The main objective of this research was to investigate the influence of curing temperature on the phase transformation, mechanical properties, and microstructure of the as-cured and sintered kaolin-ground granulated blast furnace slag (GGBS) geopolymer. The curing temperature was varied, giving four different conditions; namely: Room temperature, 40, 60, and 80 °C. The kaolin-GGBS geopolymer was prepared, with a mixture of NaOH (8 M) and sodium silicate. The samples were cured for 14 days and sintered afterwards using the same sintering profile for all of the samples. The sintered kaolin-GGBS geopolymer that underwent the curing process at the temperature of 60 °C featured the highest strength value: 8.90 MPa, and a densified microstructure, compared with the other samples. The contribution of the Na₂O in the geopolymerization process was as a self-fluxing agent for the production of the geopolymer ceramic at low temperatures.

Kaolin, theoretically known as having low reactivity during geopolymerization, was used as a source of aluminosilicate materials in this study. Due to this concern, it is challenging to directly produce kaolin geopolymers without pre-treatment. The addition of ground granulated blast furnace slag (GGBS) accelerated the geopolymerization process. Kaolin–GGBS geopolymer ceramic was prepared at a low sintering temperature due to the reaction of the chemical composition during the initial stage of geopolymerization. The objective of this work was to study the influence of the chemical composition towards sintering temperature of sintered kaolin–GGBS geopolymer. Kaolin–GGBS geopolymer was prepared with a ratio of solid to liquid 2:1 and cured at 60 °C for 14 days. The cured geopolymer was sintered at different temperatures: 800, 900, 1000, and 1100 °C. Sintering at 900 °C resulted in the highest compressive strength due to the formation of densified microstructure, while higher sintering temperature led to the formation of interconnected pores. The difference in the X-ray absorption near edge structure (XANES) spectra was related to the phases obtained from the X-ray diffraction analysis, such as akermanite and anothite. Thermal analysis indicated the stability of sintered kaolin–GGBS geopolymer when exposed to 1100 °C, proving that kaolin can be directly used without heat treatment in geopolymers. The geopolymerization process facilitates the stability of cured samples when directly sintered, as well as plays a significant role as a self-fluxing agent to reduce the sintering temperature when producing sintered kaolin–GGBS geopolymers.

The effect of different sintering parameters on the mechanical properties of sintered kaolin-GGBS will influence the variation of mechanical properties of sintered kaolin-GGBS geopolymer. Based on previous research, the samples have major cracking and many large pores due to the sintering temperature and holding time during the sintering process. The first objective is to study the effect of different sintering parameters on the mechanical properties of sintered kaolin-GGBS geopolymer and the second objective is to correlate the strength properties of sintered kaolin-GGBS geopolymer with microstructural analysis. In a solid-to-liquid 2:1 ratio, kaolin and GGBS were combined with an alkali activator. The kaolin-GGBS geopolymer was then cured at room temperature for 24 h. The samples were then cured for 14 days at 60 °C, followed by using double-step sintering at temperatures of 500 °C and 900 °C with varying heating rates and holding durations. The compressive strength and shrinkage of the kaolin-GGBS geopolymer were evaluated, and the morphology was examined using a scanning electron microscope. In comparison to other samples, the sintered kaolin-GGBS geopolymer with a heating rate of 2 °C and a holding duration of 2 h had the optimum compressive strength value: 22.32 MPa. This is due to the contribution of MgO from GGBS that refines the pore and increases the strength. The 13.72% shrinkage with a densified microstructure was also obtained at this parameter due to effective particle rearrangement during sintering.

The fabrication of hydroxyapatite (HAp) ceramics prepared by existing conventional sintering requires high-temperature sintering of 1250 °C to 1300 °C. In this paper, the activated metakaolin (MK)/HAp specimens were prepared from varied mix design inputs, which were varied solid mixtures (different amounts of MK loading in HAp) and liquid-to-solid (L/S) ratios, before being pressed and sintered at 900 °C. Phase analysis, thermal analysis, surface morphology, and tensile strength of the specimens were investigated to study the influences of the Al, Si, Fe, Na, and K composition on the formation of the hydroxyapatite phase and its tensile strength. XRD analysis results show the formation of different phases was obtained from the different mix design inputs HAp (hexagonal and monoclinic), calcium phosphate, sodium calcium phosphate silicate and calcium hydrogen phosphate hydrate. Interestingly, the specimen with the addition of 30 g MK prepared at a 1.25 L/S ratio showed the formation of a monoclinic hydroxyapatite phase, resulting in the highest diametrical tensile strength of 12.52 MPa. Moreover, the increment in the MK amount in the specimens promotes better densification when sintered at 900 °C, which was highlighted in the microstructure study. This may be attributed to the Fe₂O₃, Na₂O, and K₂O contents in the MK and alkaline activator, which acted as a self-fluxing agent and contributed to the lower sintering temperature. Therefore, the research revealed that the addition of MK in the activated-HAp system could achieve a stable hydroxyapatite phase and better tensile strength at a low sintering temperature.

The sintering behavior of hydroxide/metakaolin hydroxyapatite alkali-activated material, the resulting volume shrinkage and microstructure analysis are influenced by metakaolin contribution and alkali activator ratio. The alkali-activated material is initially produced by alkaline activation of hydroxyapatite and metakaolin with alkaline activator solution and sintered at from 875°C to 950°C with a dwell time of 2 hours. It has been found that, the optimum volume shrinkage of 41.92% at 900°C sintering temperature for sample activated by 1.0 alkaline activator ratio. As increasing the ratio of alkaline activator ratios up to 1.25, the rough surface with larger pores are obtained on the microstructure surface. The evolution of microstructure on hydroxyapatite/metakaolin alkali-activated material at higher sintering temperatures up to 1250°C, are analyses. Porosity were found as a key role in determining the properties of sintered hydroxyapatite/metakaolin alkali-activated material.

This study investigates the microstructural transformations of rice husk, with a focus on the amorphous-to-crystalline phase transition and the impact of acid leaching treatments. Recognizing rice husk’s potential as a source for high-purity silica materials, the research explores the influence of treatment sequences and citric acid concentrations on microstructure and impurity removal. Utilizing X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM), the study analyzes leached rice husk samples subjected to varied acid leaching conditions. Notably, XRD reveals a consistent amorphous phase in both leached rice husk (LRH) and leached rice husk ash (LRHA), independent of pre-treatment methods. SEM images demonstrate enhanced SiO2 particle morphology at different citric acid concentrations compared to raw rice husk particles. The nuanced interplay between treatment sequences, acid concentrations, and resulting microstructures is highlighted, indicating the potential for tailored high-purity silica materials from rice husk. The presence of pores in rice husk ash further underscores the efficacy of acid leaching in impurity removal. In conclusion, the study offers a comprehensive understanding of amorphous microstructural evolution in rice husks, presenting opportunities for controlled microstructure development in the production of high-purity silica materials.