic, organic and inorganic fibres and fillers in both OPC and geopolymers are discussed at length. Cementitious materials are typically characterised by low tensile strength and strain capacity and are sensitive to micro cracking. Fibres, and/or steel and Fibre Reinforced Plastic (FRP) rebar may be incorporated into cementitious matrices to overcome these weaknesses giving materials with increased tensile strength, ductility, toughness and increased durability. The mechanism of fibre reinforcement is common to OPC and geopolymers and as such the literature covering OPC-fibre composites is relevant. The mechanism of fibre reinforcement is discussed together with comments about the effects of fibres on processibility. Fibres also contribute to improvements in durability of cementitious composites such as corrosion and fire resistance. The properties and attributes of each fibre type are outlined with respect to the result achieved in the cementitious matrix. Fibres can reduce plastic cracking in fresh concrete and improve the post crack ductility of hardened concrete. An extensive range of available fibres is covered; natural and synthetic, inorganic and organic as well as a section on carbon based fibres. Hybrid fibre blends, typically steel and polypropylene, can give synergistic effects.

Introduction to Geopolymers

Geopolymers, also referred to as Aluminosilicate Inorganic Polymers (AIP) and Alkali Activated Cement (AAC) are based on alkali soluble aluminium and silicon precursors (aluminosilicates). Structural differences and resulting properties of geopolymers can be explained by variation in the source silicon to aluminium amorphous molar ratio, alkali metal cation type and concentration, water content and curing regime amongst other variables in the geopolymer synthesis.