Microwave Enhanced Drying and Firing of Geopolymers

Abstract

The feasibility of using microwave energy to dry and fire pre-cured geopolymers was experimentally demonstrated, and supported by analysis of published microwave dielectric data for geopolymers. Dielectric loss tangents and half power microwave absorption depths were calculated from published room temperature dielectric constant and loss values of various geopolymer compositions. The published data indicated that geopolymers would heat at room temperature with microwave energy. Several laboratory experiments were performed to test the heating behavior of sodium and potassium based geopolymer compositions. Experiments demonstrated more vigorous microwave heating with sodium geopolymers than with potassium geopolymers. Both compositions were dried in less than 10 minutes with pure microwave heating. Further heating with pure microwave energy resulted in non-uniform, rapid heating, or “thermal runaway”, with localized melting of the geopolymer. Hybrid microwave heating with susceptors resulted in uniformly fired geopolymers, without melting. INTRODUCTION Interest in geopolymers has accelerated in recent years due to their possible use in structural applications, dentistry, and hazardous waste stabilization, while maintaining a trivial environmental impact compared to traditional building materials. Many useful geopolymer compositions are fabricated by low temperature chemical reaction based curing processes. Some recent research has been reported for using microwave energy to enhance curing and drying . Other research has focused on high temperature firing of geopolymers, which can develop stronger glass-ceramic materials. In situations where heat is required, as in drying and firing, microwave energy provides an energy efficient alternative in place of conventional heating. Traditional radiant heating methods rely on thermal conduction to deliver heat throughout a material. Microwaves generate heat throughout the volume of the material, allowing faster, more uniform heating to occur. Volumetric microwave heating helps to overcome sluggish endothermic phase transitions, such as evaporation of water or decomposition of kaolin to metakaolin through the loss of hydroxides. Both drying and dehydration occur in the firing of geopolymers. In traditional heating, these reactions often require slow heating, as the endothermic reaction prevents heat from progressing into the product until the reaction completes first at the surface. Microwaves can generate heat throughout the part despite an endotherm. When microwave energy is the sole source of heat, a material that absorbs microwave energy will heat volumetrically, but cool from the surface. This situation creates an “inverse temperature profile” in which the sample is warmer inside and cooler at the surface during heating. This is opposite of traditional radiant heating where the material will be cooler in the center. In some materials, the inverse temperature profile can lead to thermal runaway – where the hotter center heats better than the cooler surface, and in turn heats better in the microwave. The thermal runaway can lead to molten centers with unfired surfaces. The most practical way to prevent thermal runaway is through hybrid microwave heating. Hybrid heating combines a radiant heat source with microwave energy, providing a uniform temperature profile throughout the sample as heat is generated at the interior while heat conducts from the exterior. This combination results in a uniform temperature profile and in turn improved properties. Two types of hybrid heating are susceptors, and Microwave Assist Technology (MAT). Susceptors function like wireless microwave heating elements, efficiently converting microwave energy into heat. When used with insulation thermal packages, susceptors can be used to fire ceramics even in standard kitchen microwaves. MAT is based on traditional kilns and uses gas or electric radiant heat, which is controlled independently of the microwave energy. MAT generally requires less microwave power, which reduces equipment costs and simplifies scale-up. Dielectric Properties The effectiveness of microwave heating for materials is generally determined by the dielectric properties at the microwave frequency. Two values represent the dielectric properties of a material, dielectric constant, e’, and dielectric loss, e”. Dielectric constant represents the ability for ions and dipoles within a material to polarize in response to an alternating electric field, and also determines the wavelength of the microwave energy within the material. The dielectric loss represents the degree to which an alternating field is converted to heat energy. From these two values can be derived the loss tangent, tan δ, and the half-power depth. A loss tangent between 0.01 and 1 generally indicates that a material will heat well with microwave energy. Below 0.01, materials tend to be microwave transparent, while above 1 materials become reflective. The half power depth measures the distance at which 50% of the microwave energy passing through a material is dissipated. The equations for loss tangent (1) and half power depth (2) are expressed below. In equation 2, DHP is the half power depth, c is the speed of light, ω is the angular frequency (2πf), and eo is the permittivity of free space.