Effects of Li Substitution on the Microstructure and Thermal Expansion Behavior of Pollucite Derived from Geopolymer

The purpose of this work was to study the role of lithium in cesium-based geopolymers and the thermal evolution during heat treatment together with thermal expansion behavior of the resulting geopolymer ceramic. A series of lithium-substituted cesium-based geopolymers, Cs(1−x)LixGP (where x = 0, 0.1, 0.2, and 0.3), were prepared. All the geopolymer samples were heated at 1300°C for 2 h and thermal evolution on heating was studied by a variety of techniques. Phase composition, microstructure evolution, and thermal expansion behaviors of the ceramics derived from the geopolymers were characterized. All the geopolymer specimens exhibited similar thermal evolutionary trends. With increases in lithium content, overall mass loss increased gradually due to the higher hydration energy of Li+ than Cs+. Thermal shrinkage of these specimens can be divided into four stages, i.e., structural resilience, dehydration, dehydroxylation, and sintering, according to the dilatometer results. The introduction of Li results in two-step sintering behavior for the lithium-substituted cesium-based geopolymers. The average thermal expansion coefficient (CTE) of Cs(1−x)LixGP ceramics decreased from 4.80 × 10−6 K−1 (x = 0) to 3.61 × 10−6 K−1 (x = 0.3) with increase in lithium substitution. The reason can be attributed to the presence of spodumene after thermal treatment, which has a relatively low thermal expansion coefficient compared with pollucite. Meanwhile, molten spodumene could serve as a buffer phase between pollucite crystals also conducive to the decline of CTE of this system.