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. INTRODUCTIONInterest 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 1 " 3 . Some recent research has been reported for using microwave energy to enhance curing and drying 4 ' 5 . Other research has focused on high temperature firing of geopolymers, which can develop stronger glass-ceramic materials 6 " 8 . 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 9 .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 sur...

Doğal puzolan-esaslı jeopolimer üretiminde mikrodalga kürünün rolü

“…Yani, mikrodalgada kritik bir kür süresi mevcuttur ve bu sürenin ötesinde, Şekil 3'te gösterildiği gibi, özellikler olumsuz yönde etkilenmektedir. Mikrodalganın yoğun ve doğrudan malzeme çekirdeğine nüfuz edici ısıtma özellikleri nedeniyle, jeopolimer numunelerin sıcaklığı birkaç dakikalık mikrodalga ışıması ile 200℃'nin çok üzerine çıkabilir [48,49]. Isıtma sırasında, nispeten düşük bir sıcaklık aralığında (ortam sıcaklığı ila 100℃) fiziksel olarak bağlı su kolayca bünyeden uzaklaşmaktadır.…”

The Use of Rice Husk Ash-Based SiC Particles as Reinforcement in Geopolymer Composites

“…dayanıklı ve asit dirençli 3-boyutlu bir yapıdır [1][2][3][4][5][6][7][8][9][10]. Geopolimerleşme, alümina silikatın yüksek yakıcı ortamda çözünmesi, su yardımı ile taşınımı, çapraz bağlı üçboyutlu amorf bir yapı içerisinde çoklu yoğuşması aşamalarını içeren ekzotermik kimyasal işlemdir [5,11].…”

“…Geopolimerler, portland çimentoya göre atmosfere daha az karbondioksit (%80) salmaları nedeniyle çevre dostu olmakla beraber hafif, dayanıklı (portland çimentonun yaklaşık 1.5-2 katı basma ve eğme dayanımı) ve kısa sürede üretim avantajlarına da sahiptirler [1,[3][4][5][6][7]. Geopolimerlerin uçucu kül [8], kömür cürufu [9], çeşitli killer ve tarımsal atıklar [10] gibi birçok ucuz hammaddeden sentezlenebilmeleri de ekonomik açıdan bir diğer önemli avantajıdır.…”