In Situ Mechanical Properties of Chamotte Particulate Reinforced, Potassium Geopolymer Sean S. Musil and Waltraud M. Kriven †, ** Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Geopolymers are an inorganic polymeric material composed of alumina, silica, and alkali metal oxides. Monolithic geopoly- mer is brittle and susceptible to dehydration cracking at ele- vated temperatures. The addition of a reinforcing phase not only improves strength and toughness but also maintains the structural integrity of the material at elevated temperatures. For this study, potassium-based geopolymer (KGP) is rein- forced with varying weight percent of chamotte particles. Chamotte is kaolinite grade clay calcined at 1350°C to pro- duce 38% crystalline mullite, as well as metastable cristobalite and quartz. The chemical composition of the chamotte is almost identical to that of the metakaolin used to create the geopolymer, however, its crystalline nature prevents reactivity with the caustic potassium silicate solution and it remains as a particulate reinforcement. Flexural strength is evaluated at room temperature and in situ at elevated temperatures to just below the leucite crystallization temperature. Reinforcement with 25 wt% chamotte has shown a two-fold increase in room- temperature flexural strength. Flexural strength is also evalu- ated at room temperature after heating above the leucite crys- tallization temperature to determine if the chamotte aids in maintaining structural integrity during the volumetric contrac- tion and destructive transformation from cubic to tetragonal symmetry upon forming leucite. I. Introduction G EOPOLYMERS are an inorganic polymeric material com- posed of alumina, silica, and an alkali metal oxide. They are synthesized as a liquid, or more specifically a fluid mixture or particles and liquid, allowing them to be cast into any desired shape, and cure at room or slightly elevated tem- peratures. 1,2 Geopolymers form an X-ray amorphous alkali aluminosilicate tetrahedral framework of zeolites, which are inherently fire-resistant. 3 Numerous supplementary cementi- tious materials (SCMs) (e.g., slag, silica fume, fly ash, metakaolin, and limestone) have also been utilized as addi- tives to ordinary portland cement (OPC)-based systems to reduce the overall environmental impact of its production. 4 The use of SCMs can reduce CO 2 emissions, as well as utilize waste products that would otherwise require disposal. Geo- polymers, including fly ash-based geopolymer, have been used as alternative alkali-activated binders to OPC-based sys- tems. 4,5 Setting time is faster and environmental impact dur- ing production is significantly lower as compared with OPC. They also significantly outperform OPC in strength. 3 The worldwide abundance of aluminosilicate materials also gives geopolymers the potential to be more cost effective than OPC. 1,2 Geopolymers have demonstrated good adhesion proper- ties, bonding strongly to a wide range of ceramics, metals, and even polymers. 6 Geopolymers are also resistant to heat and oxidizing environments. Potassium-based geopolymer remains chemically stable up to 1000°C. 7 Potassium geopoly- mer of other chemical compositions, for example, potassium polysialate, has shown thermal stability up to 1400°C. 8 Between 1000°C and 1100°C, potassium geopolymer crystal- lizes into cubic leucite. 9,10 Leucite, a ceramic of composition K 2 OÁAl 2 O 3 Á4SiO 2 , is desirable in its own right as it retains strength up to 1200°C. 7 However, during the heating process the geopolymer must first shed its free water which occurs up to 400°C. 9 This dehydration process is quite destructive for unreinforced potassium geopolymer. Heated samples develop microcracks that increase in size with increasing temperature. Microcracks develop due to stress gradients that occur as a result of dehydration when free water is forcefully extracted by capillary forces and capillary contraction through the ~40 vol% of porosity that occurs during the curing pro- cess. 10–12 The addition of a reinforcement phase has proven to inhibit this destructive dehydration process by bridging cracks as they develop and also by providing pathways for graceful dehydration. 13 Geopolymers are well suited as a matrix or binder for composite materials due to the ease of synthesis and rein- forcement adhesion. The good adhesive behavior of geopoly- mers allows for maximum load transfer to the reinforcement resulting in increased strength. Mechanical properties of the geopolymer can be tailored through the choice of reinforcing phase to either add strength or toughness, depending on the application. Reinforcements can range from high cost, high strength metal oxide or silicon carbide fibers to low cost, abundant and renewable natural fibers, or even particulates. Particulate reinforcement can offer modest strength or durability improvements over monolithic materials without significant added processing difficulty. In the case of a geo- polymer binder, it is important to select a particulate rein- forcement that will be unreactive with the geopolymer constituents during curing and thus reduce the degree of geo- polymerization, as well as being chemically resistive to the high alkalinity of the geopolymer paste. Previous works have used quartz sand, crush porcelain, alumina, ceramic spheres, ground up refractory brick particles, 14 calcite, or dolomite particles 15 as reinforcement in geopolymer binders. For high- temperature applications, thermal stability is also an impor- tant consideration. Detrimental phase changes or incompati- ble thermal expansion can lead to unfavorable mechanical properties at elevated temperatures. For these reasons, as well as low cost and availability, chamotte particulates were selected for investigation as a reinforcement to KGP binder. Chamotte is produced by calcination of kaolinite clay in a rotary kiln. It has been used for ceramic bonding for refrac- tories and creep resistant applications due to its mullite con- tent. 16 Chamotte has also been mixed with clays to reduce the sintering stresses that produce cracks and flaws. 17 J. Biernacki—contributing editor Manuscript No. 33625. Received August 11, 2013; approved October 23, 2013. **Fellow, The American Ceramics Society. Presented in part at the 37th International Conference and Expo on Advanced Ceramics, January 31, 2013 (ICACC-FS1-014-2013). † Author to whom correspondence should be addressed. e-mail: kriven@illinois.edu 907 J. Am. Ceram. Soc., 97 [3] 907–915 (2014) DOI: 10.1111/jace.12736 © 2013 The American Ceramic Society J ournal