The role of kyanite in the improvement in the crystallization and densification of the high strength mullite matrix: Phase evolution and sintering behaviour

Fine particles of kyanite were used as reinforcement in mullite matrix composite. Mix of metakaolin and meta-alumina (calcined bauxite) received different fraction of fine kyanite particles: 37.5, 42.5 and 50 mass%. The objectives were to improve the crystallization of kaolin-based mullite matrix reducing the amorphous content and the grain growth. Packing process and densification were enhanced, leading to the action of kyanite at low temperature (1350 °C), the kyanite particles are decomposed to mullite still without glassy phase and the metastable alumina presented in the mix reacts with amorphous silica to produce additional mullite. The sintering behaviour, the thermal expansion behaviour of ~0.5 % between 200 and 1000 °C and the coefficient of the thermal expansion of 5.92, 5.15 and 6.92 × 10−6/°C for KY3, KY4 and KY5, respectively, between 200 and 1000 °C, and their decrease at higher temperature demonstrated the optimum refractoriness of the mullite matrix composites developed

Thermal behaviour and microstructural evolution of metakaolin and meta-halloysite-based geopolymer binders: a comparative study

Two calcined clays (halloysite and kaolinite clays at 700 °C) were used as solid precursors for geopolymer synthesis. This study compares the physicochemical properties of the both resulting geopolymer series heated at 200, 400, 600 and 800 °C. The end specimens were characterized using Optical Dilatometer, XRD, FTIR, MIP and SEM analyses. Results revealed that the flexural strengths were 18.10 and 21.74 MPa for meta-halloysite- and metakaolin-based geopolymers, respectively. After subjected to high temperatures, the flexural strength drastically decreased from 18.10 ± 1.06 to 6.7 ± 0.23 MPa and 21.74 ± 1.20 to 4.63 ± 0.24 MPa, respectively. The maximum shrinkage recorded on metakaolin and meta-halloysite-based geopolymers was 14 and 16% around 950 °C, respectively. The thermal conductivities decreased with increase in heating temperature from 0.78 to 0.19 Wm−1 K−1 and 0.96 to 0.26 Wm−1 K−1, respectively. This reduction is linked to the additional voids and microcracks that occurred within the geopolymer network. The cumulative intrusion in both geopolymers increased with increase in heating temperature up to 600 °C, leading to the degradation of geopolymer network that affected the mechanical strength evolution. Both synthesized geopolymer series are potential candidates for insulation materials or refractory applications

Mechanical strength and microstructure of metakaolin/volcanic ash-based geopolymer composites reinforced with reactive silica from rice husk ash (RHA)

The physical, mechanical and microstructural properties of metakaolin/volcanic ash-based geopolymer composites were investigated, as well as the influence of the incorporation of a reactive silica from rice husk ash (RHA) in their performance. The geopolymer composites were designed by replacing metakaolin with volcanic ash, VA (10–30 wt%) and RHA (0–20 wt%). Physical and mechanical testing, FTIR, XRD, optical microscopic and SEM were used to characterize the geopolymer composites. The results showed that the mechanical strengths and physical properties were mostly affected by the VA content as well as RHA content where the optimal mechanical strengths (19.3 and 60.73 MPa for flexural and compressive strength, respectively) were obtained with the composite sample containing 20 wt% of VA and 10 wt% of RHA. This was due to the additive that enhances the compactness and microstructure of geopolymer matrices obtained and also the increase of SiO2/Al2O3 ratio with the addition of RHA. Synergistic use of metakaolin, volcanic ash, and rice husk ash for construction materials through alkaline activation looks like the upcoming trend to valorize these materials

Mechanical and microstructural properties of geopolymer mortars from meta-halloysite: effect of titanium dioxide TiO2 (anatase and rutile) content

Abstract: This study aimed to investigate the effect of Titanium Dioxide TiO2 (anatase and rutile) on mechanical and microstructural properties of meta-halloysite based geopolymer mortars namely GMHA and GMHR series. Meta-halloysite received 2.5, 5.0, 7.5 and 10 wt% of anatase or rutile as addition before calcination and geopolymerization. The raw materials and the end products were characterized using XRD, FTIR, ESEM and MIP analyses. The flexural strength increases from 6.90 to 9.13 MPa and from 6.90 to 12.33 MPa for GMHA and GMHR series respectively. The cumulative pore volume decreases from 102.2 to 84.2 mm3 g−1 and from 102.2 to 51.3 mm3 g−1 for GMHA and GMHR products respectively. Both matrices present micrographs with very low capillaries pores and fractured surfaces that confirmed the enhancement of the mechanical properties. It was concluded that TiO2 in both forms is beneficial for the reduction of porosity and densification of geopolymer matrices. Rutile enabled more compact and denser geopolymer structure compared to anatase. The aforementioned results showed the efficiency of both fine TiO2 particles to improve the geopolymer network significant for its durability. Graphic abstract: [Figure not available: see fulltext.]