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Microporous MgO–Mg(Al, Fe) 2 O 4 refractory aggregates were prepared by the in-situ decomposition synthesis method using the magnesite, Al(OH) 3 and Fe 2 O 3 as raw materials. The effect of raw material composition (theoretical Mg(Al, Fe) 2 O 4 contents were 0–55 wt %) on their microstructure and strengths was investigated. When the theoretical Mg(Al, Fe) 2 O 4 contents were relatively low (0–5.5 wt %), the number of neck connections between the particles in the microporous MgO–Mg(Al, Fe) 2 O 4 refractory aggregates was small. As the theoretical Mg(Al, Fe) 2 O 4 contents increased to be 11–22 wt %, the number of neck connections increased and the compressive strengths were enhanced. When the theoretical Mg(Al, Fe) 2 O 4 contents increased to be excessive (33–55 wt %), the inter-particle pore size further increased due to the increase of volume expansion caused by the formation of more spinel, resulting in a decrease of compressive strength. Overall, when the theoretical Mg(Al, Fe) 2 O 4 contents were 11–22 wt %, the microporous MgO–Mg(Al, Fe) 2 O 4 refractory aggregates showed the excellent performances with the median pore sizes of 17.37–25.46 μm, the apparent porosities of 23.4–28.1%, the bulk densities of 2.57–2.79 g/cm3 as well as the compressive strengths of 41.2–75.8 MPa. [ABSTRACT FROM AUTHOR]

Microporous MgO-Mg(Al, Fe) 2 O 4 refractory aggregates were prepared using magnesite, Al(OH) 3 and Fe 2 O 3 applying an in-situ decomposition synthesis method. At 1400–1600 °C, there was a Mg(Al, Fe) 2 O 4 with Fe3+, which had two structures. One was a ring structure formed from Al(OH) 3 pseudomorph particle as a template and a low content of Fe3+. The other was the dot and strip structures precipitated in magnesite pseudomorph particles with a high content of Fe3+. Besides, at 1550–1600 °C, microporous MgO-Mg(Al, Fe) 2 O 4 refractory aggregates had an excellent compressive strength (75.8–81.5 MPa) and apparent porosity (26.8%−28.2%). [ABSTRACT FROM AUTHOR]

Seven microporous MgO–Al2O3 ceramics with an Al2O3 content of 15–90 wt% were prepared using Al(OH)3 and calcined magnesite as raw materials. A wet mixing process was employed during sample preparation to transform the calcined magnesite with a larger particle size to smaller Mg(OH)2 particles. The in situ decomposition synthesis method and the Kirkendall effect were utilized to produce and control the pore structure of the microporous MgO–Al2O3 ceramics. There were two kinds of pores in the microporous MgO–Al2O3 ceramics. The first one resulted from the in situ decomposition of Al(OH)3 and Mg(OH)2 particles, which were small and equally distributed. Another one originated from the position of the Mg(OH)2 particles due to the Kirkendall effect caused by MgO diffusion. They were similar in size to the Mg(OH)2 pseudomorph particles. Simultaneously, the Al2O3 content affected the packing behavior and the spinel formation, which changed the characteristics of the pores and necks among the particles. These mechanisms also affected the strengths of the microporous MgO–Al2O3 ceramics. Thus, when the Al2O3 content was 45–90 wt%, the microporous MgO–Al2O3 ceramics had a high compressive strength (10.0–18.3 MPa) and apparent porosity (52.2%–58.4%). [ABSTRACT FROM AUTHOR]

In this investigation, microporous MgO-MgAl 2 O 4 refractory aggregates were prepared by utilizing Mg(OH) 2 powder and α-Al 2 O 3 micro-powder as raw materials through employing an in situ decomposition synthesis method. We focused on the impact of α-Al 2 O 3 micro-powder contents on the microstructure and properties of the aggregates. As the α-Al 2 O 3 micro-powder increased from 0 to 6 wt%, the volume expansion of the MgAl 2 O 4 formed remedied the shrinkage of the pseudomorph particles, resulted a decrease in the inter-particle pore size and the development of MgAl 2 O 4 necks. The apparent porosity of the aggregates was reduced, while enhancing the flexural strength. With the α-Al 2 O 3 micro-powder content continues increased to 21 wt%, the inter-particle pore sizes increased. This was because the significant expansion of the spinel formed among the pseudomorph particles, which led to a decrease in the necks among them. The progressive decrease of the flexural strengths can be attributed to its responsibility. In summary, the best performing specimen was microporous aggregates of MgO-MgAl 2 O 4 refractory with the α-Al 2 O 3 micro-powder content of 6 wt%, which owned flexural strengths before and after thermal shock of 49.10 MPa and 37.43 MPa, a bulk density of 3.01 g/cm3 and a thermal conductivity of 6.54 W/(m K) at 800 °C. The thermal conductivity decreased by 36.5% at 800 °C with the comparation of commercial sintered magnesia. • The microporous MgO-MgAl 2 O 4 refractory aggregates have nano-sized pores at high temperature. • New lightweight microporous MgO-MgAl 2 O 4 refractory aggregates with a higher strength were fabricated. • More MgAl 2 O 4 necks formed between the Mg(OH) 2 pseudomorph particles using α-Al 2 O 3. • The increase of the MgAl 2 O 4 formed decreased the inter-particle pore size. • α-Al 2 O 3 was used to reduce the cost of the microporous MgO-MgAl 2 O 4 refractory aggregates preparation. [ABSTRACT FROM AUTHOR]

• Ettringite, monosulfate and aluminum hydroxide gel constitute the main hydration products of B-CSA cement. • SEM combined with nitrogen adsorption experimental characterization technology to evaluate the micro-morphology and pore evolution of B-CSA cement. • The changes in PH, turbidity and electrical conductivity further explain the ion dissolution and the transformation of AFm and AFt during the early hydration of B-CSA cement. • A new hydration mechanism of Ba-doped B-CSA cement is proposed. Belite-calcium sulphoaluminate (B-CSA) cement is regarded as one of the most promising alternatives to ordinary Portland cement (OPC). The production of B-CSA cement from industrial solid waste is a critical technology for maximising mineral resource utilisation. The morphology and microstructure of hydration products produced during the hydration process of B-CSA cement have a significant impact on the mechanical properties of the cement. In this paper, B-CSA cement prepared by electrolytic manganese residue (EMR) and barium slag (BS) was taken as the research object, and the changes of PH, turbidity and electrical conductivity in the early hydration process were monitored. Furthermore, the phase composition and microstructure of hydration products of cement paste were analysed using X-ray diffraction (XRD), Fourier transform infrared (FT-IR), thermogravimetric analysis (TG), solid state MAS-NMR spectroscopy, nitrogen adsorption test, scanning electron microscope (SEM) and electron microprobe analyzer (EPMA). The results show that with the sharp change of pH value, turbidity and electrical conductivity in the early hydration process of B-CSA cement, a large amount of ettringite is formed during the rapid hydration of ye'elimite, the metastable AFt part is transformed into AFm and the hydration reaction of belite was inhibited. The hydration products are mainly ettringite, monosulfate and AH 3 , which are responsible for the strength development of B-CSA cement. The hydration rate of belite in the early stage is slow, and the C-S-H gel phase can be formed in an aluminum-rich environment. The microstructure evolution of cement paste was revealed by nitrogen adsorption tests and SEM. The crystal state of ettringite changes from columnar to fibrous to construct the hardened framework of disorderly distributed cement paste, and the gel phase fills in the void formed by the ettringite skeleton to form a dense structure and reduce the porosity. The interface between the unreacted belite phase and the AFt skeleton is clear, and the gel phase plays a role in connecting them. Mn2+ was enriched on the C-S-H gel phase to form the Ca-Si-Mn compound and solidified. Ba2+ preferentially forms BaSO 4 with SO 4 2− which precipitates at the interface between the ettringite phase and C-S-H gel phase. Based on the analysis of the test results, the hydration characteristics and microstructure evolution of B-CSA cement were determined. [ABSTRACT FROM AUTHOR]